Thursday, December 26, 2019

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.

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.  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.

* * *

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