Cable losses are a reality which affects all of our RF installations.
It reduces the amount of forward power from the power amp to the antenna. It also reduces the amount of reflected power from the antenna reaching the amplifier, making the SWR appear better that it actually is, for an SWR meter mounted at the amplifier end. This effect is what is commonly known as the cable loss masking effect.
Giulio, James and Yesie (9V1FC, 9V1AR, 9V1SQ) were having suspicions regarding some of their cables in their satellite tracking setup. We had already discovered some connectors which had come loose on their heavy LMR400 cables. On the other hand, I had with me some of the old cables from the decommissioned SARTS repeater which had been in service for more than 10 years, and I was curious about their state.
If you scan the literature, you will find that there are 2 main methods of measuring RF cable losses. I added the word “RF”, because for low frequencies e.g. audio, telephony, one of the methods is not applicable.
The 2 port method shown below is the most intuitive method. You inject the signal into the cable and measure the output power at the other end. Of course, in order to minimize the measurement uncertainties, the Z at both ports of the instrument must match the Zo of the cable. Min Than 9V1XY drew the following 2 diagrams for me, depicting the actual equipment we used.
The 2nd common method is the one port method illustrated in the following diagram. This method is not applicable to low frequencies. The cable is terminated at the far end with an RF SHORT; an OPEN could also be used, but the results will not be as accurate due to the fact that an OPEN radiates. The injected signal travels down the cable and is fully reflected by the SHORT. The signal reaching the driven end of the cable has thus be subjected to 2 losses – a loss in the forward direction, as well as a loss in the return direction. The cable loss is then derived from the S11 term by the instrument.
We measured a total of 7 cables: Cables 1,2A,3A and 3B were LMR400s belonging to Giulio. Cables A,B and C were the old cables mentioned before, some of doubtful pedigree. Each cable was measured first by the 1-port (reflection) method, followed by the 2-port (transmission) method.
Here are the results:
Follow up work by Yesie 9V1SQ:
I suggested to Yesie 9V1SQ that since she had an AIM UHF, she should try to verify my test results, which she did.
The results of our measurements are summarised in the following spreadsheet. All loss values were normalized to dB/m by dividing by the cable length.
You will note that the results of the 3 different methods:
- 1-port S11 using the Anritsu 331E
- 2-port S21 using the Anritsu 331E
- 1-port S11 using the AIM UHF
agreed to a large extent.
The graphs for the 2-port measurements had ripples which were not seen in the 1-port graphs, probably due to:
- Poor quality adapters used at port 2
- Not exact load match between the (cable-adapter) combination and port 2 of the S331E
Thanks for reading up to here!