Ham Radio

In our final installment of tests to compare the ‘UHF’ connector with more recent offerings I present the graphs below.

I presented these results to members of my local ARA and opened some eyes. One thing learned during the extensive discussions at this presentation was how materials for the PL259, SO239 and related connectors have changed over the years.

The best point heard at the meeting was the sampling of ‘UHF’ connectors for these tests were arguably too small to statistically prove any point. I agree and a plan for future tests with many samples of each connector type is in the works. For now, however, you are free to make your own conclusions with the data as presented in this post.

Graphs
Five different graphs are used below: Return Loss, SWR, Mismatch Loss, Insertion Loss and Intrinsic Loss.

Return Loss (aka -S11), SWR and Mismatch Loss are related to each other and can be calculated from each other with formulas. In this test, SWR and Return Loss are directly measured with a calibrated VNA. Mismatch Loss is calculated from Return Loss.

Insertion Loss is simply the S21 measurements from the VNA and indicates how much energy did not make it through the connector(s) under test.

Intrinsic Loss is the final graph and is calculated as the difference between energy bounced back from the test item (Mismatch Loss) and that which does not make it through (Insertion Loss). This difference is missing energy and is energy likely turned into heat via dielectric material losses.

The multi-color graphs shown below detail the actual values for each type of UHF connector plus N, SMA, TNC, and BNC connector types. First, however, let’s examine the two groups against each other because the results are revealing. The following graph shows the various UHF connectors in red, the other connectors in blue and our perfect baseline in black…

The stunning result is all the UHF connectors in the test have worse performance than all the other connectors. One immediate conclusion concerning ‘UHF’ connectors is they will function at these higher frequencies, but one must decide if using the PL259 or SO239 is worth it in an age where its deficiencies have been made moot by ALL connector designs since WWII.

Now let’s get into the colorful graphs which are the same data as above, but with each line identified to the device under test. The following table shows the ties the abbreviation in the graphs to the item under test.

Baseline:	Direct connection of test cables using superb SMAs.
SMA:	Same as baseline plus two more SMA barrels.
BNC:	Two SMA to BNC Converters plus one BNC Barrel in between.
TNC:	Two SMA to TNC Converters plus one TNC Barrel in between.
N:	Two SMA to N Converters plus one N Barrel in between.
UHF 1″ Cl:	This is a 1″ Amphenol Model PL-258 Barrel with clear dieletric.
UHF 1″ Wt:	El cheapo 1″ SO-239 Barrel with White dielectric.
UHF 2″ Cl:	A 2″ SO-239 Barrel with Clear dielectric.
UHF 3″ Cl:	The combination of the Amphenol 1″ and the 2″ using a Male-Male joiner. It is really about 3.5″ long.
UHF T Wt:	A UHF T adapter with two SO-239 and one PL259 and White dieletric. The PL259 port was left open.
UHF Ft Cl:	A 12.5″ long SO-239 Barrel. This is the thing used to stick through walls.
PL/SO:	One SMA to N Converter directly mated to one PL259 to TNC converter. TNC is adapted to SMA.

Note: In all the graphs below, you can click to reveal a more detailed graph out to the absurd test frequency of 1 GHz. No one would ever use a UHF connector at this high a frequency, I hope, but the resulting patterns are interesting nonetheless.

Let us start our examination with Return Loss…

…we see the various UHF barrel lengths have different effects. The PL/SO line is simply one PL259 mated with one SO239 with as little other interaction as possible. Indeed the PL/SO performs the best of the UHF bunch.

Being a ratio of incident energy vs. reflected energy, larger Return Loss values indicate better performance. It’s a bit like the “Yes, we have no potatoes” statement. Yes, we have lots of power not returning to the source. The straight S11 measurement makes more sense. “The power reflected back is 10dB down from the power applied” means 1/10th power is reflected back to the source. A Return Loss (or S11) of 0 dB is awful as this means ALL the power is being reflected back (aka as an open or short at the end of the transmission line).

There is another way to look at the S11 data. What we really care about is how much system loss occurs because of this reflected energy. This leads us to something called Mismatch Loss and is shown in this graph…

Now we have something we can relate to. A mismatch loss of 1 db means that, because of the energy reflected back to the source, 1 dB of energy will never make it to your load. It is possible the source will re-reflect the energy back to the load (similar to what happens when you use a transmatch). However, all RF sources do have an impedance for this reflected energy which likely winds up simply as heat in the transmitter.

Look at the blue line for the 3″ SO-239 barrel. It measures a max of 1 dB loss due to the impedance bump.

It should be interesting to the reader the N, SMA, TNC and BNC connectors are on this graph, but in the thin pastel smear on the 0 dB line.

Now let’s look at good ol’ SWR of the same data…

This graph looks a lot like Mismatch Loss doesn’t it. Note the blue line again. What is 1 dB loss due to impedance issues shows as more than 2.5 SWR.

In the above graphs did you notice the gray line response of the 1 foot long SO239 “barrel?” Just like a broken “analog” clock is correct twice a day, so it is with transmission lines of differing impedance when they are 1/2 or multiple 1/2 wavelengths long. With the first low point frequency of the foot long barrel and its length you can compute the velocity factor if you like (reader exercise). The same holds true for the shorter UHF barrels, but you will need to click the 1 GHz plots to see this better. The 3″ barrel (blue plot) 1/2 wavelength point is evident in the 1 GHz plots.

At long last its time to show the energy that actually did not make it through the connectors in the S21 test.

The above graph shows us the real cost of using UHF connectors. Being a measure of total losses, it is the combination of Mismatch losses plus any heating losses in the dielectric materials.

You finally can see some imperfections in the N, SMA, TNC and BNC connectors. However, they all handily beat every UHF component.

So how do we measure actual heating loss of an RF system? Several folks in the Amateur Radio world have devised clever laboratory assemblies to directly measure this heat. Unfortunately, they incorrectly assume all losses turn into heat within the connector body.

My lab does not have a way to measure heat so, instead, I calculate it as simply the difference of Insertion (Total) Loss less Mismatch Loss. Here it is…

Once again all the UHF connectors have a bit more “heat” loss than the other modern connectors.

Conclusions

What are we to make of all this? Well, even with this arguably small sample size a trend emerges suggesting UHF connectors have inherent design issues that were nicely solved by later connector designs. Indeed, these later designs were likely developed to directly solve the issues in UHF connectors.

Many suggest the UHF connector is usable at VHF and UHF frequencies. Indeed it will function, but with some cost. It is likely completely unfair to suggest the UHF connector was designed to work above 30 MHz. It served a purpose before WWII and will still work quite well, electrically, at 30 MHz or below. 50 MHz is not too much of a stretch either.

Amphenol maintains the UHF connector, with good quality materials, will do quite well up to 300 MHz. Note, however, one of the two 1 inch UHF barrels with the Clear (Cl) dielectric is an Amphenol product – Model number PL-258. Judge for yourself.

Some claim since mobile dual-band radios use the SO-239 connector, it is designed to work well at UHF – specifically the 70cm band. Indeed, my Kenwood TM-D710 has the SO-239 connector. However, did you know the European version of the D710 uses the N connector for both 2m/70cm antenna jack? Sigh. I have a permanent PL259 to TNC adapter on my D710.

Another good example radio is the Icom 910H which uses an N connector for the 70cm antenna jack. It seems when performance counts, the ‘UHF Connector’ is not used for UHF.

It would seem American hams have a love affair for the UHF connector and are, unfairly, giving it properties it may never have had. A mind-blower for sure. However, to be fair, the UHF connector has, in the past, been seen to work reasonably well up to 500 MHz as shown by A. B. Crites[1] in his extensive analysis and tests of various dielectric materials in the UHF configuration. Are the UHF connectors in my sample pool of the same quality as used by Crites? I don’t know, but I suspect not. A better funded test with larger sample sets of various currently available offerings is in order.

By all means use the PL259 and SO239 connectors, but understand and accept the limitations as measured and shown above. Personally, I use N and TNC for anything above 30 MHz whenever possible.

References

1. “An Analysis of the VSWR Characteristics of Amphenol Series 83 UHF R.F. Connectors” – A. B. Crites, September 14, 1955