An interesting case study – in service evaluation of coax loss

A poster sought advice of the forum experts about in service evaluation of the loss of some coax feed lines…

Has anyone tested old coax cable to see if the loss increased over time? I just tested two different coax cables at 146 Mhz with the use of a Bird Model 43 Wattmeter. Power measurements were taken at the input of each cable followed by the output. The load in both cases was a 146 Mhz Ground Plane.

The test results seem to show losses similar to new coax although Berk-Tek foam coax may have had a lower loss when new.

1. Berk-Tek 6211 RG-8X Ultra Flex Foam Coax – 68 feet

Measured 25 watts in and 11.7 watts out which represents a 3.3 db loss. …

Assuming that the stated measured power is in fact the indicated forward power on the Bird 43 directional wattmeter and given that the actual Zo of the line should be very close to the calibration impedance of the Bird (50+j0Ω), then the Matched Line Loss (MLL) is very close to 10*log(PfIn/PfOut)=10*log(25/11.7)=3.3db which is significantly above the expected 2.6dB for ‘ordinary’ RG-8/X and warrants re-measurement as it suggests that the cable might have degraded a little. In fact, the OP later reports 10.7W out for 25W in which is MLL of 3.7dB against spec of 2.6dB… a more convincing case for replacement!

The Transmission Loss (10*log(POut/PIn)=10*log((PfIn-PrIn)/(PfOut-PrOut)) cannot be calculated from the above data alone as the Pr terms are not given above.

A forum expert offered If you wish to get accurate loss calculations, you need to terminate the transmission line under test with a 50-Ohm J0 load. An interesting form of expression, but it is true?

Well no, it is not necessary that the line be terminated in 50+j0Ω to find MLL with a suitable directional wattmeter.

The calculation given above for MLL has very low error if actual Zo is close to the calibration Zo of the instrument (as it is in use of a nominal 50Ω instrument on a nominal 50Ω cable) at low VSWR, insignificant error in terms of other source of measurement error for most practical cables above about 10MHz.

It is the MLL, not the Transmission Loss, that should be compared to cable specification to assess its performance.