On testing two wire line loss with an analyser / VNA – part 1

This article series shows how to measure matched line loss (MLL) of a section of two wire line using an analyser or VNA. The examples use the nanoVNA, a low end inexpensive VNA, but the technique is equally applicable to a good vector based antenna analyser of sufficient accuracy.

Above is a short piece of the line to be measured. It is nominal 300Ω windowed TV ribbon. It has copper conductors, 7/0.25, spaced 7.5mm, though as can be seen the spacing is not perfectly uniform. The dielectric is assumed to be polyethylene… but later measurements suggest is has slightly higher loss than polyethylene. The test section length is 4.07m.

Fixture

The Noelec 1:9 balun (or perhaps Chinese knock off) is available quite cheaply on eBay and provides a good hardware base for a 1:1 version.

Above is a modified device with the original transformer replaced with a Macom ETC1-1T-2TR 1:1 transformer. The replacement is not exactly the same pads, but it is sufficiently compatible to install easily.

See A 1:1 RF transformer for measurements – based on noelec 1:9 balun assembly for more detailed info.

The system is OSL calibrated at the fixture load pins.

Performance of the fixture is crucial to valid measurement.

Method 1

This method gives a good approximation of MLL if Ro and Rin of a resonant section near the desired frequency (146MHz) are known. See (Duffy 2016) for more information.

Above is a chart of Zin to the test section near the frequency of interest with OC termination. At exactly half way between the resonance and antiresonance, \(R_0 \approx |X_{in}|\), in this case we will take Ro to be 298Ω.

Above is a zoomed in scan of Zin near the resonance, and we will take Rin to be 7.35Ω.

We can calculate MLL from these values.

Above, estimated MLL is 0.053dB/m.

Continued at On testing two wire line loss with an analyser / VNA – part 2.

References

  • Duffy, O. Feb 2016. Calculation of Matched Line Loss from measurement of Zin at resonance or antiresonance of a short circuit or open circuit transmission line section. https://owenduffy.net/files/MllFromZin.pdf.
Last update: 18th February, 2022, 5:15 PM