Shawn, KB1CKT, set about measuring the matched line (MLL) loss of a roll (~152.4m) RG6/U on the roll and published his results in an online discussion.
A word of warning: measuring on the roll can give unusual results if the cable is deformed significantly on the roll. A higher risk for foamed dielectric cables, and this is one of that type.
The technique he followed was to make a series of measurements of Rin at low Z resonances of the length of line with open circuit at the far end, and to calculate the MLL using Calculate transmission line Matched Line Loss from Rin of o/c or s/c resonant section.
Impedance measurements were made using a nanoVNA which was OSL calibrated in several ranges through the series of measurements. Also notable was that there were several coax adapters used to connect the RG6 to VNA.
Though the calculator can use high Z resonances, the high Z resonances are very narrow and it is very difficult to measure Rin at the true resonant frequency. So, only his reported low Z measurements will be used.
So. let’s graph the measured Rin at zero crossing of Xin.
The circled measurements appear out of line with the others. They were made with a different OSL calibration that the ones immediately below and above, so that hints something may have gone wrong in the calibration. Two of the possible explanations are:
- it is vital with the nanoVNA that calibration is RESET before starting a new calibration; and
- it is also vital for both calibration and measurement that connectors are properly tightened.
The circled measurements are suspect, it is not known if they are faulty. It is a good idea to plot measurements as they are made as it allows investigation of apparent anomalies.
Above is the calculated MLL from the measured Rin. The calculation can be done in the spreadsheet, \(MLL=\frac{-10log|\frac{R_{in}-R_0}{R_{in}-R_0}|}{length}\). Also show are two datapoints (+) from what appears to be the datasheet for this line. The measured points are not a lot different to the two datapoints from the datasheet.
The shape of the curve is not the classic shape that one might expect, but this is a CCS line and copper like performance will not be observed at lower frequencies where the copper thickness is insufficient.
Above is a comparison of the measured MLL with that from the datasheet for Belden 1694A which uses a solid copper centre conductor. There is a growing departure from copper like performance below about 30MHz.
References
- Measuring matched line loss
- High and low Z responses of a resonant shorted / open transmission line section
Conclusions
- The method used works, there is reasonable reconciliation.
- Great care needs to be taken to measure Rin:
- choose low Z resonances;
- make sure the instrument frequency step size is sufficiently small;
- check the results for sanity.