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]

An ultrasonic superheterodyne receive converter (e.g. "Bat Listener")

In the mid 90s I decided to throw together what I called a "Bat Listener" - a simple receiver used to convert ultrasonic sound down to the audible range.

Figure 1:
The exterior of the ultrasonic receiver, complete with fancy
labeling!
Click on the image for a larger version.

Two types of circuits:

There are two common ways to convert a higher (ultrasonic) signal to the audible range, whether this is done using analog or DSP (Digital Signal Processing) techniques.

Frequency division

There are several ways to do this, the simplest being the "divider" type which digitally converts ultrasonic frequencies to audible by integer division of the input to a lower frequency.

The problem with this simple approach is that it does not preserve the amplitude (loudness) of the original sound since it must take the input signal, amplify/convert it to a series of logic-level pulses - which loses any amplitude reference - and do a brute-force digital division.  Additionally, if there are multiple signals present, for the most part only the strongest one will be converted down.

Clearly, one cannot "tune" this type of circuit:  A signal at 40 kHz will always be divided down by a fixed integer amount,  Let's say that the circuit digitally divides by 32:  That 40 kHz signal will be at 1.25 kHz.

Additionally, the direct "A-B" frequency differences between ultrasonic signals is lost, instead being "(A-B)/N" where "N" is the number of divisions.  In other words, the relative frequency differences between signals is not preserved.

Heterodyne conversion

The other way to do this is to convert the frequency.  In this technique, two signals - the ultrasonic to be converted - and another generated by the device (the "local" oscillator) are mixed together.  The result is an arithmetic shift in frequency.

The biggest advantages of this method are the fact that that not only are the differences in frequency preserved (e.g. two tones 1 kHz apart at ultrasonic will appear as two tones 1 kHz apart at audio) but the relative amplitudes (loudnesses) of the received signals are preserved as well.

Frequency conversion:

I chose to build a heterodyning receiver to convert the input frequency to a lower one.  This can preserve the amplitude and frequency relationships  - plus it is fully tunable, allowing one to choose the frequency range to convert to audible sounds - and since it is a simple conversion, multiple signals present will also be preserved.

When it comes to frequency conversion, there are two ways:  The simplest - direct conversion - would involve mixing a variable oscillator with the incoming signal and filtering/amplifying the resulting audio.  This has the advantage of being the easiest, and it is the method described in this article:

     April, 2006 QST article, A Home-made Ultrasonic Power Line Arc Detector - link)

A similar device - the MFJ-5008 - was made by MFJ Enterprises, but as this company is no longer doing business, it is found only on the used market.

While I could have easily built something like this a decade before the above article was published, as I'm sometimes wont to do I decided to make it a bit more complicated, constructing a superheterodyne converter.

While a direct-conversion receive simply mixes an oscillator with the desired signal to cause a frequency conversion, a superheterodyne receiver operates like a conventional AM or FM radio:  The desired signal is first converted to an IF (Intermediate Frequency) - and this IF is then converted to audio.  The advantage of the superheterodyne scheme is that filtering may be applied at the IF to limit the receive bandwidth - and since the IF is fixed, its width remains constant over the tuning range, just like that in a conventional radio/receiver.

Circuit description

Figure 2:
Schematic diagram of the superheterodyne ultrasonic receiver.
See text for a circuit description.
Click on image for a larger version.

 

As noted above, this circuit is more complicated than it needs to be, so make of it what you will!

VCO:

The heart of the unit is U1, the VCO (Voltage Controlled Oscillator) which uses the venerable CD4046 PLL chip.  Often used for frequency synthesis, we are using (only) the oscillator portion, which provides a linearly-tuned and fairly stable frequency source, adjusted by the voltage applied via R101 (and scaling resistor R102).  The values were chosen to provide an approximate frequency range of 125 to 185 kHz (more on this later) to allow tuning of audio signals from (ostensibly) 0 to about 60 kHz.  The actual tuning range is closer to 115-190 kHz as a bit of extra margin for the frequency range.

The only critical component here is C101 which should be a frequency-stable capacitor.  I used a polystyrene capacitor, but an NP0 (a.k.a. C0G) or silver-mica could be used, instead.  When I reverse-engineered this device, I noted that the marked capacitance value was unreadable, but back-of-the-envelope calculates indicate that a value of "about 150pf" should be in the ballpark.

R103, connected to the "R1" pin of U1, sets the approximate center frequency range while R104, connected to the "R2" pin - sets the lowest frequency - which important, since we want to constrain the tuning to 125-185 kHz.  Additionally, the low end of the tuning range was further refined by R102 on the "ground" side of the tuning potentiometer, which sets the minimum voltage that may be applied to the "VCOIN" pin.

The VCO output, a square wave, is buffered by U2, a hex inverter, and several sections are used to provide both a VCO signal and its inverted version to drive the mixer.

While the 4000 series CMOS chips throughout this receiver will happily run from 3-15 volts, they are operated from a regulated 5 volt supply - mainly to improve frequency stability and to provide a nice, stable voltage for a few other low-level circuits and to provide isolation from the main battery supply which will vary a bit, particularly at higher receive volumes:  This variance, if it gets back into some earlier stages, could cause instability of the receiver in the form of "motorboating" (low-frequency oscillation or pulsing) or some other type of feedback.

BFO:

Another circuit is the BFO (Beat Frequency Oscillator) which is used to convert the IF signal back down to audio - both being processes that we'll discuss shortly.  This uses an inexpensive 500 kHz ceramic resonator to form an oscillator using one of the sections of U2C, the signal being buffered by U2B.  This signal is divided-by-two using U3A, one half of a 4013 dual flip-flop - and then divided by two again using U3B, yielding a stable 125 kHz signal.  As with the VCO, two phases of this signal (normal and inverse) are available, this time using the "Q" and "!Q" outputs of the 4013.

Input signal path:

J1, a disconnect-type 3.5mm stereo jack is wired so that an internally-mounted electret "capsule" microphone is connected by default.  This microphone element (M301) is of the "2 wire" type or electret microphone in which a bias voltage is applied to the same pin from which audio is drawn - this voltage being applied via R301 from the 5 volt regulated supply.  The specific make/model of this electret element is unknown as it was selected from a small collection to find the best performer at ultrasonic.

Using a MEMs microphone

Since this article was first written, I replaced the electret with a more modern MEMs microphone as described in Another article:  Improving my ultrasonic sniffer for finding power line arcing by using MEMs microphones - link.  These types of microphones are capable of detecting from low audio frequency (a few 10s of Hz) and much higher - often through 100 kHz.

The schematic in Figure 2 now shows how an ANALOG MEMs microphone (see the link above for more information) may be integrated into the unit.  As these microphones are not tolerant of 5 volts, I used some "normal" (not "high brightness") GaAsFET Green LEDs:  D401 limits the voltage to 1.8-2.0 volts on the microphone's power supply while D402 prevents "glitches" from damaging the MEMS device that might get through via coupling capacitor C402 when an external microphone is connected/disconnected.  When everything was done, there was a very dramatic improvement in sensitivity above 15 kHz and several switching supplies near the workbench - which I couldn't hear before - suddenly became very audible.

Microphone amp and filtering

The signal from the microphone is applied to U4A which is wired as a unity-gain buffer.  For this, an LM833 is used, an inexpensive, low-noise dual op amp:  An LM358 or many other types may be used here as well - just make sure that it is is fairly low noise:  I'd avoid the use of the LM1458 here as it is quite noisy by comparison!

Section U4B amplifies the signal voltage by 10 (20 dB of power gain) and this signal is applied via R305 to a simple L/C high-pass filter consisting of C303, C304, L301 and L302 the latter two components being inexpensive 18 milliHenry inductors.  Certainly, an R/C-based high-pass filter could have been constructed using U4B, but I chose not to do that for some reason.

Figure 3:
Inside the ultrasonic receiver, constructed on
prototype board and having been modified
several times over the years, prior to the
installation of the MEMS microphone.
Click on the image for a larger version.

In simulation, the C303/C304/L301/L302 filter has a -3dB roll-off of about  23 kHz, it's down by 10dB at about 19.5 kHz, by 20dB at about 16 kHz and by 40 dB at 9 kHz and with the values shown, it's flat to within 1 dB between about 24 and 100 kHz.

The output of the filter is amplified by U5B - and then even more by U5A (which has a bit of roll-off from C307) to yield a whole lot of gain.  It's very possible that I over-did the gain here, but unless the signal source is quite close, there is no clipping observed on the output of U5A.

Its worth noting that a mid-supply voltage is created using R309/R310 to provide a "virtual ground" for the op amps and to maintain stability, it is heavily filtered by C306 and C302, each located near the respective op amp shown on the diagram.

Mixer and band-pass filter:

It is this next section that may seem unfamiliar to some - the use of a CMOS analog switch as a signal mixer.  For this, a CD4066 is used which consists of four separate analog switches.  The filtered and amplified ultrasonic input signal from U5A via C308 is applied to pins 2 and 10 of U6A/U6D.  When the respective signals on the control pins "VCO_A" and "VCO_B" go high, the switches are activated, and because VCO_A and VCO_B are inverts of each other, each of these switches is closed in turn.  The result of this is that the inputted signal is chopped up at the rate of the 125-185 kHz VCO and this produces two mixing products.  

For example, let's assume that there is a 40 kHz signal is present on the input that we wish to hear.  If the VCO is tuned 40 kHz above the 125 kHz IF (again, more on that momentarily) - to a frequency of 165 kHz - the switching action of U6A and U6D produces both the sum (165 + 40 = 205 kHz) and the difference (165 - 40 = 125 kHz).

T301 is a filter/transformer that passes only the 125 kHz signal - the difference signal in this case.  This transformer consists of two separate windings, each resonated using its internal capacitors and the externally-added 820 pF capacitors on each winding (e.g. C309/C310) to "pad" it down to 125 kHz.  This forms a fairly wide (8-10 kHz) filter that rejects signals outside the immediate vicinity of its 125 kHz frequency.  Because this filtering is at a fixed frequency, it does not vary with input tuning which means that its bandwidth is constant over frequency.

Of all of the components in this device, this transformer is unique:  It was originally a 262.5 kHz IF transformer from a 1970s/1980s Philco (Ford) AM-only car radio.  While I could have certainly used the original 262.5 kHz frequency - or even 250 kHz, when I built this I decided to pad it down to 125 kHz using C309/C310  - a frequency that is conveniently 1/4th of the 500 kHz resonator.

It's been so long since I built this, I don't recall why I didn't simply divide the 500 kHz by two and readjust that transformer to 250 kHz.  Practically speaking, I could have also up-converted to 455 kHz and used either transformers or ceramic filters from a modern AM radio as 455 kHz ceramic resonators were certainly available at the time - but I didn't do that.

Each half of T301 has a center tap and to this, a bias voltage is applied via R315 to assure that the voltage on these switches was in the middle of the supply range, away from the protection diodes on the 4066's I/O pins, which could cause clipping/distortion should they be allowed to conduct if the signal voltage got too near the ground or supply rails.  To prevent coupling between the two halves of the transformer via the center tap, R314/C311 was added, the resistor adding isolation with the capacitor bypassing the remainder of the signal.  Practically speaking, being able to adjust the bias voltage was unnecessary as a simple resistive voltage divider to set the bias at 2.5 volts (1/2 the supply voltage) would have been just fine.

On the "other" side of the transformer is the other half of U6 (e.g. U6B/U6C) - this time, clocked from the fixed 125 kHz oscillator.  From this, the signal - previously converted up to 125 kHz is now converted back down to audio.

Post-mixer amp/LPF:

The output of the down-converting mixer is applied to U7B via R316, a 1k resistor and a 0.001uF capacitor, both of which form a simple R/C low-pass filter to attenuate any high-frequency leakage signals from the mixer.  Because the mixing process itself is a bit lossy (about 25% efficient) as is transformer/filter T301, U7B boosts the signal by a factor of 10 (20dB) and then applies it to U7A, which is configured as a variable gain amplifier section.  The output of this is then boosted again by U8, an LM386 which is capable of driving headphones or even a small speaker.

A few comments about the design:

Originally, the circuit lacked U7 at all, but it was added when the gain of U8 (the audio amplifier), by itself, was found to be inadequate.  Since U7 was "patched" into place, this explains the odd gain distribution:  If I were rebuilding this from scratch, I'd certainly not need two post-mixer amplifier sections and I could have likely eliminated one full dual op-amp package.  As it is, I may add a "high/low" gain switch somewhere around U5 to allow reduction of the gain somewhat when in the presence of possibly-high ultrasonic signal levels to prevent clipping prior to the band-pass filter which would surely degrade overall performance.

If I were to build this again I would likely use a 455 kHz IF, instead as noted.  While not as plentiful, 455 kHz ceramic resonators are available to use for the BFO as are either transformer or ceramic-based band-pass filters.  I would also likely reconfigure U4B or U5 to perform the high-pass filter function rather than using harder-to-find inductors.

Again, I built this unit in the mid 1990s and have since lost my original notes, but I do recall that I modified it a few times since, simply tacking changes onto the old circuit rather than completely revising it.

Use as a longwave receiver:

While primarily intended to "hear" ultrasonic sounds such as those produced by bats, insects, leaking pipes, arcing power lines, etc., it is just a longwave radio receiver connected to a microphone:  If one connects a few 10s of feet/meters of wire to to J1 - and provides an Earth/ground reference to its shield connection - one can easily tune in the high-power transmitters used for submarine communications (around 20-30 kHz) plus the WWVB time signal at 60 kHz.  This must, of course, be done away from man-made noise sources such as power lines.

Alternatively, I have used a loop of about 1 foot (25cm) diameter of a dozen or so turns of wire along with a 10uF capacitor in series (to optionally block DC from R301) and been able to hear such signals - even in suburbia - but with this arrangement you'll also likely hear plenty of similar signals from the myriad switching supplies that likely inhabit your house as well!

Final comments:

The reader should be under no illusion that this is an optimized circuit or that I would do it this way again:  It was assembled fairly quickly to suit a need and to test a few random ideas, just to see if they would work.  Will I rebuild it at some point?  I don't know - it works as it should, so I don't plan to re-make something that is currently fit for purpose.

While I've heard very few bats with this - probably due to the deficiencies of the electret microphone at ultrasonic frequencies (which explains the future switch to MEMS-type microphones) - I've used it to find powerline noise (arcs are noisy at ultrasonic) and to test longwave receive antennas.

This page stolen from ka7oei.blogspot.com

[End]

A (semi)-typical suburban E-field whip receive system for the 630 and 2200 meter amateur bands

Even though the general availability of the 630 meter (472-479 kHz) and 2200 meter (135.7-137.8 kHz) bands to U.S. amateurs is a recent phenomenon, I've had interest in these frequency ranges for about as long as I can remember.  Back in the "old" days (the 1980s, for me) I would listen in these low-frequency ranges (10kHz to 530 kHz) using my modified Drake TR-7 which has an "LF input" on the back panel.

From the very beginning, I discovered a few things that did not work well for receiving these frequencies:

• Simply connecting an end-fed random wire to the "Low Frequency" input.
• Using my 40 meter dipole.
• Anything that was indoors.

Attempts to do any of the above resulted in either no audible signals other than a racket of power mains "buzz" that would drown out anything that I could hope to hear.  I quickly realized that there were a few signals that I could hear without too much trouble - mostly the very high-power VLF transmitters between 17 and 30 kHz and WWVB at 60 kHz, which is only few hundred miles/km away - and I knew that unless I could hear those signals really well that there would be little hope of hearing anything that was actually weak.

The "discoveries":

Figure 1:
The LF-400B active e-field whip on my
roof.  The antenna is about 5 feet (1.5
meters) above the roof, mounted to a vent
pipe.  The red ground wire connected to
the coaxial cable's shield at the bottom
of the antenna can be seen along with
choke.
Click on the image for a larger version.

Even as a teenager with limited experience and knowledge in such things I realized that at such long wavelengths even a rather long piece of wire as a receive antenna would be akin to putting a paper clip in the antenna connection of an HF rig and expect to "hear the world" - but I also knew that it was possible to hear low frequencies quite well on a very short antenna:  The short whip on my car could hear the entire AM broadcast band pretty well - so it was possible if done correctly.

These realizations told me several things:

• I would probably have to match the "short" antenna to the receiver input to be able to hear anything.  I determined that this could be done with a series inductor or some sort of high-impedance amplifier - or a combination of both.

• When in a car, I could be well-away from interference sources - such as power lines and noisy appliances - and could hear weak AM stations.  Somehow I had to keep the interference from things in the house from finding their way into my receiver.

Rummaging around in my junk box I found a large, variable inductor - probably from a scrapped TV - that I placed in series with my wire antenna and receiver - and over a limited frequency range (dictated by the adjustment range of this inductor) I noticed a dramatic improvement in the signal strength at about the frequencies that the combination of the coil and antenna provided a semblance of matching - although the noise was still substantial.

The next "breakthrough" was to wind a simple 1:1 transformer on a chunk of ferrite - probably the flyback transformer core of an old TV -  that allowed only magnetic coupling between the radio and its chassis, and the antenna and a connection that went directly to my kludgy system of buried ground rods.  By doing this, the "noisy" ground of my receiver - which was connected throughout the house with its noisy devices - was no longer referenced to the antenna.  Because the antenna must have a "ground" of some sort to "push" against I knew that if that "ground" was the radio itself, which was connected to the noisy house wiring, that this noise would, in effect, appear on the wire antenna.  This transformer effectively decoupled the two, using, instead, the comparatively "pristine" ground rod for the antenna to "push" against.  (For a depiction of this method see the external link to a paper by DL1DBC at the bottom of this page.)

Between the above two tricks an entirely new world opened up as I could now hear the (now defunct) Omega transmitters between about 10 and 14 kHz and a myriad of "NDBs" (non-directional beacons) and similar signals in the range from 190 through just below 530 kHz.  To be sure, I had to do most of my listening at night when TVs and lights were turned off, but that's when most of these frequencies propagated best, anyway!

The "LowFER" band:

Somewhere around this time I learned of the so-called 1750 meter "LowFER" band - a spectral slice from 160 through 190 kHz where legal, unlicensed operation (according to FCC §15.217 - read more here) could occur with some very strict limitations (e.g. an antenna that was, at most, 15 meters "long" and a maximum of 1 watt of input power.) but the challenge of both transmitting a usable signal with these limitations and receiving it via conventional techniques (e.g. CW) had its appeal.

It was at about this time - in the mid 1980s - that I purchased an LF Engineering LF-400B - a commercially-available active E-field whip antenna that seemed to have decent reviews in the various longwave-related newsletters to which I then subscribed.  This antenna, with a built-in amplifier and a strong low-pass filter to remove signals above 500 kHz, was much more convenient than trying to string a long piece of wire and matching it as it was rated from "3 kHz to 500 kHz".  One slight disadvantage of this - or any active antenna - is that it needs power, supplied in this case by a "power inserter" that ran from an external power supply or a pair of contained 9 volt batteries.

Being an E-field whip antenna it was still sensitive to the direct radiation of interference from the household and neighborhood wiring and appliances, but provided that I located it away from the house and "decoupled" its cable by winding as many turns as could fit on the core of a flyback transformer from a scrapped TV and grounding the shield at the antenna, it seemed to hear the background static very well - and if I could hear the background noise, there was hope that I could hear the weak signals buried within.

It was during this time period that I actively listened on the LowFER band, managing to hear a number of stations that were 200-700 miles (about 300-1100 km) away and, on one winter evening, hearing a station halfway across the continent - about 2000 miles (3200 km) away.   I also set up my own LowFER beacon that, although very modest, was occasionally heard, on CW, up to 700 miles (1100 km) away.

Comment:

Another antenna to consider for MF/LF/VLF reception is a shielded H-field loop.  By its nature, it is less-sensitive to nearby E-field energy - often that which emanates from electrical devices' interference radiating from wiring.

Another advantage of a loop is that it has a "figure-8" pattern with two nulls, allowing the possibility of rotating it such that one of these nulls is oriented toward an interference source.  The obvious disadvantage is that a loop should have provisions for rotation to steer it into the null for the worst interference - or take care of those instances where the desired station happens to be in the direction of the null.

Shielded loops are available and they can be constructed fairly easily, typically using a piece of coaxial cable.  Unless they are rather large and/or actively tuned to the receive frequency, they - like a short E-field whip - must have an amplifier that is externally powered.

Fast forward to the 21st century:

As it happens, I still have the L-400B and it has been outside, on a roof, for most of the time since the mid 1980s.  Other than having to repair it a time or two (usually due to condensation and related corrosion) it still works as well as it ever did.  While I had not been as active on LF as I once was, I'd been maintaining that receive antenna system and with the recent availability of the 630 and 2200 meter bands, interest has been rekindled.

To this end, I decided to document my receive antenna installation, showing what "works for me."

The antenna on the roof:

I will admit to a luxury that most others will not have:  My house has a metal roof.

Figure 2:
A close-up of the coax choke at the antenna.  This
choke consists of 10 turns wound on a large
ferrite bar.  The coax used is solid-dielectric RG-58.
The use of a solid dielectric rather than a foam
dielectric - such as that found in RG-6 - allowed
a very tight radius winding without worrying much
about the center conductor "migrating" and shorting
to the shield.  A cable like RG-174 would have also
been usable, allowing a tight radius and more turns.
At these frequencies, the loss of the coaxial cable is
insignificant.
Click on the image for a larger version.

The metal roof not only acts as an excellent ground plane, but it is also an effective barrier between what is "inside" my house and the "outside world".  This means that at VLF and LF frequencies, things in my house that generate noise (light dimmers, switching power supplies, TVs) are fairly effectively isolated from this antenna on the roof - at least in terms of direct radiation of energy from these devices.

If you are not "blessed" with a metal roof on your house - but you are willing to go through a bit of hassle - you could lay down a suitable ground plane:  Many people have been known to put down a layer of chicken wire on the roof or an interconnected grid of wires to act as an effective shield.  Practically speaking, it need not cover the entire roof, but if the radius of this plane is 1-2 times the height of the antenna over the roof, it will probably have reasonable effectiveness.

Somewhere this plane must be grounded and it is best that this is done via its very own ground system - which could be as simple as a ground rod - which is preferred over tying into the house's "noisy" electrical ground.

As can be seen from the picture in Figure 1 the whip antenna is mounted to a vent pipe at a height of approximately 5 feet (1.5 meters) above the roof - which happened to be the length of the piece of aluminum that I'd found to mount the antenna.  When experimenting with mounting this antenna I found that if I placed it just above the metal roof, it was very quiet and relatively insensitive - but much of that was due to the fact that the very E fields to which the antenna is sensitive decrease significantly with proximity to "ground".  By raising the antenna above the roof the signals increase very dramatically, but still seemed to be within the "cone of silence" afforded by the metal roof.

Decoupling the coaxial cable at the receive antenna:

At the time that I bought the LF-400B antenna it was offered only with a permanently attached RG-174 feedline, but after about a year of use, often hauling it into the wild to listen, away from the city, the coaxial cable fatigued and broke, so I carefully disassembled it and installed a BNC connector (later versions of this antenna have a choice of connectors as an option.)  This modification allowed me to connect a ground directly to the bottom of the antenna.

Figure 3:
A block diagram of the antenna and receive system showing the grounding and coax chokes.
Note that the "roof ground" - which is, in my case, the metal roof itself, but it could be a grid of wire or fencing material laid on the roof and is grounded elsewhere - is connected directly at the shield of the e-field whip itself, "before" the coaxial choke.  At the ground level, in close proximity to the building entry is another connection to a "clean" local ground such as several ground rods and/or some buried ground radials.
The "Inside coax choke" has the most inductance and does most of the isolating of common-mode noise currents that could otherwise "light up" the antenna with electrical noise that would be conducted from the radio system's ground connection to the power mains.  The DC power inserter puts DC on the coaxial cable to the antenna - but not on the coax to the receiver - to provide power.
Click on the image for a larger version.

As mentioned earlier, one of the "tricks" to a quiet E-field antenna is to prevent electrical noise from being conducted from the receiver and "lighting up the ground" of the antenna itself - a problem that is arguably worse than the antenna itself picking up noise, directly.  One of the better ways to to do this is to "decouple" the coaxial cable between the antenna and receiver using a large amount of inductance on the feedline - and I chose to do this several ways.

As can be seen from figure 1 there is a (red) wire connected directly to the antenna's connector that, in turn, connects to the "local ground" - that is, the metal roof itself.  By doing this, the "ground" of the antenna and the roof are at the same RF potential and the interception of "local" interference by the antenna is reduced.

Also visible in figure 1 - and in more detail in figure 2 - is an inductor in the form of a portion of the connecting coaxial cable being wound around a large ferrite rod from a discarded AM radio.  The location of this inductor places it between the antenna and the receiver and its inductance adds common mode impedance to signals that would be conducted along the coaxial cable, but will not affect the desired signals within the cable itself.  A better choke for this location would be like that depicted in Figure 4 (and described below) as it has higher effective resistance at the frequencies of interest, but since I'd already installed this one, I left it in place.

Figure 4 shows the other end of the cable just after it enters the house.  Just as it enters through the window there is another BNC connector, and connected to the shield at that point is a wire that goes directly to a grounding system that is located immediately outside the window.  Between this grounding point and the inside of the house where the connection to the radio is made the coaxial cable is wound around a much more substantial choke - this one consisting of as many turns of the RG-58 coaxial cable as will fit on a TV flyback transformer ferrite core that was scavanged from a discarded CRT TV or computer monitor.  The details of the locations of these chokes and the grounding points is detailed in Figure 3.

It is this second "inside" choke that does most of the work:  Consisting of about 20 turns, it has a measured inductance of about 15 millihenries.  In running the math we can see that this large amount of inductance is what is required to effectively isolate the coax at LF and VLF frequencies, as in:

Where inductive reactance is calculated using the equation:

Z = 2 * Pi * F * L

Where:

Z = Reactance in ohms
F = Frequency in Hz
L = Inductance in Henries

Because we are dealing with milliHenries and kHz, the "10s" parts cancel out, so:

At 500 kHz:


500 kHz * 15 milliHenries * 6.28 = 47100 ohms

Because this is a linear equation, we can then re-run the numbers which tells us that at 50 kHz, the reactance is 4710 ohms and that at 5 kHz it would be 471 ohms.


What this shows us is that even at very low (VLF) frequencies, the impedance of our rather substantial inductance is still effective, so it will work nicely at both 630 and 2200 meters - and everything in between!

Figure 4:
The indoor coax choke consists of 20 turns of  RG-58 wound on a TV flyback transformer core.  This choke, with a measured inductance of about 15 milliHenries, provides excellent isolation even down below 10 kHz.  If a flyback transformer core cannot be found a suitable choke can be wound on a high-permeability ferrite core using smaller (e.g. RG-174) coaxial cable as described below.  Note that to be effective at these frequencies this choke really does need to have at least several milliHenries of inductance!   See the links at the end of this page for sources of suitable ferrite devices.
Click on the image for a larger version.

Obtaining the inductance:

While "current-mode" 1:1 baluns that isolate the feedline in the manner we desire are readily available, unless they were specifically designed for LF and VLF use they do not have enough reactance to operate effectively at these low frequencies!  What this means is that unless a suitable product is offered by one of these companies that is has been designed for LF and VLF use, they will not work well!  This means is that you will probably need to construct your own coaxial choke.

Using flyback transformer cores:

Many years ago it was pretty easy to scavange flyback transformer cores from old CRT-based TVs or computer monitors, but these are getting harder to find - but this is mostly a good thing since these transformers were part of the very device that caused a lot of interference at VLF and LF frequencies!  Once one manages to get the core out of an old flyback transformer in the first place (sometimes a trick in and of itself!) the fact that these cores are in two pieces makes it easy to wind the coaxial cable over one half and then assemble it.  When I come across a flyback transformer, I often resort to putting it in a toaster oven and heating it so that the glue softens.  Often, the core breaks - but ferrite typically breaks very cleanly and the two pieces can be rejoined using a drop of cyanoacrylate (e.g. "super") glue with little change in performance.  Before using the ferrite core, make sure that any plastic or fiber shims between the two halves are removed:  These were important in the role as a flyback transformer, but they will reduce the inductance of our choke and aren't helpful to us in our quest to quash QRM.

If a flyback transformer core is not available, what can be used, instead?

Using high-permeability toroidal cores:

While not as convenient as a flyback transformer core - which can be disassembled during winding - a ferrite toroidal core can be used, instead.  To maximize the number of turns, smaller coax such as RG-174 would be used and the connectors installed/connected after winding was complete.

Take, for example, a common ferrite material designed for low frequencies - "Mix 75" (sometimes called "Mix J") with a typical permeability of about 5000.  A reasonably large toroidal core would be the FT-240 (the complete part number would be either "FT-240-75" or "F240-75").  Note that the ferrite mixes that one would normally use for things like HF baluns aren't ideal for this purpose as they have lower permeability.

Extrapolating from a data sheet and rewriting the equation we can see that if we can manage to wind 30 turns on this particular toroidal core, we can expect:

L = Al * (T/1000)²

Where:

L = Inductance in mH
T = Turns
Al = mH per 1000 turns from the spec. sheet - 6850 for an FT-240-75

So,

6850 * (30/1000)² = 6.165mH

Clearly, this is a bit less than half as much as I'd measured on my discarded TV flyback, but if we use the equation above we still get 194 ohms at 5 kHz and over 5 kohms at 2200 meters - a respectable amount of reactance!  Using this size of core (an inside diameter of 1.4 inches/3.5cm) it is likely that more than 30 turns of RG-174 could be wound on it - and if you make this type of core, by all means, put as many turns on at as you can!

Unfortunately, the "Mix 75" toroids are not as easy to find as typical toroids designed for higher (HF) frequencies and if we use a more common type such as Mix 31 the result will be between a quarter and a fifth of the inductance for the same number of turns whereas "Mix 77" will, for the same number of turns, yield about 1/3 of the inductance as Mix 75, but this would still imply between 1 and 2 kohms at 2200 meters - still quite good.

Where does one get this sort of toroid?   Toroids can be found at a number of places, including:

• Palomar Engineers (link) 
• Amidon Associates (link). 
• Another distributor of some of these devices is the web site kf7p.com - link.

Again, while "Mix 75" is preferred, "Mix 77" is the second choice - and cores may be stacked to increase the inductance for a given number of turns.

Another possibility - Common-mode chokes:

While a coaxial-based choke is preferred, there are other devices - possibly in your junk box - that may be suitable:  A common-mode choke used for power supply filtering.  The best place to find these is from scrapped switching power supplies - such as those used in computers.

Figure 5:
An assortment of power line filtering chokes and devices.  In the upper-left
is a self-contained AC line filter, but it is not suitable for this purpose as it
is designed to block all RF - both differential and common-mode.  All of
the other devices are dual-winding common-mode chokes that allow
differential currents to pass, but will block common-mode currents - but
not all of these devices are suitable for our purpose - see text.
Click on the image for a larger version. 

Figure 5 shows an assortment of typical devices - but not all of them are suitable.   As noted in the caption, the self-contained power line filter (upper-left) blocks all RF and wouldn't work, but the other devices allow differential currents to flow while blocking common-mode currents - which is what we want.

In order for these devices to be suitable for our purpose, they need to have:

• Adequate inductance.  As we noted above, we need milliHenries of inductance to effectively choke out interference at LF and VLF frequencies.  The smaller toroidal chokes shown - typically those wound on toroidal cores - have hundreds of microHenries of inductance which may be suitable at 630 meters, but could be marginal at 2200 meters.  For example, a choke with 100 microHenries per winding will offer about 295 ohms of reactance at 630 meters, but only 86 ohms at 2200 meters.  Because we want as much reactance as possible - at least in the many hundreds of ohms - we would hope to do better!
• Good balance.  All of these chokes consists of two identical windings and the idea is that if a common mode signal appears across both windings, they will be suppressed.  If, however, the two windings are not identical, this suppression will be incomplete.  It is likely that the "transformer-looking" chokes (e.g. those that do NOT look like toroids) will have reasonable suppression at 2200 meters - and maybe even 630 meters - but as one goes up in frequency even more, the imbalance will grow.
• Low loss to differential signals.  The reason that we can pass a signal through a coaxial cable wound on a large piece of ferrite without affecting the signal being carried by that cable is that the coaxial cable, by its very nature, is fairly low loss to the signals carried within where the signal on the inside conductor of the coax is precisely equal and opposite to that carried on the shield.  If one has separate windings, each carrying an equal and opposite signals, imperfections in these two windings - sometimes the same as those that cause imbalance - can cause degradation of those signals.  As one goes up in frequency these ferrite cores - which are formulated to block low frequencies - can start to get lossy - and this doesn't include the self-capacitance of the windings which can cause other things to happen, such as strange resonances or coupling.  In other words, they may work find at low frequencies, but "fall apart" at higher frequencies such as 160 meters (1.8 MHz) and up.

Figure 6: 
An example of how a bifilar (or similar) choke would be
connected to a coaxial cable.
The diagram above depicts how the two windings would be
connected, keeping straight which is the "center", and that
which is the shield of the coaxial cable.  The dots indicate "phasing" -
that is, same ends of the two windings connect to the antenna side and
the other ends connect to the receiver side.  On the "antenna" side's
coax shield would be connected our "quiet ground".
Click on the image for a larger version.

In short, the suitability these devices for our purpose is best determined experimentally.

How it is would be connected:

Figure 6 shows how such a device would be connected to coaxial connectors.  Note that the winding for the shield on one side of the choke connects to the same shield on the other side.  In theory, this wouldn't matter at RF, but because we may need to conduct DC to power the active antenna, we would also need to preserve the polarity.

Not also that both sides of the input and output coaxes connect to the same "side" of the dual winding choke as indicated by the dots - in other words, the two windings are in phase with each other:  Were either one of the windings (ground or center conductor) "flipped", this choke would do exactly opposite that which we desire - that is, the signal on the coax would be blocked, leaving only noise!

For an inductor such as that depicted in Figures 5 and 6 that is not wound with coax, it doesn't matter which side is the shield and which is the center - just as long as the windings are "shield-to-shield" and "center-to-center".

Another example of feedline choking and grounding:

Figure 7:
RG-174 coaxial cable wound on a TV flyback core.  About
55 turns fit on this core yielding a measured inductance
of a bit over 1.5 milliHenries.  If this core
had equal gapping in both of its "legs" (see text)
the inductance would have been higher.
Click on the image for a larger version.

Figure 7 shows another example of how an E-field antenna's feedline (not mine) was isolated.

In this case - which just happened to be another LF-400B - RG-174 coaxial cable was permanently attached to the antenna.  This cable was cut, leaving about 20 feet (approx. 6 meters) of it still attached to the antenna and, leaving a "service" loop of about 1.5 feet (35cm) the remainder was wrapped on the flyback core of a discarded computer monitor.

It is worth noting (again) that these flyback cores are usually "gapped" - that is, a small - usually plastic - insulator is placed between the two halves of a core to prevent it's being saturated.  On some of these cores there are two equal gaps - one on each of the two mating surfaces and if these are removed, the two ferrite surfaces mate closely.  In the case of the core in Figure 7, only one of these mating surfaces had a gap, meaning that one side mated closely while there was a gap on the other side:  Sometimes the cores are symmetrical and one can be "flipped" to eliminate this problem, but that was not the case with the core in Figure 4, so a slight gap was inevitable and this has the inevitable result of reducing the total inductance of the core.

In the case of the flyback pictured in Figure 7 the plastic gapping material was carefully retained and a very thin layer of epoxy was put on the two sets of mating surfaces and the metal bail holding the two together was reinstalled, the cores being worked back-and-forth to squeeze out extra epoxy.  Once this was done epoxy was applied to the wire bail itself to keep it in place.  After the epoxy was allowed to cure, the remaining RG-174 coaxial cable was wound on it, filling it up.

Figure 8:
A wire attached to the shield to permit grounding, necessary
because the coaxial cable was permanently attached to
 the antenna, preventing a connection from being made
at then antenna, between it and the choke.  See text.
Note that the ground wire emerges from the "downstream"
side of the coax.  When installed, this ground wire will
face down to reduce the probability of moisture ingress.

Click on the image for a larger version.

Figure 7 shows this core mounted in a plastic "pull" box intended for non-metallic electrical conduit.  On one end of the box is mounted an "F" connector to which the end of the RG-174 is soldered while the other end - connected to the whip antenna - emerges through a plug:  If you do it this way you will surely want to pull the coax through the plug before winding it on the core!  After it was assembled, it occurred to us that we should have put the "F" connector on the same side as the plug so that they could both be faced downwards.  Between the receiver and the choke box, ordinary RG-6 TV coax will work:  The impedance mismatch/loss is unimportant at this frequency, in this application.

Figure 8 shows a bit of detail about the grounding of the antenna.  This particular antenna has a permanently attached cable and the owner didn't wish to modify the antenna to add a coaxial connector to it.  That which follows was done before the coaxial cable was wound on the ferrite core.

The ground connection needed to be made directly to the cable's shield and this was done by carefully baring a bit of the shield by removing a small amount of the outer jacket and then using a hot soldering iron to quickly make the connection without melting the inner dielectric - a bit of a trick to do if one isn't skilled in the art of soldering!  To make this weather proof the connection was covered with a thin layer of thermoset (e.g. "hot melt") glue and a small piece of heat shrink tubing was slid over the joint and shrunk.  Over the top of this a thin layer of RTV ("Silicone") sealant was spread over the entirety of the connection and another, slightly longer piece of heat shrink tubing was installed and shrunk - and then another thin layer of RTV and slightly longer heat shrink tubing.

While this sounds like overkill, it should prevent moisture from finding its way in between the jacket of the coax and the tubing.  Finally, this connection should be oriented at the time of installation such that water runs away from it - which is to say, the part with the wire coming out from underneath the tubing should be facing down.

More information about interference reduction:

While the above techniques will go a long way to reduce the amount of noise picked up by an E-field antenna - and, to a degree, any antenna - it is too-often the case that there will be some device that simply radiates a lot of noise.  While at HF frequencies and higher it is possible to reduce this noise with the application of large ferrite devices on cables, power cords, etc. this tactic simply does not work well at VLF/LF/MF frequencies because it takes so much reactance (inductance) to introduce enough effective resistance in the wire conveying this noise and a "snap-on" choke simply cannot do this.  Even if a device contains "good" noise suppressing components (not all do!) they simply may not be very effective at VLF/LF/MF frequencies.

If you are interested in listening on the LF and MF amateur bands, the necessary first steps are outlined above:  Do what is necessary to prevent noise from being conducted out, onto the antenna in the first place.

Once that is done, you may need to "seek and destroy" devices that are particularly egregious when it comes to generation RF "grunge" - and the typical suspects are switching power supplies, light dimmers and some brands of LED lights.  Plasma TVs are notoriously bad interference generators, but since they are no longer being made, their contribution to the miasma of QRM is slowly decreasing as they die off.

The best way to find noise that you can do something about is to power the receiver from a battery (NOT including an inverter!) and turn off all of the power to the house - including shutting down any UPSs that you might have.  If the noise decreases or goes away, turn on one circuit at a time until it returns and upon finding the circuit, isolate the specific device that causes the problem.  If the noise is just the same with your power off as it is on, there may be a noisy power line nearby and/or a neighbor may have a noisy device - and how you deal with those two entities is up to you!

If you find a device (or devices) that generate lots of interference, they might either be replaced with "quieter" ones or modified to be quiet.  Unfortunately, the latter can be a challenge and the links below include techniques for doing this.  If your goal is interference reduction at VLF/LF/MF - and you are constructing better filtering - remember that the higher-inductance chokes will be best!

How well does my receive antenna system work?

In the late evening and overnight, I can easily hear the "band noise" - that is, the sounds of the ionosphere and propagated storm static.  During the day time the noise level is typically lower as it seems as though propagated noise from a wide geographical area is suppressed somewhat - possibly by the formation of the ionospheric E-layer.  During the "busy" hours - particularly from, say, 5 to 11 PM, there can be a bit of interference from other peoples' TVs, appliances and whatnot, but it is usually not severe enough to completely quash reception.

In my ham shack I have some track lighting over the workbench that is equipped with LED floodlights and is controlled by a light dimmer.  While I do not "hear" the LED's switching power supplies, I do get a significant "buzz" on 2200 meters from the dimmer itself - but I don't hear it on 630 meters.  The work-around for this is to use a smaller work light near the workbench - both of them being fluorescent - one having an iron ballast and the other electronic - but neither of them causing detectable interference on either 630 or 2200 meters.

For the past 5 years or so there have been a number of Canadian amateur stations (who have had access to the frequencies around 630 and 2200 meters for a while) plus some U.S. based "experimental" stations that have also operated on a number of other frequencies and in this time, I've been able to "receive" these stations which are typically using a digital mode like WSPR or a more analog-like mode like QRSS (slow-speed Morse code) - both typically being detected by computer.  The operational frequencies of these stations has varied from above 500 kHz to below 30 kHz, depending on the authorized frequencies of the various experimental stations and I've generally been able receive such signals including a number of stations operating in the 470-500 kHz range across the U.S. and an experimental station operating near 29 kHz (yes, 29 kHz!) from New York state to my QTH in Utah - a distance of about 2000 miles (approx. 3200km).

In the relatively short time since U.S. amateurs have been allowed to operate on the 630 and 2200 meter bands I've heard several stations on both bands - some well enough to have copied using Morse code via ear and, possibly, even SSB voice.  As the northern hemisphere descends into winter - and as more amateurs receive authorization and put their systems on the air - I expect to hear even more stations.

One device in my arsenal is a "Line Synchronous Noise Blanker" - that is, a device that will mute the antenna signal when an interfering pulse - which is usually in sync with the power mains - comes in.  This devices is adjusted manually and can go a long way to knocking out this type of noise.  This device is described on this page:  A Line-Synchronous Noise Blanker for VLF/LF/MF use - link.

Links to other articles about power supply noise reduction:

• Completely containing switching power supply RFI - link.  Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave.
• Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - link.  Electronic light ballasts, like many switching power supplies, operate in the LF frequency range so "cleaning them up" at VLF/LF/MF frequencies can be a challenge.
• Quieting high current switching power supplies used in the shack - link.  This page describes techniques that can be used to reduce the amount of RF energy produced by switching power supplies that you may be using to power your radios.  Again, higher-inductance chokes may be required at VLF/LF/MF frequencies.
• Reducing switching supply racket - link.  This describes techniques that can be used to beef up the filtering for switching supplies in general.

In case you get intermodulation distortion (e.g. overload) from nearby AM broadcast stations:

•  An effective low-pass filter for 500 kHz and below - link.

Other information about the use of active antennas at VLF, LF and MF frequencies:

• Discussion from the DL1DBC web site about active antennas, including their operation and installation - link.
• Construction and installation of a PA0RDT whip by VK6YSF - link.
• Discussion of E-field whip antennas by PA3FWM - link.

Once again, here are some links to suppliers of suitable ferrite devices:

• Palomar Engineers (link) 
• Amidon Associates (link). 
• Another distributor of some of these devices is the web site kf7p.com - link.

While "Mix 75" is preferred, "Mix 77" is the second choice - and cores may be stacked to increase the inductance for a given number of turns.

Final comments:

The L-400B still seems to be available - at about twice the price as it was when I bought mine in 1986-7.  The page with information on this and similar products may be found here - link.

In addition to the L-400B, there are now other active whips, including the AMRAD active whip, the PA0RDT "Mini-whip" and variants on those designs which may or may not include a low-pass filter to remove mediumwave signals.  All of these are reported to work well, but be aware that some receivers have difficulty dealing with signal from strong, local AM broadcast transmitters.

I have built my own copy of the PA0RDT mini-whip and I tried it only briefly:  When I put it up, it was badly overloaded by my own HF WSPR transmitter - so I took it down and put the LF-400B back up as its built-in low-pass filter makes it pretty immune to being overloaded nearby HF transmissions.

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This page stolen from ka7oei.blogspot.com