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:
With an ordinary reflectometer, we can detect attenuation from a few tenths to a few decibels, čo je práve rozsah, ktorý nás zaujíma. Pri meraní je vhodné na výstup transceiveru vložiť útlmový článok, aby sa minimalizovali odrazy v transceiveri, ktoré by skreslili meranie. Na rýchlu orientáciu pomôže tabuľka a graf:
| 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 – on the commonly measured PSV with a ham shack device. Napríklad ak nameriame v shacku PSV=2 a kábel bude mať straty 3dB, skutočné PSV na svorkách antény bude 5. Ak by mal koaxiál útlm až 6dB, nielenže ku svorkám antény sa dostane len malá časť výkonu, ale aj v shacku nameriame PSV=1,5 čo by mohlo viesť k domnienke, že anténny systém je v poriadku.
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