V prípade, že máme kvalitný PSV meter, we can quite quickly find out the attenuation of the coaxial cable exactly at the frequency at which we want to use it.
Princíp merania it is based on the fact that the reflected wave of a coaxial cable terminated by a load with PSV infinity (disconnected or shorted cable) is weakened by the losses of the coaxial cable. That is the reason why we do not measure PSV infinity on a coaxial cable with losses, but a lower value.
PSV is expressed mathematically by the formula where:
Po – power of the reflected wave in watts
Pp – "forward" wave power in watts
PSV – standing wave ratio value
However, the power of the reflected wave is reduced by the power lost by the attenuation of the coaxial cable. If the coaxial cable is terminated by a load with PSV infinity, it is true that:
Po – power of the reflected wave in watts
Pp – "forward" wave power in watts
Pstr – power lost in the coaxial cable in watts
From the previous calculations, it is possible to determine that the losses are calculated according to the formula:
Bežným reflektometrom môžeme zistiť útlm od niekoľkých desatín do pár decibelov, which is the range we are interested in. When measuring, it is advisable to insert an attenuation element at the output of the transceiver in order to minimize reflections in the transceiver that would distort the measurement. The table and graph will help you quickly navigate:
| 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 |
Zaujímavý je aj druhý pohľad – na bežne merané PSV prístrojom v ham shacku. For example, if we measure PSV=2 in the shack and the cable has losses of 3dB, the actual PSV at the antenna terminals will be 5. If the coaxial had an attenuation of up to 6dB, not only will only a small part of the power reach the antenna terminals, but we will also measure PSV=1.5 in the shack, which could lead to the assumption that the antenna system is fine.
The described method has its shortcomings, but it allows a quick check of the attenuation of the coaxial cable with common measuring devices.
Literature:
[1] Checking the attenuation of the coaxial cable with a reflectometer - OK2BIU; Registration number 2/1985
[2] Ocena tlummienosci fidera poprzez swr measurement – SQ5GVY