PL-259/SO-239 Return Loss and Attenuation Test

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This previous post in this series gave some history concerning the classic “UHF” connector also known as the PL-259 and SO-239. As we continue the research into the “real” performance of the PL-259 and SO-239, we come to this post where the test configuration and the items under test are revealed.

Great laboratory equipment was used in this test, but some good amateur grade equipment is available for much less price and can verify some of the findings here. As you will read below the S11 measurement is really a form of SWR reading with a one port instrument. Check out Array Solutions for some possible choices in gear. The S21 measurement will be more difficult to accomplish with amateur level gear; However, used equipment may be available. I tested to 500 MHz, but the issues with the UHF (PL-259) connector system becomes apparent at around 70 MHz.

The Two Tests
There are two individual tests in this experiment: S11 Return Loss and S21 Attenuation.

S11 Test
S11 is analogous to the familiar SWR check, but with a bit more information. Return Loss measures how much energy is bounced back from the load to critique how well it absorbs power from the source – the more negative number in dB the better. For the most part, SWR and Return Loss can be calculated from each other. For completeness, I will show both SWR and Return Loss plots for the S11 test.

Figure 1 - Test Setup for Connector S11 Test
Figure 1 – Test Setup for Connector S11 Test

Figure 1 shows the simple connections involving one cable to the VNA and a 50 ohm terminator load on the other side of the device under test (DUT).

S21 Test
S21 is simply how much power makes it through the device under test. Losses are revealed in dB increments. Figure 2 shows the extra cable for measurement of the power through the device under test.

Figure 2 - Test Setup for Connector S21 Test
Figure 2 – Test Setup for Connector S21 Test

Frequency Test Range
Both the above tests will be performed from about 70 MHz (the point where losses began to appear) to 500 MHz.

A good Vector Network Analyzer (VNA) calibrates the test cables as close to the DUT as possible and the one used for this is no exception. It’s good… trust me.

Note the VNA’s test cables have SMA connectors at the ends. Where needed, adapters are used to convert from SMA to the DUT. Sometimes, this means multiple conversions occur. Such is the case for testing the PL-259 and SO-239 as shown below.

Comparative Connectors
When critiquing any component it is always a good idea to compare it with other types of devices in its class. For this test I include samples from SMA, BNC, N, TNC and, of course, the PL-259.

Contender #1 – The SMA Connector
Because the VNA’s connectors are already SMA, a simple SMA barrel was used. No picture is needed for describe this I think.

Contender #2 – The BNC Connector
Who hasn’t seen or used this connector over the years. This use to be the standard connector for HT antenna ports. It is still the connector of choice for QRP radios. Originally designed as a smaller bayonet version of the Neill connector (of N connector fame), the Bayonet Neill–Concelman (BNC) connector is wildly popular.

Figure 3 - Adapters for BNC Connector Test
Figure 3 – Adapters for BNC Connector Test

Figure 3 shows the conversion necessary to get a complete test setup. SMA is converted to BNC and then goes into a bulkhead silver plated BNC barrel. One side connects to one port of the VNA and the other side connects to either a load for S11 or the other port of the VNA for S21.

Contender #3 – The N Connector
With the exception where lots and lots of power are delivered to the load, the N connector (named for Paul Neill [1]) reigns as the supreme consumer grade RF connector of choice for amateurs and many professionals.

Figure 4 - Adapters for N Connector Test
Figure 4 – Adapters for N Connector Test

Figure 4 shows the same sequence of adapters to get to the N with two SMA to N converters. An N barrel about one inch in length completes the test set.

Contender #4 – The TNC Connector
It looks like a BNC and acts like a BNC. In fact it is identical to a BNC except for the bayonet. This is the Threaded Neill Connector or TNC. This is likely the least popular connector in this test. It is my standard connector for everything in my ham shack less than 1kW, but the reasoning is for another post.

Figure 5 - Adapters for TNC Connector Test
Figure 5 – Adapters for TNC Connector Test

Figure 5 shows the adapters for the TNC connector test. It is almost identical to the BNC configuration.

Contender #6 – The PL-259 with 1 inch SO-239 Barrel
I found several UHF barrels around the lab for testing and this is the shortest one.

Figure 6 - Adapters for 1" PL259 SO239 Connector Test
Figure 6 – Adapters for 1 in. PL259/SO239 Connector Test

Figure 6 shows the tortured path to get to the UHF barrel. I had to convert from SMA to TNC and then use a TNC to PL-259 adapter.

The UHF connector system presents a varying impedance to the signal that is dependent on frequency; This is the primary problem with this connector. The disturbance to the signal should be proportional to the length of the impedance deviation. To confirm this, differing lengths of the SO-239 barrel were used… and here is one…

Contender #7 – The PL-259 with 2 inch SO-239 Barrel

Figure 7 - Adapters for 2" PL259/SO239 Connector Test
Figure 7 – Adapters for 2 in. PL259/SO239 Connector Test

This is the same configuration as before substituting a 2″ barrel for the shorter version.

Contender #8 – The PL-259 with a T Connector
Just for fun, I tried the T connector shown on the previous post. The SO-239/SO-239 part is between the two barrels above. Of course the transmission line has the PL-259 stub sticking out. I did not take a picture of this configuration, but it is exactly like the previous two.

Intuition suggests the T portion should really mess with the transmissive properties – perhaps the most. We will see.

S11 Test Terminator
All the above configurations include the 50 ohm terminator on one side for all the S11 tests as shown in the TNC example in Figure 8.

Figure 8- Adapters for 50 Ohm Load Test

Perform the Test
I calibrated the VNA and performed the S11 and S21 test to all the above configurations.

In the next post I will show the results starting with tests of the post World War 2 connector designs.

In the last post in this series, I compare the above with the UHF connectors.


  1. Paul Neill – Helped invent BNC, TNC and N Connectors –

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