Coaxial cable attenuation check
In case, that we have high quality PSV meter, we can detect the attenuation of the coaxial cable exactly at the frequency relatively quickly, on which we want to use it.
Measurement principle based on that, that the reflected wave of a coaxial cable terminated by a load with PSV infinity (disconnected, resp. skratovaný cable) is weakened by coaxial cable losses. That is why we do not measure PSV infinity on a coaxial cable with losses, but the value is lower.
PSV is mathematically expressed by a formula, where:
After - power of the reflected wave in watts
Pp - "forward" wave power in watts
PSV - value of standing wave ratio
However, the power of the reflected wave is reduced by the power, which is lost by coaxial cable attenuation. If the coaxial cable is terminated with a load with PSV infinity, applies, that:
After - power of the reflected wave in watts
Pp - "forward" wave power in watts
Pstr - power lost in coaxial cable in watts
It is possible to determine from previous calculations, that we calculate losses according to the formula:
With a common reflectometer we can detect attenuation from a few tenths to a few decibels, which is the scope, which interests us. When measuring, it is advisable to insert an attenuation element at the output of the transceiver, to minimize reflections in the transceiver, which would distort the measurement. A table and a graph will help you find your way around:
| PSV | 1,1 | 1,3 | 1,5 | 1,8 | 2,0 | 2,3 | 2,5 | 2,8 | 3,0 | 3,5 | 4,0 | 4,5 | 5,0 |
| Attenuation in dB | -13,2 | -8,9 | -7,0 | -5,4 | -4,8 | -4,1 | -3,7 | -3,3 | -3,0 | -2,6 | -2,2 | -2,0 | -1,8 |
The second view is also interesting - the commonly measured PSV device in a ham shack. For example, if we measure PSV = 2 in the shack and the cable will have a loss of 3dB, the actual PSV at the antenna terminals will be 5. If the coaxial had an attenuation of up to 6dB, not only do only a small part of the power reach the antenna terminals, but also in the shack we measure PSV = 1.5 which could lead to the assumption, that the antenna system is fine.
The described method has its drawbacks, but allows quick control of coaxial cable attenuation with conventional measuring instruments.
Literature:
[1] Coaxial cable attenuation check with reflectometer - OK2BIU; RZ 2/1985
[2] Assessment of the fider's dampness by measuring the SWR - SQ5GVY
