Measuring Coax Loss


How to measure coax loss without expensive equipment

There are two ways of doing it. Firstly, ensure all connectors are properly put on and rely on the attached excel spreadsheet (use only the frequency and coax length portions) to calculate the loss. What of course this does not do is test an old piece of cable you may suspect of being faulty (especially if it has had water climb into it). Unless you have a radio test set or something similar one could be a little lost as there are very few instruments that work down at a half watt, when they do work at this power they're usually expensive.

The known method is to inject a known power in the one end and read the power at the other e.g. if 10 watts was plugged in and 5 watts read at the other the loss is 3dB. If this corresponds to the calculation then all is well. If it differs drastically then the cable or connectors are faulty.

The second method is to set up a receiver with a very accurate 'S' meter and a transmitter preferably about 500 metres away. Put the transmitter into constant TX and "worsen" the antenna on the transmitter until about -70dB is measured on the receiver (the antenna on the receiver must be a good one and correctly mounted). Then add in (i.e. put in series with what you already have) the piece of suspicious cable and take another measurement. If for example it is -73 then the cable has a 3dB loss. Simple? Please note that you may have to first "calibrate" your receiver using a signal generator and find a range that is accurate. Once done do not try to do this type of "loss test" out of this range. I do hope this all explains and helps.

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