A question was asked in an online forum specific to nanoVNA as to how the use the nanoVNA to
check the attenuation loss in some old & weathered RG-6 (75 ohm) cables for the TV signal frequencies. Excuse the term
attenuation loss, lets assume the poster is asking for matched line loss (MLL).
The assembled experts are offering solutions to transform the ports to 75Ω and make a measurement, deducting the loss of the transformation (minimum loss pads were suggested).
There is a very simple solution that should be quite practical for the scenario described. Let’s work through two examples using 35.5m of unbranded quad shield RG6 with CCS centre conductor (of unknown quality) for the DUT.
The cable is connected to the nanoVNA Port 1 (Ch0 in nanoVNAspeak), and the far end is open circuit. A swept impedance measurement is made around a frequency where the cable is an integral number of quarter waves electrically and the R value at resonance noted.
Above is the impedance sweep, and R at resonance is 468.76Ω.
Lets now calculate MLL using Calculate transmission line Matched Line Loss from Rin of o/c or s/c resonant section . We need to know the length and Zo to fairly good accuracy. We will assume that at 28MHz, Zo is very close to 75Ω.
Above, the calculation returns 0.042dB/m, or 4.2dB/100m. That is a little higher than copper cable datasheet loss, possibly a result of insufficient copper cladding for the frequency.
Above is the impedance sweep, and R at resonance is 23.07Ω.
Lets now calculate MLL using Calculate transmission line Matched Line Loss from Rin of o/c or s/c resonant section . We need to know the length and Zo to fairly good accuracy. We will assume that at 145.5MHz, Zo is very close to 75Ω.
Above, the calculation returns 0.078dB/m, or 7.8dB/100m. That is very close to datasheet loss.
Sometimes, when you have a hammer in your hand, every problem you see looks like a nail. The method described here is a different application of the VNA (and could be done with a one port analyser or other instrument that can measure Z), but a bit of lateral thinking is needed to look beyond the obvious.
The method does not allow arbitrary choice of frequency, the cable must be resonant at the measurement frequency. That is not a great restriction when used for the purpose of checking that the cable is close to specification.
The method is simple, and does not involve uncertainty of tranformation elements etc.