Making LED headlights RF-quiet

TL;DR

 If you have LED headlights that are causing RF interference, ferrites alone will probably not be enough to completely solve the problem:  You will probably need to put the offending switch-mode LED controller in "RF Jail" as described below.  While VHF/UHF is the target of the efforts noted below, they should be equally effective at HF.

* * * * *

We live in a world of RF - and an increasing amount of this is from devices that are not intended to produce radio-frequency energy.  These devices have proliferated in the past several decades and surround us at home, work and in our cars.

Figure 1:
The vehicle in question with LED headlights -
which are now RF-quiet on all bands.
Click on the image for a larger version.

Generally speaking, modern internal combustion vehicles are in RFI terms "pretty quiet" - especially compared to their electric and hybrid counterparts - and someone in such a vehicle will experience less RF noise when they are "out and about" in a rural-ish area than they will at home.  For amateur radio operators, this is a good thing as mobile operation often includes weak signals - whether this is on HF, or on VHF/UHF with weak signals from distant repeaters or during simplex operation, with intervening terrain.

Effective mobile operation is therefore contingent on a vehicle that is intrinsically "RF-quiet", but this also means that any accessories that you might add to this vehicle also be RF-quiet as well.  These days there are any number of things that you might throw in your car that can spoil an otherwise-clean RF environment and the short list includes USB chargers¹ , GPS receivers and extra lighting ², to name but a few.

Real-world case - Aftermarket LED headlights

A friend of mine recently installed aftermarket LED headlights in his older Honda CRV ³.  It took a while to correlate the cause, but he eventually noticed that when the headlights were turned on, he lost significant weak-signal sensitivity on 2 meters:  Around town and with stronger repeaters, the effect wasn't really noticeable, but when the repeater was distant - or when communicating simplex (e.g. direct - with no repeater) with a weak/distant station - turning on the headlights dramatically reduced received ⁴ signal quality and range.  As it happens, he has more than one VHF/UHF radio in his vehicle and although both were affected, the one with its antenna mounted to the front fender - much closer to the LED headlights - was more severely impacted.

Retrofit LED headlights typically consist of two modules:  The LED module itself and the "controller" - which itself is a switch-mode power supply to regulate the current to the LEDs as well as switching arrays of LEDs for high/low beam - and even color temperature in some models.  Sometimes these units are combined, but in this case, they were separated with a short cable, the "controller" being separate from the LED module that is mounted in the headlight housing.

Even knowing that it was futile, we tried putting clamp/snap-on ferrites on the cables to/from the LED modules and its controller, but all that we could manage was a slight reduction in interference that was hard to quantify, taking the problem from being "terrible" to just "awful".  This was not unexpected:  Under the very best conditions ferrites alone may provide 15-20dB of reduction in conducted energy (2-3 "S" units) but at VHF/UHF getting anywhere near that much attenuation is very difficult - and measurements indicated that even if we did achieve 15dB reduction across the board, the "jamming" of weak signals by the headlights' switch-mode controllers would still be significant:  It would be like taking an "10-over S-9" interfering signal down to just "S-8" - still pretty bad!

Methods of filtering

As noted earlier, simply putting ferrite devices on the conductors can reduce the amount of conducted energy, but their effect is typically limited - likely 15-20dB in the best case when this is the only method employed and ferrites alone are likely to be effective only when the interference is very slight to begin with.  Ferrite devices - such as beads - simply add inductance (and thus loss or impedance) to RF energy while leaving DC and low-frequency signals alone, but these devices have limitations:  Properties such as self-resonance and the permeability of the magnetic material vary wildly with frequency and high levels of attenuation are difficult to attain - particularly at high frequencies (e.g. VHF/UHF) where even short conductors carrying RF currents can radiate with reasonable efficiency - particularly when the noise-generating device and the receive antenna are in close proximity.

As discussed previously on this blog ⁵ one sure-fire way to quash such interference is to put the offending device in "jail" - that is, enclose it completely in a metal box and use both inductive and capacitive filtering on each and every wire to prevent RF energy from being conducted and the metal enclosure can prevent direct radiation from leads themselves - particularly important at VHF/UHF but less-so at HF.  Done properly, this method can be "completely" effective ⁶ in preventing interference.

Figure 2:
Simplified diagram of the method of filtering.  The inductance - provided by the ferrite beads ("L")
provide "choking" impedance to the RF currents being carried on the wires from the LED controller
(the "noisy device") before they connect to the feedthrough capacitors ("FT") in order to maximize
their efficacy.
Click on the image for a larger version.

Figure 2 shows a very effective method of dealing with this problem and it involves inductance ("L") and capacitance in the form of "feedthrough" capacitors (marked "FT").  The inductance is in the form of a ferrite bead installed on each of the conductors between the "noisy device" and the capacitors that increase the impedance at radio frequencies on that conductor.  The capacitors are then used to shunt the remaining RF energy to the local "ground" which, in this case, is the partition on which the feedthrough capacitors are mounted (this will be discussed shortly) and also the metal enclosure in which the noisy device is mounted.

The intent here is to prevent RF currents flowing through to the "external connections" where the wires themselves will act as antennas to radiate the RFI generated by the noisy device.  By shunting RF to the partition - and the metal box itself - the remaining RF energy will be minimal and confined within the enclosure.  The above configuration is easily capable of attenuating RF energy from HF through UHF by 30dB or better (more likely 50-60dB at some frequencies) - a value far higher than ferrite alone.

The metal box containing the electronics and filtering offer another important benefit:  As even short conductors a few inches/cm long can radiate at VHF/UHF, placing the noisy device and its conductors within the metal enclosure will prevent this.

An important feature of the design is that the inductances ("L") are located between the noisy device and the feedthrough capacitors.  As these inductances (ferrite beads) offer 10s to 100s of Ohms of impedance to the RF signal, this allows the very low impedance of the feedthrough capacitors at those same frequencies (which is likely an Ohm or less at higher frequencies) to more-effectively shunt that energy.  If the ferrite beads - or some sort of inductor - had not been installed, the shunting of the low-impedance RF energy from the noisy device would have been far less effective.

Figure 3:
An assortment of feedthrough capacitors.  The top two
rows are of the "screw-in" type, typically mounted to chassis-
walls and bulkheads while those on the bottom row are the
"solder-in" type as used in the partition in this project.
Click on the image for a larger version.

In this case, we were preventing RF from a noisy device from leaving the enclosure - but if we were trying to protect a sensitive device FROM RF energy from a nearby transmitter we would place the inductances on the conductors coming from the outside world as well to allow the capacitors to better-perform their function.

Feedthrough capacitors and the partition

A bit more needs to be said about "feedthrough capacitors".  Even if you are "into" electronics, you may not have seen these devices for the simple reason that they are a bit esoteric - and, perhaps, they are not quite as prominent as they have been in the past.  Figure 3 shows an assortment of feedthrough capacitors:  The top two rows are chassis-mounted types that are held in a pre-drilled hole by a nut while the three on the bottom row are of the "solder-in" type.

A feedthrough capacitor has a wire that passes through its center with the "capacitance" surrounding this wire over the length of the of its body and the other "plate" of this capacitor is the body of the feedthrough capacitor itself.  By being constructed this way, there are no wires or leads between the "capacitor" part  and either the signal or ground wires and as such, any series inductance - which would reduce the efficacy of the capacitor - is minimized.

Figure 4:
Solder-in type feedthrough capacitors soldered
to the brass partition.  This large sheet of metal
provides a low-impedance RF path to the
common "ground" (e.g. case) to contain RF
entirely within the metal case.
Click on the image for a larger version.

Compared to a "normal" capacitor with wires, such a capacitor is far more effective at bypassing RF energy to "ground" and it also suffers much less from parasitic issues like self-resonance - a property in which the capacitor and its internal inductance form a resonant circuit can cause it to practically "disappear" from the circuit (e.g. cease to be effective) at certain frequencies.  For this application - where it's particularly important to reduce RF interference at VHF and UHF - the use of feedthrough capacitors is - along with some inductance - a nearly foolproof method of attenuating such energy without resorting to surface-mount components and/or a specially-designed PC board. ⁷

Figure 4 shows nine feedthrough ⁸ capacitors soldered to a brass partition (the soldering to the plate is on the opposite side) and as can be seen from the photo, these are capacitors that have a wire that runs through them.  As such, they have no "ground lead" aside from the outside of the body of the device and all of them are tied together on the same piece of metal.  This method assures a low impedance RF path between all of the capacitors and since the partition itself is bolted to the aluminum case (see Figure 5), it, too, is well-bonded.

Putting it in the box

Figure 5:
The LED controller in the box w/filtering.  By
containing RF currents within the box, both
common-mode and differential RF currents
on the leads are reduced to near zero.
Click on the image for a larger version.

To eliminate direct radiation from even the very short leads, the LED's controller which can cause problems at VHF/UHF, ferrite beads ⁹ and feedthrough capacitors are contained within a box as seen in Figure 5.

Toward the top of the image we see the switch-mode controller for the LED headlights, bolted inside the case (which also helps dissipate heat) and farther down we see that all nine wires (three for power, the remaining six to the LED module itself) connect to the feedthrough capacitors on the partition. Each of these wires has its own ferrite bead and these wires go directly to their respective feedthrough capacitors on the brass partition, which is held in the case with screws.

Below the partition (see Figure 5) are the wires that connect to the outside world:  On the right are the three wires that go to the power supply (e.g. the original connector to the headlights) while the gray cable on the left goes to the LED module.  The original LED retrofit had very short leads - on the order of 5" (13cm) for the headlight connector and another set of similar length to the LED module - and this made the installation a bit challenging as there was just enough wire to make the connection between the controller and the capacitors on the partition and install the rather large ferrite beads.

The box containing everything is die-cast aluminum and it's a bit larger than necessary - but it was the only size for which I had two identical cases and also large enough to accommodate the LED controller and the filtering.  As this box is quite a bit larger than the original controller, rather long wires had to be used to allow it to be placed where there was room in the rather crowded engine compartment, somewhat away from the headlights.

Figure 6:
As the original cables were very short (about 5", 13cm) longer
wires had to be spliced to allow placement of the large die-
cast box.  This shows the male headlight connector and the
LED module spliced to the cable and covered with nylon web.
Click on the image for a larger version.

At the opposite end of the wires it was necessary to splice the added cable to the LED unit's headlight connector and LED module and this was carefully done using soldered connections insulated with head-shrinkable tubing, all of which was covered by woven nylon braid for protection and a neater appearance.

It's worth noting that there is no effort to "ground" the box itself to anything.  While the LED controller happens to be connected to the box with a mounting screw, the fact that RF currents are contained within the box by means of the bypass capacitor means that there are no such potential differences outside the box.

The result

With the added length of the cables, there were nooks and crannies into which the die-cast box with the LED controller could be placed within the engine compartment of the Honda CRV.

The real test came when a distant 2-meter repeater was keyed up to cause it to send its ID:  The return signal was very weak and noisy - as hoped and expected - but there was no difference in the way that it sounded when the lights were switched on and off.  While admittedly unscientific, this test tells us pretty much everything that we needed to know:  Whatever RF interference there is that might be escaping the box and its filtering is well below the level at which it can be detected and the problem is considered to be solved!

* * * * *

Footnotes:

1. The topic of "very noisy" plug-in USB chargers effectively "jamming" VHF/UHF reception was discussed on this blog several years ago - see:  "How USB car power power adapters can ruin 2 meter mobile reception" - link and its follow-up article:  "A 'quiet' 5 volt USB car power supply" - link.
   
2. This same friend frequently volunteers in public service events involving runners and cyclists on roads where it is required that yellow/amber lights be used to minimize hazards.  Certain makes/models of these lights have been observed to produce tremendous amounts of RF energy that effectively quashed all 2 meter reception,so they were sent back to the seller until he found a unit that was "quiet".
3. Check your local regulations regarding retrofitting of headlights with equipment other than that of the type provided by the original manufacturer.
4. A low-level increase in the noise floor in the proximity of the vehicle would have absolutely no effect on the transmitted signals, but the result of this interference is that the station in his vehicle became an "alligator" - all mouth, no ears - meaning that he was able to "talk" much farther than he could hear.
   
5. Whereas simple capacitor (shunt) or inductor (series - and this includes ferrite devices) may reliably attenuate an offending signal by 15-20dB or so at best (very generally speaking) combining both types of reactance - "L" (inductor) and "C" (capacitor) - appropriately can provide many 10s of dB of attenuation if done properly - easily 30-60dB for simple circuits.  This greater amount of attenuation is far more likely to be able to put the interference from the device well below the noise floor of the receive system.  This is the technique used in footnote #1 (above) and explained in some detail in the blog entry "Completely containing Switching Power Supply RFI" link.
6. "Completely" eliminating conducted RF is actually impossible, but reducing it by 30-60dB is likely to attenuation it below the level of detection.
7. The use of surface-mount components - like capacitors - with their lower parasitic reactance than their counterparts with leads - can be used very effectively to filter RF, but several cascaded stages of such capacitors and inductors - and careful layout of a PC board - are likely to be required to obtain sufficient attenuation.  "Feedthrough"-type surface-mount capacitors are also available - which have excellent performance - but these, too, require a properly-designed PC board.  With the solder-in feedthrough capacitors, a brass partition was used as it was easier to drill and solder to than one of copper.  If screw-in feedthrough capacitors were used, the partition could have been aluminum.
8. Nine wires are needed for the headlights:  Three wires are from the vehicle itself (low beam, high beam and "common") with two more wires for the fan and two each wires for the pair of LED emitter arrays.  As an aside, this and many other retrofit LED headlights are polarity-insensitive (e.g. it doesn't matter if the "common" lead is at V+/battery+ or ground) since different vehicle manufacturers connect the headlight's "common" pin to either V+ or ground.
   
9. The ferrite beads used in this project were Fair-Rite 2643000801.  These use "43 mix" ferrite and are 0.295" O.D., 0.297" long and 0.094 I.D. (7.5x7.55x2.375mm) and are able to accommodate the wires + insulation of the conductors from the LED controller.  This material has a typical impedance of about 94 ohms at 100 MHz and cost about $0.24 each in single quantity at the time of writing.  I used them primarily because they were on hand.
   

 * * * * *

This page stolen from ka7oei.blogspot.com

[END]

Frequency response of the RX-888 SDR at the high and low ends (Above 30 MHz, below 1.5 MHz)

Figure 1:
The RX-888 Mk2.

For information about improving reception below 1.5 MHz, scroll down to the text surrounding Figure 2.

The RX-888 Mk 2 (hereafter referred to as the  RX-888 or '888) is a versatile device, essentially providing a means by which "all of HF" (0-30 MHz - or even 0-60 MHz) may be sampled and presented to a computer for processing via a multi-gigabit USB3 interface.  As it has no onboard signal processing, this device is practically "future proof" in that as all computations are performed on the host computer and there are no frequency or bandwidth limitations regarding the sort of signals - or how many - may be processed, presuming adequate processing capacity.

Comment:

I've seen at least three different "sub-versions" of the RX-888 Mk2 - each one looking slightly different (different circuit board color and other minor differences) - since this device was released.  It's also very likely that components also vary a bit with different manufacturers so the actual frequency response of units of different "builds" may also change.  Unfortunately, I only "have what I have" and haven't been able to compare possible variations in frequency response with these different "builds" - if any.

The highs and the lows

Like any receiver, it has limits of its frequency response - both at the upper end where the high-pass filter dominates and at the bottom end where the component selection as well as the design itself will limit low-frequency response.

Let's look at the low end first.

The lows

The low end limit to the frequency response (somewhere below 1 MHz) of the '888 has not previously been well defined.  This low frequency response is set by component limitations within the HF signal path, including:

• Coupling capacitors.  DC blocking capacitors in series with the signal path will act as high-pass filters, rolling off the low frequencies.
• The Bias-Tee inductor.  The RX-888 has the ability to supply power via the antenna port to an amplifier.  This inductor has a finite inductance and it, too, will force a high-pass response as well.  This inductor's value was measured as being 10uH (nominal) which presents a reactance of 50 ohms at about 800 kHz.  This is the major contributor to low-frequency roll-off as discussed below.
  
• The coupling transformer.  The RX-888 has a transformer that couples the input of the variable gain amplifier (VGA) from the attenuator.  As with any transformer, this, too, has defined low-frequency response.  This transformer was measured and found to have an inductance of 125uH of its primary (a reactance of 50 ohms at 64 kHz) with the secondary (the side facing the VGA) being about 760 uH.
  

This low-frequency roll-of is not uncommon in broadband receivers:  Most amateur transceivers suffer severe performance degradation at LF and VLF frequencies for the simple reason that the designers presume (correctly!) that very few of the users of that gear would ever be interested in that range - and making this assumption simplifies the design somewhat and reduces cost. 

Using a signal generator with a constant output, the response of the RX-888 (Mk2) was measured, using the signal strength at 1500 kHz as a reference, with and without the Bias-Tee inductor removed:

Frequency (kHz)	Attenuation (db) Unmodified unit	Attenuation (db) Bias-Tee inductor removed
1500 (Reference)	0	0
1250	0.3	0
1000	0.4	-0.1
750	1.0	-0.2
500	2.6	-0.3
475 (630 meters)	2.9	-0.3
400	4.5	-0.2
300	7.4	-0.2
250	10.9	-0.2
200	19.9	-0.2
150	17.9	0
137 (2200 meters)	14.7	0
100 (Loran C)	9.7	0.3
75  (DCF77 approx.)	7.5	0.7
60  (WWVB, JJY)	6.8	1.1
50	6.7	1.9
40 (JJY)	7.4	5.3
30 (Submarine comms)	11.3	7.4
25 (Submarine comms)	12.8	10.5
20 (Submarine comms)	17.5	15.0
15	22.5	22.7
10	30.7	32.8
7.5	36.4	40.0
5	45.5	49.5
2.5	61	60
1	80	87

Table 1:  Attenuation measurements at 1.5 MHz and below using both an unmodified RX-888 and the same one after the bias-Tee inductor was removed.

Comments about the frequency response of the unmodified unit:

As can be seen from the table above, the "stock" RX-888 is flat within about 2 dB or so across the AM broadcast band (520-1700 kHz) but it falls off precipitously between 100 and 300 kHz with a bit of "rebound" in the 40-150 kHz area, likely due a very low "Q" resonance of inductance and capacitance of the aforementioned components (inductors, transformers) in the signal path.

In the "VLF" range (30 kHz and below) the unmodified receiver may be somewhat usable when using an active antenna to overcome losses, but at 20 kHz and below the response drops off like a rock and, as the chart shows, it's pretty much unusable below 5-10 kHz.

The factors above conspire to prevent a flat frequency response at lower frequencies - say, those below 1.5 MHz.  For the table below, my reference amplitude and frequency is 1.5 MHz as it seemed to be more or less representative of the amplitude response above this, in the HF range - and it seemed to be comfortably above that at which the aforementioned high-pass effects of the components were having a significant effect.

Can anything be done to improve LF/VLF response?

Figure 2:
The red arrow points to the location of the
10uH bias-Tee inductor.  As seen in
Table 1, its removal can significantly improve
MF and LF performance.  The yellow arrow
points to the 0.47uF bypass capacitor that may
need to be removed even if the Bias-Tee
jumper (if equipped) were to be removed.
Click on the image for a larger version.

YES, it is possible to modify the RX-888 to improve the low and frequency response by removing the Bias-Tee inductor (see Figure 2) from the HF port and as can be seen from the above data there is a dramatic difference in usable sensitivity at frequencies below 1 MHz - particularly below 400 kHz.

This is the easiest modification as it entails the removal of a single component and the red arrow in Figure 2 shows the location of this inductor.  It may most easily be removed with a hot-air rework tool, but it should be possible to carefully use a solder-wetted iron to heat it and remove with a pair of tweezers (temporarily remove any thermal pad below that portion of the board if it's present) or a very sharp pair of diagonal flush-cut pliers to remove it (perhaps destructively) as well.

If your RX-888 has a "Bias-Tee" Jumper:

Some versions of the RX-888 have a jumper near the location of the red arrow in Figure 2.  

While removal of this jumper will prevent DC from being applied to the "HF" antenna port, it MIGHT not remove the 0.47uF bypass capacitor - depending on the version of board.  There are apparently two different versions that have this jumper.

• In one version, removing the jumper leave the capacitor in the circuit as depicted in Figure 2.  In other words, removing this jumper does nothing to help low-frequency performance.  If you remove the bias-Tee jumper and don't wish to remove the inductor, you may also remove the 0.47uF bypass capacitor, indicated by the yellow arrow in Figure 2.  (Note that even though Figure 2 shows the version without the jumper, the bypass capacitor is still in the same location.)  If you remove the inductor, you need not remove the capacitor.
• In another version, the jumper is located between the capacitor and inductor.   On this version, removing the jumper DOES improve low-frequency response and no further work is necessary.

Unfortunately, if you have a jumper, it is up to you to determine which of the above versions you have to find out if simply removing the jumper will improve reception.  The easiest way to do this is to connect an antenna, tune to an AM radio station at the low end of the band (below 600 kHz) - or a signal below this such as a 630 meter transmission, NDB or even a time station (WWVB, DCF77, MSF, JJY, etc.) or even just noise - and carefully compare signal levels with and without the jumper.  

* * *

There are other ways by which the low frequency response may be improved, including:

• Replacing the coupling transformer.  The transformer used in the RX-888 is likely specified for a low-end frequency response of 1 MHz or so, so it's not surprising that this may be the worst offender (once the bias-tee inductor has been removed) in low-frequency roll-off.  Replacing it with a different unit with larger inductance (a commercial or hand-made unit) would certainly help.  It may also be possible to simply replace the transformer with coupling capacitors (say, 0.1uF) - but this would be at the expense of sensitivity and performance across the entire frequency range, something that might be acceptable if one's primary interest was in the MF/LF/VLF spectrum.  As the inductance of the transformer's primary is known to be about 125uH, we can see that this is likely the main cause of attenuation below 60 kHz.
• Increasing value of coupling capacitors.  The coupling capacitors in series with the signal path are likely not ideal for coupling VLF frequencies.  A value such as 0.1 uF or larger would be suggested.

For VLF use (30 kHz and below) if you have interest in this frequency range you may be better off not trying to use the RX-888 - at least directly.  Some possibilities include:

• Use a VLF up-converter.  Converting the frequencies 0-30 kHz to a higher frequency range will put this spectrum within the useful range of the RX-888 and practically any other modern receiver.  There have been a number of VLF up-converter units for sale in the past, but I don't have a specific recommendation.  If this up-converter is clocked from the same source as the RX-888's clock (e.g. using its onboard 27 MHz oscillator, or both from a common, external clock) then frequency drift could be minimized.
  
• Use a sound card.  A modest computer sound card with a 192 kHz sample rate and a 16 to 24 bit A/D converter is perfectly capable of ingesting frequencies up through at least 80 kHz and down (nearly) to DC.

Having a receiver capable of VLF (3-30 kHz) or ELF (300-3000 Hz) is one thing, but having an antenna system capable of this is a different matter altogether.  There are many available E-field active whips that will work well down into the 10-20 kHz region, but below that frequency you are into the realm of specialized gear - and listening at "audio" radio frequencies in all but the most rural areas devoid of power lines and other forms of civilization can be fraught with frustration and disappointment due to the likely pick-up of mains-related energy and its harmonics.

Here are a few links related to equipment for LF/VLF reception.  Note that I have not necessarily built, bought or used the equipment described below, so your mileage may vary.

• The BBB-4 Natural Radio Receiver.  This is a simple circuit is intended for "Audible" RF frequency ranges of a few hundred Hz to a few 10s of kHz and is suitable for detecting "spherics" like whistlers and chirps.
• Some thoughts on E-Field Whistler Receiver Design.  This article describes design aspects of ELF and VLF receivers
  
• An up-converter for receiving long and very long waves.  This article describes an upconverter that may be built.
  
• AMRAD low-frequency up-converter.  This is another upconverter design for VLF/LF use.  The low-end frequency response is limited by T1 in this design.
  
• Heros VLF-LF-MF up-converter.  This is a commercial LF/VLF upconverter. 
• The AMRAD Active LF Antenna.  This article - from the September, 2001 QST - describes a high-performance antenna capable of reception down to at least 10 kHz.  In lieu of the CP666 transistor, one may also use the Russian KП903 (a.k.a. "KP903") power JFET which may be found on EvilBay and other sites.
  

The effective reception of signals in the LF, VLF and ELF frequency range is highly contingent on having a "quiet" receive site, largely free of local noise sources and also on scrupulous attention to detail when it comes to decoupling the feedline (going to the "noisy" chassis of the receiver) from the antenna to prevent unwanted signals from being conveyed - but that's a topic of its own!

See the article A (semi)-typical suburban E-field whip receive system for the 630 and 2200 meter amateur bands - link. for a few details on how this might be done.

Real-world observations

At the Northern Utah WebSDR - where there are, at the time of writing, full-time WSPR receivers - it so-happens that there are currently some KiwiSDR and RX-888 based receivers sharing the exact, same signal path.  The KiwiSDR - which is capacitively coupled (e.g. you can hear "tinny" audio from the receiver tuned to 0 Hz and you apply the source to the antenna connector) has quite good response well into the VLF range.

Compared to the RX-888, the KiwiSDR performs noticeably better on the 2200 meter amateur band (137 kHz) in decoding WSPR and FST4W signals in which the '888 is about 15dB down.  As the '888 based system can't hear the 2200 meter signals as well, this indicates that signal levels feeding the '888 are a bit too low for it to "hear" the noise floor of the antenna system - but it also indicates that, perhaps, a few dB of boost in the signal path may remedy this:  This RX-888 has NOT had its bias-Tee inductor removed - but that's on the "to do" list:  After the bias-Tee inductor is removed I expect that it will perform comparably to the KiwiSDR at 2200 meters and I'll update this web page after having done so.

As the "LF/VLF" antenna system at the Northern Utah WebSDR is separate from that of the HF signal path - being combined in a special filter/amplifier module - boosting only the LF/VLF path would be the most beneficial as it wouldn't compromise HF reception by potentially overloading the A/D converter as would boosting everything.

The Highs

The RX-888's specifications state that it contains a "60 MHz" low-pass filter - but the precise nature of its response is not noted.

Comment about sample rates and aliasing - and the need for additional low-pass filtering

The use of the 60 MHz low-pass filter implies that the designers intended an A/D converter sample rate of more than twice that frequency - and since the RX-888 will happily sample at more than 130 MHz, this fits the need.  Many users do not operate their RX-888 at 130 MHz, however, as their interest does not extend beyond HF and operate it, instead, at around 65 MHz to reduce CPU and power loading.

A bit of warning here:  With a 65 MHz sample rate, the '888 will happily respond to signals above the Nyquist frequency (half of the sample rate, or 32.5 MHz) and these signals - spectrally "inverted" - will naturally appear at lower and lower frequencies as the original source signal's frequency increases.  Since the '888s low-pass filter is set at around 60 MHz, it will do nothing to prevent this:  The far right column of Table 2, below, shows the aliases of the test frequencies.

What this means is that users of the RX-888 using it at a sample rate lower than 130 MHz should be using an outboard low-pass filter.  With a sample rate of 65 MHz, a good-quality 30 MHz Low-Pass filter is strongly recommended and will suppress aliased signals that would otherwise appear above Nyquist.  Such filters may be found online via the usual retailers, but do not overlook an old 30 MHz transmit-type low-pass filter of the sort used to prevent interference to analog TV by an HF transmitter - often found at amateur radio swap meets or on EvilBay for cheap.

Even if you do run the RX-888 at 130 MHz sample rate, the low-pass filter is rather mediocre (only 30-40dB across most of the band) and you will likely need to add an FM broadcast-band filter to your receive system if there's even a modestly-strong transmitter near you.

 The amplitude response, relative to 30 MHz, is shown below:

Frequency (MHz)	Attenuation (dB)	Alias frequency (MHz) @130 MHz sample rate	Alias frequency (MHz) @65 MHz sample rate
30 (Reference)	0	-	-
40	0.8	-	25
50	3.8	-	15
54	5.5	-	11
60	8.5	-	5
64	10.5	-	1
70	14.1	60	5  (double alias)
75	17.7	55	10  (double alias)
80	21.7	50	15  (double alias)
85	27.1	45	20  (double alias)
90	32.5	40	25  (double alias)
95	37.8	35	30  (double alias)
100	42.8	30	30  (triple alias)
105	48.0	25	25  (triple alias)
110	52.9	20	20  (triple alias)

 Table 2:  Sensitivity response of the RX-888 relative to 30 MHz

Table 2 shows the amplitude response of the RX-888 (Mk2) relative to 30 MHz.  The third and fourth column show the resulting aliased frequencies at sample rates of 130 and 65 MHz, respectively.

"Could I intentionally use aliases to receive higher frequencies than my sample rate would allow?"   After reading this, you might ask yourself "If I operate at a sample rate of 65 MHz, could I intentionally do this to receive spectrally-inverted 6 meter signals between 15 and 11 MHz?"   The answer is yes, you could - and as the chart above shows, they would be only 3.8-5.5dB down from the "real" signals across that same 15-11 MHz range.  Intentionally allowing aliases to occur is often done to allow the detection of signals well above the sample rate.  The caveat here is that one would want to sharply filter the source of the "above Nyquist" frequencies to limit them to the band of interest as well as prevent noise on the aliased frequency (15-11 MHz in this example) by filtering those frequencies as well.   Doing this works just fine as long as proper filtering is done to keep out the "unwanted" signals (at the higher and lower frequencies) along with appropriate amplification make up for losses.   In the example above, the lower part of 6 meters would appear just above the 20 meter band - but if one adjust the sample rate, the alias could be moved farther away from 20 meters and, with proper filtering, one could receive both 6 and 20 meters on the same receiver hardware.

What the above table above also shows is that the 60 MHz low-pass filter isn't very good:  By the time you get to the bottom of the FM broadcast band (88 MHz) we know that the attenuation is only around 32 dB.  Here in North America it's common for an FM broadcast station to have many 10s of kilowatts of ERP which means that if you live anywhere near such a station - even if you are using an antenna that wasn't designed to receive FM broadcast frequencies - you may experience some interference around the alias frequencies noted in Table 2.

No matter the sample rate at which you operate your RX-888, it's recommended that you carefully check for aliased responses of FM transmitters.  If you find them - and even if you don't - I'd recommend a separate FM broadcast band blocking filter be installed to quash ingress from strong signals:  Without it you'll probably get some leakage of moderate-to-strong signals in the 22-42 MHz range (frequency-inverted) if you are running at a 130 MHz sample rate or in the 23-32 MHz range if you are running at a sample rate of 65 MHz.

Figure 2 also demonstrates why - if you operate the '888 with a sample rate of 65 MHz - you should really be using a good 30 MHz low-pass filter with it:  Any signals above 30 MHz - including noise - will be attenuated only to the extent shown in the table and will interfere with the desired 0-30 MHz signals.

* * * * *

Other RX-888 related posts at this site:

• Measuring signal dynamics of the RX-888 -  This page discusses the gain distribution of the RX-888, its apparent sensitivity and steps that one should take to maximize performance when used for simultaneous "all of HF" reception.
• Improving the thermal management of the RX-888 (Mk2) - The internal power dissipation of the RX-888 exceeds its ability to get rid of the heat that it produces, reducing reliability - particularly in environments with elevated temperature.  This page discusses what to do to remedy this.
• Using and external clock with the RX-888 (Mk2) - Although the RX-888's TCXO is pretty good, you may wish to use an external reference to provide very high frequency accuracy and stability - and this page gives advice and warnings about doing so.
• Repairing a dead RX-888 (no A/D converter clocking) - While external clocking of the RX-888 (Mk 2) is desirable, it must be done with a bit of care to protect the circuitry involved.  If you do manage to damage your '888, this page may be helpful in its repair.
  

 

This page stolen from ka7oei.blogspot.com

[END]

Characterizing the RTL-SDR Blog (Version 3) for HF reception using the "direct" input.

An inexpensive option for SDR (Software Defined Radio) reception on the HF (low frequency) bands is a device sold by "RTL-SDR Blog" - the current iteration being Version 3.  Originally intended for digital VHF/UHF TV reception - and that of FM broadcast - the hardware is also capable of tuning much lower frequencies.

Figure 1:  An RTL-SDR Blog V3 USB receiver "dongle".

Unlike most other inexpensive RTL-SDR dongles, this has - via a single SMA port - the ability to operate in "direct" mode where RF below the VHF frequencies is passed straight to the A/D converter rather than via a down-converter, allowing reception from (theoretically) a few hundred kHz to around 30 MHz.

How does it do this?

The typical RTL-SDR Dongle actually consists of two tunable devices:

• The Rafael R820T.  This is simply a frequency converter, capable of handling an input signal from somewhere below 60 MHz into the GHz range and converting it to what we'll call an "IF" (Intermediate Frequency) which is much will likely be below 12 MHz.  In addition to having a programmable oscillator and mixer for frequency conversion, his device has some built-in filtering that provides some protection to strong-ish off-frequency signals, and it has an AGC (Automatic Gain Control) that can adjust the level being output from it to prevent overload of the A/D converter as well as some front-end attenuation control to reduce the likelihood of overload on the input.
  

• The Realtek RTL2832U.  The down-converted output of the R820 chip is passed to this device, which consists of two 8 bit A/D (Analog-to-Digital) converters that are clocked at 28.8 MHz (meaning that signals above 14.4 MHz will be "aliased"), a USB interface, a (reported) 8051-type microcontroller and a digital frequency "converter" that is also capable of being "tuned" to produce a quadrature "baseband" signal that is output onto the USB port - the rate being programmable from around 250 ksps to 2880 ksps.  This device does NOT have an AGC or gain/attenuation control.
  

VHF/UHF operation:

Briefly, reception on the VHF/UHF frequencies is done the following way:

• A simple high pass filter/diplexer passes only the VHF/UHF signals (e.g. those above approximately 40 MHz) directly to the R820T.
  
• The filtering of the R820T is programmed for the desired characteristics at the operating frequency.
• The frequency converter in the R820T is offset from the input signal to provide an output (IF) signal in the 1-14 MHz range - more likely somewhere between 3 and 6 MHz.
• The level of this output signal may be automatically controlled by the AGC system of the R820T to keep it within the optimal range of the A/D converter.  Similarly, input gain adjustment on the R820T can prevent it from being overloaded by strong signals.
  
• The tuner within the RTL2832U is set to about the same frequency being output by the R820T (likely in the 5-12 MHz range) so that it is within the range of output ("baseband") sample rate, and output via the USB port.

HF operation:

• The signal to be received is applied via the RF antenna port and diverted from the R820T to a very simple low-pass filter/diplexer to an RF amplifier.  This diplexer's effect begins at approximately 22 MHz - See the discussion near table 1, below.
  
• The output of from the RF amplifier is passed through additional filtering and applied to one of the two A/D inputs of the RTL2832U - typically the "Q" input.
• The tuner within the RTL2832U is set near the frequency to be received so that it is within the range of the output ("baseband") sample rate, and output via the USB port.
• Because the sample rate of the RTL2832U is 28.8 MHz, any signal above 14.4 MHz will also appear "below" it as well - see the discussion below.
  

There's a penalty to pay for simplicity:

For those familiar with receiver topology - digital or analog - several things the above description of the HF operation of the RTL-SDR dongle without proper measures (e.g. filtering, gain control) should give cause for concern - namely:

• There is NO bandpass filtering at all.  Whatever is being intercepted by the antenna system will be input directly to the A/D converter via the preamplifier.
  

• ANY signal applied to the antenna input - no matter the frequency - can contribute to overload.  Because the inputted RF goes into the A/D converter, a strong signal well away from where you are listening can cause overload.  For example, if you are listening to 14 MHz, a strong AM broadcast signal (e.g. mediumwave) could well be the culprit if you are experiencing overload.
  

• There is no AGC in the HF ("direct") signal path.  Considering that the A/D converter is only 8 bits, this means that compared to even low-end shortwave receivers, the range of signal levels over which this device will operate is very narrow.  The lack of an "AGC" (automatic gain conttrol) means that there will likely not be enough gain when signals are very weak and overload of the A/D converter is likely if the signals are very strong.
  

• The A/D converter's sample rate is 28.8 MHz.  What this means is that the Nyquist limit - the frequency above which the digitized output can no longer represent the input signal - is half this, or 14.4 MHz, uncomfortably close to the 20 meter amateur band, and entirely below the 17, 15, 12 and 10 meter bands - and this does not even consider the fact that the sample frequency is within the 10 meter band itself. 

What this means is that there are unsuppressed image responses all across the HF spectrum.  For any frequency you tune below 14.4 MHz, you can also hear any signal above 14.4 MHz, the frequency of which is calculated by subtracting its frequency from the sample rate (e.g. 28.8 MHz).  For example, 15 meter signals will also appear, spectrally inverted (e.g. USB = LSB) 7.80-7.35 MHz.  Image frequencies appear in table 1 - along with MDS and clipping values - below.

In other words, if you are tuned to any HF frequency, you are actually "hearing" TWO frequencies at the same time unless you have specific filtering to prevent such image response.  See the right-hand column of Table 1 (below) for the frequency at which you will see an image.

The above puts strict limits on the performance of the RTL-SDR dongle as an HF receiver and these realities must be considered when the configuring any system that might use them.

The usable dynamic range:

In theory, 8 bits of A/D sampling would indicate about 48dB of useful signal range, but the reality is more complicated than this.  Compared to the bandwidth of the narrow signals typically sought on HF, the overall sample rate of the A/D converter - and even the rate after the RTL2832U's converter has reduced the signal to the sample rate being sent to the USB port - are very much higher, effectively improving the bit depth via oversampling - and the fact that the HF spectrum is backgrounded with what amounts to white noise works to our benefit, helping to spread discrete, spectral artifacts that are the inevitable result of the imperfect signal acquisition.

This also works the other way:  Very strong, discrete signals (e.g. SWBC transmitters) can also cause mixing products which can find their way in other signals - particularly weak ones - but this effect may be minimized if one manages to keep the input level "high enough" so that at least several bits of A/D conversion are involved and also limited such that A/D converter overloads are minimized as much as practical - a balancing act that makes it difficult to handle both weak and strong signals at the same time.

All of this makes the actual, usable range a bit difficult to divine.  To this end, an RTL-SDR (V3) unit was put on the workbench, using the "HDSDR" program as a receiver, and its operation was analyzed by observing CW (unmodulated) signals on narrow (SSB) bandwidths.

Methodology:

Ideally, we would be able to determine the RF level at which the A/D convert clipped directly, but the HDSDR program does not provide a means to see the peak A/D level, requiring us to infer it by noting the level from a known-accurate signal generator at which the S-meter starts to decrease at the same rate that the signal level is reduced - and also by noting the disappearance from the waterfall many of spurious signal caused by overload.  Typically, this is about 3dB below the "maximum" S-meter reading.

For sensitivity, DL4YHF's "Spectrum Lab" program was used to measure the SINAD in a 500 Hz bandwidth set by the HDSDR program, the "minimum discernible signal" being equivalent, in this test, to 3dB S/N.

Table 1:  Measured signal levels for A/D clipping and MDS, along with corresponding image frequencies on HF.  Because the sample rate of the RTL-SDR is 28.8 MHz, ALL signals above half this frequency (e.g. 14.4 MHz) are, by definition, Nyquist images.  The far-right column is "apparent" dynamic range - see the discussion below.

Frequency (MHz)	Clipping (dBm)	MDS (dBm)	Image  (MHz)	Apparent DR (dB)
0.5	-17	-97	28.3	80
1.0	-24	-107	27.8	83
2.0	-27	-109	26.8	82
4.0	-27	-116	24.8	89
7.0	-29	-117	21.8	88
10.0	-30	-117	18.8	87
14.0	-30	-117	14.8	87
15.0	-30	-116	13.8	86
18.0	-30	-116	10.8	86
21.0	-29	-114	7.8	85
24.0	-25	-109	4.8	84
28.0	-14	-95	0.8	81
29.0	-16	-94	0.2	78
35.0	-16	-90	6.2	74
40.0	-2	-84	11.2	82

Discussion:

Table 1 tells us several things:

1. As mentioned in the documentation found at the RTL-SDR Blog web site, signals below 2 MHz - and especially below 1 MHz - are rolled off by the "Bias Tee" blocking choke which has insufficient inductance at frequencies below the AM broadcast band.  To a degree, this effect can be mitigated by removal of the RF choke which will remove the capability to inject DC onto the cable.  This table includes the effect of this choke.
   
2. The RTL-SDR has a low-pass filter to diplex the HF and VHF and above frequencies to separate signal paths, and the effects of this filter are becoming evident above 15 meters (21 MHz).  This also means that by itself, the RTL-SDR becomes "deaf as a post" on the 12 and 10 meter bands.
   
3. Note that at the very low frequencies, the sensitivity appears to be somewhat reduced.  The effects of high-pass roll-off - likely caused by coupling capacitors and the input bias inductor - appear to be evident through at least 7 MHz.
4. There is clearly no image rejection at all, considering that the sensitivity through 21 MHz is comparable to that below 14 MHz.  For example, 15 MHz WWV will also appear on a receiver program at 13.8 MHz, at the same apparent signal strength.
   
5. Low-pass effects are evident by 24 MHz, most likely a result of the diplexer used to split the HF (direct) and VHF/UHF signal paths.  This limits sensitivity on the 12 and 10 meter amateur bands.
6. With only 8 bits of quantization, additional noise will be generated due to the imprecise nature of the process.  This "noise" will show up as spurious signals and, less obviously, as a rise in the overall noise floor - depending on the nature of what is being digitized.  In short, the fewer the number of bits, the less likely it is that weak signals will coexist (and be audible) in the presence of strong signals.
7. Across most of the HF spectrum, the RTL-SDR will overload signal at about -30dBm - which is approximately equal to an S-meter reading of "40 over S-9".  While this seems like a fairly strong signal, this power level represents the total amount of RF energy - no matter the frequency!
8. Note that the power at which the A/D converter starts to overload is approximately -30dBm across much of the HF spectrum.  This represents the TOTAL amount of RF power required, at all frequencies combined, that will result in overload.
9. Across much of the HF and MF spectrum (0.5-28 MHz) the apparent dynamic range is on the order of 80 dB.  This number should be taken with a grain of salt as it was measured in the absence of any other signals -  hardly a real-world equivalent.  Practically speaking, this number is indicative of the best possible performance under ideal conditions.
   

Points 7 and 8, above, should be considered very carefully in terms of its implications:

• You cannot simply connect an RTL-SDR Dongle to even an "average" performing HF antenna and expect reasonable results as the total power from ALL signals reaching the receiver are likely to exceed the -30dBm signal level - particularly if you have any AM (mediumwave) transmitters anywhere nearby (e.g. within 20 miles/30km).
  

• Particularly on the lower bands (80, 40, 30 meters) the signal levels of amateur and especially shortwave broadcast signals can, by themselves, exceed the -30dBm overload level - particularly in Europe and the eastern U.S. On some bands, the shortwave broadcast band adjacent to the amateur band (e.g. 41 and 40 meters) are too close to effectively filter out and it may be that an RTL-SDR is simply not usable on these bands when propagation favors reception on these frequencies. 
  

• As seen in Table 1 (above) the RTL-SDR dongle becomes increasingly deaf on the higher HF bands (particularly 12 and 10 meters) making them unusable at these frequencies without additional amplification AND filtering.  What's worse is that the lower HF frequencies (e.g. below 10 MHz) are typically very noisy while the higher frequencies are quiet by comparison.  If you connect an antenna to the RTL-SDR dongle with no band-pass filtering and try to tune in, say, the 15 meter band, you will likely hear only noise (and signals) around 4.8 MHz, which will likely overwhelm any weak, 15 meter signals.
  

• If you plan to use an RTL-SDR for HF reception and expect even mediocre performance, you should precede it with a band-pass filter for the frequency band of interest.  For the highest HF bands (15, 12 and 10 meters) the typical noise floor in a quiet location is around -120 dBm (in a 500 Hz bandwidth) - which is well below the noise floor of the RTL-SDR at these frequency meaning that a preamplifier (along with a bandpass filter) will be required for reasonable performance.
  

• It's worth remembering that unlike an analog receive system, one cannot always use all 8 bits for digitization:  The signal input must be kept well below the "full scale" level (at and above which "clipping" will occur, causing distortion and signal degradation everywhere else) to accommodate for random fluctuations that are ever-present on signals input from the antenna.  What this means is that the A/D must be under-driven overall and that fewer bits are actually being used most of the time.  In order to maintain suitable margin, it's typical to drive the A/D converter at between 1/4 and 1/2 full scale, meaning that for 8 bits of A/D conversion, 2-4 bits are typically being used - and fewer, still, when the band is "dead" and signals are weak.
  

Conclusions:

• DO NOT simply connect an RTL-SDR to your HF antenna and expect it to work as well as even a low-end shortwave receiver:  If it doesn't get overloaded by local AM broadcast (mediumwave) signals, it will get overloaded by strong shortwave broadcast signals when conditions are favorable on certain bands.  This isn't to say that you won't hear anything if you do so, but know that normal signal levels present on even an "average" antenna will be enough to overload the RTL-SDR dongle.
  

• ALWAYS precede an RTL-SDR with a band-pass filter that is specific for the frequency range of interest. For example, if you are interested in 40 meter reception (7.0-7.3 MHz for ITU region 2) your filter should pass only frequencies in this range, and this filtering will prevent unwanted reception of signals at the image frequency around 21.8 MHz.   Unfortunately, the use of a band-pass filter precludes reception outside its design range, but this is necessary considering the limited capability of the RTL-SDR dongle in terms of handling both strong and weak signals at the same time and its unfettered response to unwanted images.
  

• In some cases - even with a mediocre antenna - the signals in the desired frequency range may exceed the signal handling capability of the RTL-SDR and cause overload.  As noted above, a single signal stronger than -30dBm can do this, but so could a number of signals whose total power can exceed this.  Typically, this overload is manifest as intermittent distortion across the entire receiver as signals fade in and out.  In such cases, it might be beneficial to attenuate the signals reaching the RTL-SDR as the degradation caused by overload is more "destructive" across the entire receive frequency range than too-little signal.
  

• Because of image response and roll-off, the HF port of the RTL-SDR is really not well-suited for 12 and 10 meter (24-30 MHz) reception.

Comment about HF upconverters:

Specifically to address some of these issues, there are upconverters available for the RTL-SDR that will upconvert the HF spectrum to VHF (typically in the 100-130 MHz range) to allow the use of that signal path, making use of the Raphael R820T converter.  This converter has the advantage of having a degree of band-pass filtering and the ability to use AGC (automatic gain control) on the signal path.

This method can be useful, but there are several caveats:

• If frequency stability is of importance, the addition of the upconverter introduces two additional frequency stability issues:  Drift of the upconverter itself, and the fact that any existing drift in the dongle itself will be multiplied because of its operation at the higher frequency.  This can be an issue in environments where the temperature is not stable and/or when a frequency sensitive mode like SSB or (especially) digital modes are used.
  

• If multiple bands and receivers are to be used, there may be the temptation to upconvert the output of the upconverter to VHF and distribute this signal to the receivers.  While this may work in many cases, it's worth noting that if the entire HF spectrum is converted, the total signal power level can be significant, potentially overloading the upconverter itself, any RF amplifiers that might be used at VHF, and/or the front end of the RTL-SDR dongles themselves.
• While a "fix" might normally be to filter out just the HF band(s) of interest, this can become impractical if the 40 meter band (7.0-7.3 MHz) is upconverted to, say, 137 MHz where it can become difficult to make an effective band-pass filter at that frequency.
• It is possible to filter for a specific HF band before the upconversion, but this means that each RTL-SDR would require its very own upconverter.  While effective, the cost of an upconverter for each band may be prohibitive.

* * *

Successfully using an RTL-SDR on HF:

As mentioned earlier, one must precede the RTL-SDR with a band-pass filter to obtain reasonable performance on HF - or any other frequency range where very strong and very weak signals will be simultaneously present at the antenna input:  Remember that, especially in the "direct" mode, all signals applied to the RF input - even those MHz away from where the receiver is tuned - will count "against" you in terms of the total amount of RF power that may be applied to the A/D converter before it clips/overloads.

To see some RTL-SDR based HF receive systems in operation - and to be able to directly compare them with higher-performance receivers using the 16 bit signal path, visit the Northern Utah WebSDR site at sdrutah.org (link).  This will take you to a "landing page" where you can select several receivers - specifically:

• WebSDR #3:  This server uses RTL-SDRs for both the 80 and 40 meter bands.
  

• WebSDR #1:  This server uses SDRPlay RSP1a receivers which have both an RF AGC and around 14 bits of A/D converter depth - plus external band-pass filtering.
  

Both of these systems use the same antenna for 80 and 40 meters and it is possible to directly compare signals between the two in side-by-side windows.  Generally, the 8 bit RTL-SDRs used on WebSDR #3 hold their own compared to those on WebSDR #1, and this is possible only because the WebSDRs are preceded with both band-pass filtering and AGC as described in the link below.

* * *

Additional resources:

• Band-pass filters are readily available in kit form from QRP Labs http://qrp-labs.com/bpfkit.html  
  

• An article on using band-pass filtering and AGC (Automatic Gain Control) to improve the performance of an RTL-SDR when used for amateur band service may be found here:  http://www.sdrutah.org/info/rtl_sdr_agc.html .  Several "test" receivers are currently in operation at the Northern Utah WebSDR that demonstrate the efficacy of doing this.

 This page stolen from ka7oei.blogspot.com

[End]

Using TV (F-connector) 75 ohm splitters and taps in 50 ohm systems on the amateur HF, VHF and UHF bands

I recently posted several articles about using commercially-available splitters link - and making one's own splitters - link - particularly for the HF frequencies and below (e.g. 30 MHz, down to a few 10s of kHz).  A comment was posted asking about how useful inexpensive 75 ohm "TV and satellite" type splitters might be for amateur radio use.

Figure 1:
The assortment of 75 ohm TV and satellite splitters and
taps tested in this article.
Click on the image for a larger version.

Implied by this question is the use of these devices in receive-only or small signal applications:  They cannot be used for transmit purposes as putting even 100 milliwatts through one of these devices is likely pushing its power-handling capability.

I've used these devices in 50 ohm circuits before - typically for VHF and UHF (2 meters, 70cm) where, along with some attenuators, combined the outputs of multiple signal generators to do "multi-tone" testing of receivers - but the question seemed to be a good one.  Rummaging around, I gathered a bunch of devices of various manufacturers and decided to test them for insertion loss and port-to-port isolation.

Note:

Please do not ask questions like "How well does a 'brand X' splitter work over the [fill in the blank] frequency range?"

There have been thousands of makes and models of these devices sold around the world over the past several decades and I simply am not able to find, locate, and measure more than the tiniest fraction of devices that have been sold.  The information given here is expected to be generally representative of the devices available from reputable manufacturers and distributors - but your mileage may vary.

Limitations of the measurements taken:

Because my VNA (DG6SAQ WVNA) was constructed for use with 50 ohm systems (the changing of  both internal hardware components and software would be required for "proper" analysis of a 75 ohm system) I was able only to analyze them in that context - but because the question was about using them in amateur radio service - which presumes a nominal 50 ohm system - I believe that the results are still useful within the limits noted in this article.

Because the emphasis of the question was interpreted as being for amateur-band frequencies likely to be encountered by the average user, the measurement range was limited to frequencies below 1 GHz - in some cases down to 100 kHz.  The nature of the equipment and methods (e.g. 50 ohm test equipment and cabling, the use of inter-series adapters, etc.) used to test the splitters and taps increasingly limits the usefulness and accuracy of these measurements at frequencies above that of the 70cm amateur band (above 450 MHz).

The variety of splitters and taps available:

There are literally thousands of brands and models of TV/Satellite splitters and taps available on this planet - some of them from recognizable names, but most not.  For those devices from sources that might be suspect (e.g. not "name" brands from reputable suppliers) you are on your own to determine the suitability of those devices for your purpose.

Although not intended as an endorsement per se, it has been observed that devices marketed by Holland Electronics appear to consistently meet their stated specifications and is one of the few brands that is likely available worldwide from a number or different sellers - including Amazon - and major suppliers of electronic components and TV/satellite supplies.

Over the years I have seen many dozens of brands and models of these devices - and the vast majority of them are what they are purported to be, but I have run across some devices that claimed to be splitters, but were simply a box with wires connecting the ports together.  In many cases, the casual user would not have noticed anything amiss, but using several of these faux devices in a larger system would certainly result in cumulative signal degradation (e.g. "ghosting" of analog signals, degrading of quality - but not necessarily signal strength - of digital signals).

General types of devices:

There seem to be three general types of these devices out there:

• "TV" and/or "VHF/FM/UHF" and/or "CATV" - These devices are typically designed to operate over the range of off-air TV stations across the world and the frequencies typically found on receive-only cable TV (with no Internet), encompassing the frequency range of about 40 MHz through 700 MHz, more or less.  While useful for use on the amateur bands from 6 meters through 70cm, inclusive, their usability on HF or above this range is limited as noted in the testing, below.

• "Satellite" splitters - These devices are typically designed to operate starting at about 900 MHz, often extending to 1500 or as high as 2500 MHz, depending on the vintage and intended use.  These devices are not usable on the 70cm amateur band frequencies and below.

• "TV/CATV/Satellite"- These devices are of a bit more recent vintage and are designed to accommodate a very wide range of frequencies - often from about 5 MHz through and above 2000 MHz - a band that includes off-air, cable and "L-Band" satellite signals - plus the "reverse" channels (sometimes called the "T" channels) often used by "cable Internet" modems that reside below 45 MHz.  These are the most useful to amateur service and can often be used on HF through 70cm.

If you do not see a specific frequency range noted on the device itself, assume the worst-case, smallest frequency range that covers that usage category - unless you can test them yourself.

* * *

General findings

For the TL;DR types, here is a summary of the results of the measurements described in more detail farther down the page.


Using 75 ohm devices in 50 ohm systems:

The most obvious issue is that TV-type consumer devices are almost universally equipped with type "F" connectors which means that one must use either an adapter or use a cable with an attached "F" connector.

Figure 2:
Left to right:  Two BNC female to male F connecitrs with an
F-type 75 ohm terminator on the right.
Click on the image for a larger version.

For receive-only systems, it's not too uncommon to simply use a 75 ohm cable like RG-6 - which is quite low loss and very inexpensive - to connect a 50 ohm antenna to a 50 ohm receiver.  The effects of this apparent "mismatch" are typically minimal as most receivers are only "approximately" 50 ohms, anyway.  In theory, the use of 75 ohm cable on 50 ohm devices will result in a 1.5:1 mismatch and commensurate losses, but this sort of mismatch is commonly observed on many antenna systems that are ostensibly designed to operate at 50 ohms and is usually of minor consequence.

When using an inexpensive cable like RG-6, it's worth noting that most of these cables use copper-coated steel (CCS) center conductors which may have implications for DC resistance of power is being sent on this cable (for a preamplifier, converter, controls) as this type of cable will have far more total resistance than one with a solid copper center conductor.  Copper-coated steel center conductors may also have implications in terms of skin effect at low frequencies (low HF and below) - but this is beyond the scope of this article - see, instead, this article by Owen Duffy from the Internet Archive.  There exist cables with copper-coated aluminum (CCA) center conductors that have lower DC resistance that CCS cables, but they tend to be more fragile due to the tendency of the aluminum center conductor to become brittle with flexure.

The device itself (splitter, tap) is designed primarily for 75 ohms and this means that its performance will be somewhat degraded in a system that is "completely" 50 ohms (e.g. 50 ohm cables with F-connector adapters) but these effects are largely as follows:

• The "through" loss may be slightly higher.  In the case of a 2-way splitter, the ideal loss will be 3dB - but even at the proper impedance, it will be slightly higher than this due to component losses, typically in the area of 3.5 dB.  Practically speaking, the main effect of using a 75 ohm splitter in a 50 ohm system was a slight change (only a few tenths of a dB) in the loss.

• Reduced isolation between ports.  The most obvious effect on splitters was that the isolation between ports (e.g. the "out" ports of a 2-way splitter) was reduced.  Compared to some specialized splitters, the isolation of inexpensive, consumer-grade "TV" splitters is lower overall.  As can be seen from the graphs, below, operating in a 75 ohm system resulted in better isolation - sometimes over 40dB at certain frequencies - but this assumes that all loads and sources are well-matched to 75 ohms, something that is not likely to be the case in a real-world installation.  Typically, isolation reduced to something in the 20dB area when operated in a 50 ohm system.  In many cases, this is "good enough".

• In splitters and taps, resistors are major components in determining their "native" operating impedance.  For example, a 75 ohm splitter or tap, depending on design, may have a 150 ohm or 37.5 ohm (2 times and one-half 75 ohms, respectively) resistor contained internally.  In theory, changing this resistor to a value appropriate for 50 ohms (typically 100 or 25 ohms) would optimize performance at 50 ohms - but doing this may or may not be worth the trouble. 

In short:

Unless your situation requires precision, the use of inexpensive, TV-type splitters and taps of the types described on this page will yield "reasonable" performance over the design frequency range - provided that the device is constructed as described by a reputable manufacturer.

The use of a (nominally) 75 ohm device in a 50 ohm system will require using connectors that are not normally used in 50 ohms systems (typically "F" connectors) which means that adapters of some sort will be needed - the expense, bulk and inconvenience of which must be considered in the overall design.

Finally, note that the above comments are for the general case:  Remember that your needs, requirements and results may vary and that you must do your own analysis and testing to verify that such components are appropriate in your specific case.

* * *

Plots of various devices:

Below are selected plots of devices representative of the types on-hand.  In general, devices with similar stated ratings performed in the same manner.  In all of these plots, the insertion loss is represented by the blue line while the complex impedance data is depicted on a Smith chart in the middle:  Numerical data at the frequencies indicated by markers is seen in the lower-left corner of the screen.  Again, remember that at higher frequencies, the nature of the 50 ohm test system, connecting cables and adapters will increasingly skew the results - particularly those depicted by the Smith chart.

The interpretation of a Smith chart will not be covered here, but there are many online resources that describe its use including this video in a series on this topic by W2AEW on his YouTube page.

A "satellite" splitter:

Figure 3:
The "through" loss of the HFS-2 splitter represented by the blue line
across the top.
Click on the image for a larger version.

This device - a "Tru Spec HFS-2" is representative of those intended for use on an (older) L-band system found in satellite receive systems, having on its label a "900-1500" MHz frequency range.  As noted above, the limitation of the measurement set-up made measurements above the 70cm amateur band (in the 440 MHz area) suspect - but the object here was to see if it was usable below that range.

At initial glance, the "through loss" of this device below 900 MHz (Figure 3) might seem to indicate that it worked below this frequency, but notice that at lower frequencies (below 50 MHz) indicates a loss less than 3dB indicating that it is not working as a proper 2-way splitter.  A look at the isolation plot (Figure 4) tells more of the story.

Figure 4:
Isolation between ports of this splitter.
Click on the image for a larger version.

As can be seen, at about 900 MHz and above, the apparent isolation between ports is reasonable but at 2 meters (146 MHz) it is only 3dB verifying the fact that at these lower frequencies, it less a proper splitter, but more equivalent to a device where the three ports are connected with a piece of wire.  The apparent isolation increase at low HF is more likely an artifact of its construction - the insertion loss being below 1 dB (in Figure 3) verifies this.

In short, these "Satellite only" splitters aren't really useful on TV and CATV frequencies or the amateur bands 70cm and below.

A "TV" splitter:

Figure 5:
The "through" loss of the Archer splitter.
Click on the image for a larger version.

I tested several splitters that were intended for general VHF/UHF/FM use - one of these being an "Archer" (Radio Shack) two-way splitter being typical of that type.  The implied frequency range is from at least 54 MHz to 700 MHz - the extent of the cable TV, FM broadcast, and off-air VHF and UHF TV frequencies at the time it was made.

Figure 5 shows the measured "through" loss in a 50 ohm system.  Compared to a plot done at 75 ohms (not shown, using resistive matching) the insertion loss barely changes across the frequency range.  In both 75 and 50 ohm systems, at least at 2 meters, down to 20 meters (14 MHz) seems to be "ok" - but the "dip" in the 3-4 MHz area - and the fact that the attenuation below it drops below 3dB - indicates that it's not likely acting like a splitter at these lower frequencies.

Figure 6:
Port to port isolation at 75 ohms for this splitter.
Click on the image for a larger version.

Figure 6 shows the port-to-port isolation at 75 ohms and we note that in the "low" and "high" VHF band (U.S. channels 2-13 - which more or less includes the 6, 2 and U.S. 222 MHz amateur bands, that the isolation is quite decent - well above 20 dB.

From this plot we can see that the "dip" in the 3-4 MHz area seen on Figure 5 is quite telling as the port-to-port isolation is pretty much gone below this frequency

Figure 7:
Port to port isolation in a 50 ohm system for this splitter.
Click on the image for a larger version.

The plot of Figure 7 shows what happens if the splitter is operated in a 50 ohm system.  The main effect is that the port-to-port isolation is reduced - being on the order of 15 dB or so from the 20 meter band through the 2 meter band (14 MHz - 144 MHz).

From this we can conclude that this splitter is quite usable from the middle of the HF spectrum through at least 2 meters - and is probably usable through 70cm.


A "TV/CATV/Satellite" splitter - preferred for HF use:

Figure 8:
Holland HFS-2P through loss in a 50 ohm system.
Click on the image for a larger version.

I have on hand several splitters that have on their label a frequency range that starts at (typically) 5 MHz with a high end of between 600 MHz and 2450 MHz.  The reason for this extended "low end" is likely due to their being designed for use in systems that have "Cable Internet" where the return (upstream) signal from the user's modem to the cable system are likely to be in the 5-50 MHz (or, possibly, a bit higher) range.  The plots included are those of a Holland Electronics HFS-2P which is a 2-way splitter/combiner that has a stated range of 5-2050 MHz and the results of this device are typical of that type.)

Figure 8 shows the "through" loss in a 50 ohm system showing a reasonable insertion loss (4 dB or below) from below 40 meters (about 5 MHz) through at least 70cm (440 MHz) - but again, the limitations of the measurement set-up make readings higher than this a bit suspect.

Figure 9:
Port-to-port isolation at 50 ohms.
Click on the image for a larger version.

Again knowing that the "isolation" measurement is the way to get the "true" story, port-to-port isolation in a 50 ohm system is depicted in Figure 9.

This verifies - to the extent that the test set-up can - the 5-2050 MHz range showing that the port-to-port isolation from 5 MHz to 1 Ghz is well over 15dB.  A port-to-port isolation measurement at 75 ohms (not shown) is slightly better (by a few dB) over the same range.

The combination of Figure 8 and Figure 9 show that this device may be usable down to the 160 meter band (1.8 MHz) provided that a slight amount of extra insertion loss (about 1dB) and lower isolation (approximately 12dB) can be tolerated.    (The Holland HFS-2D has characteristics similar to the HFS-2P down to 1.8 MHz.)

Figure 10:
The through loss, the other 7 ports being terminated with 75 ohm F-type
connectors.  The insertion loss is reasonable - between 10.5 and
11.5 dB over the range of 1.8 to 450 MHz.
Click on the image for a larger version.

An 8-way splitter:

The final splitter to be tested was the Holland Electronics GHS-8 8-way splitter-combiner.  Often, splitters with an even number of outputs greater than two contain multiple two-way splitters which means that this 8-way splitter might contain seven such devices - but I didn't break it open to check.

Figure 11:
The port-to-port isolation between two adjacent ports with the "in/out"
port and unused ports terminated with 75 ohm "F" loads.  The apparent
isolation is on the order of 35dB from 1.8 through 450 MHz - but this would
likely drop to something closer to 20dB.
Click on the image for a larger version.

Figure 10 shows the typical "through" insertion loss with the seven unused ports being terminated with 75 ohm "F" type terminators:  I don't have enough F-male to BNC-female adapters on-hand to terminate the 7 ports at 50 ohms - but if one were going to use one of these devices, it's probably more convenient to use F-type terminators on the unused ports, anyway.  The typical "through" loss is measured to be about 10.5-11.5 dB - slightly higher than the predicted "ideal" 9dB insertion loss, but typical for these devices.

The port-to-port isolation was also measured and the use of 75 ohm terminations on the other ports and the "common" in/out port likely improved this:  The isolation would likely be significantly worse if all ports were at 50 ohms, for the same reason as the other splitters tested.

Based on these readings, this device is useful down to 1.8 MHz and up through 2 meters - and probably 70cm.

Figure 12:
Coupling coefficient at 50 ohms for this tap
Click on the image for a larger version.

A TV-type signal "tap":

Likely unfamiliar to many, a signal "tap" is a very useful device in multi-drop TV installations found in hotels, hospitals and other larger buildings.  Unlike a splitter - which usually divides a signal equally to its output ports - a "tap" will siphon only a certain amount of signal off the cable and leave the majority of it intact - which is very useful for systems such as those in a hotel or hospital to distribute and split a signal hundreds of times to serve all of the devices.

In some ways it can be considered to be similar to a part of an SWR bridge where only a small amount of signal is sampled - and in only one direction - allowing the majority of the original signal to pass with minimal loss.  Several taps - all from Holland Electronics - were tested as they were what was on-hand and the "DCG-6SB" is represented in the plots. 

Figure 13:
The "reverse isolation" loss of the tap (e.g. turned "backwards") with a
50 ohm termination.
The reverse isolation is described as being the absolute amount of isolation
(e.g. that in the chart above) minus the coupling coefficient which means
that the actual forward coupling loss - which means that using Figure
12, we know that the actual reverse isolation is about 7 dB lower than
indicated by the graph above.
Click on the image for a larger version.

Figure 12 shows the "coupled" energy in a 50 ohm system:  Compared to the coupling in 75 ohm system (now shown) the insertion loss was slightly higher (about 1dB) but the frequency loss/flatness was about the same, being pretty consistent from about 1.8 MHz through 1 GHz.

Figure 13 shows the reverse isolation of the tap:  Rather than 6dB of coupling from the main line for signals going the "other way", the absolute is closer to 20dB - about 13dB lower.  (The actual reverse isolation is the absolute isolation minus the forward loss).  In a 75 ohm system (not shown) the reverse isolation was quite a bit better (closer to 30dB over the 5 MHz-1GHz range) - but this result is completely expected:  The reverse isolation is akin to measuring VSWR, and operating a 75 ohm device at 50 ohms implies a VSWR of 1.5:1 - a "return loss" of 14dB - very close to the values depicted in Figure 13 over much of the frequency range when the "forward" loss is taken into account.

On a tap there is yet another measurement to be taken - the loss between the in and out port.  Because we are measuring a 6dB tap - a device which siphons off about 25% of the signal - we would expect at least that amount (theoretically 1.25dB for 6dB) to be lost as it is coupled to the "tap" port. Figure 14 we can see that the measured loss is slightly higher than this between 1.8 and 200 MHz- a bit over 2dB.  Some of this "extra" loss is due to the intrinsic losses of the device, but a smaller amount is a result of the use of a 75 ohm device on a 50 ohm system.

Figure 14:
Through loss of the 6dB tap in a 50 ohm system.
Click on the image for a larger version.

This device - which is rated down to 5 MHz - may be useful through at 160 meters (1.8 MHz) - but the insertion loss goes up rather quickly at lower frequencies.

This device is NOT suitable for passing DC (e.g. for amplifiers, control signals) as it has a DC short across it - but that is not true of all taps.  For example, the Holland Electronics "HDCS" series does allow low frequency RF down to DC to flow through it - but like the DCG-6SB, its coupling coefficient deteriorates quickly below about 1.8 MHz.

* * *

General conclusions:

If you are going to use TV-type splitters for HF, make sure that you get devices that are explicitly rated down to 5 MHz.  Based on the (limited!) sample of devices that were tested, these devices can be expected to work into the 160 meter amateur band (down to 1.8 MHz).  While these devices may be usable thoughout the entire AM broadcast band (down to 540 kHz) expect performance to drop quickly in terms of added "through" attenuation and worse port-to-port isolation.

A "TV" type device - one that may indicate a start frequency of 5 MHz, or just any device that is claimed to work at TV (VHF/UHF) and FM broadcast frequencies will likely work from 6 meters through 70cm (50 MHz - 450 MHz).

Again, for general signal splitting and combining, these 75 ohm devices, used at 50 ohms, are quite usable for non-critical applications - provided that they be used at low power levels (a few 10s of milliwatts at most) and where one need not have precise 50 ohm matching and high port-to-port isolation.  Remember that most 50 ohm devices (receivers, amplifiers, filters) have only "approximately" 50 ohm source/load impedances - and filters in particular will, out of their design frequency range (outside the band-pass, on a notch frequency, above the low-poss cut-off, below the high-pass cut-off) will likely have anything but a 50 ohm characteristic impedance, so even a "proper" 50 ohm splitter/tap device would not necessarily yield any better performance in those situations.

For information about the design and use of splitters/combiners in general, a good reference is Mini-Circuits AN10-006, "Understanding Power Splitters" - link.

* * *

Far more data was gathered than was presented here, but that depicted above is representative of the devices that were on hand.

* * *

This page stolen from ka7oei.blogspot.com

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