Measuring coaxial cable loss by reflection with a VNA

At Measuring coaxial cable loss with a voltmeter I discussed measuring terminated coax cable loss with an RF voltmeter, and it had some real practical limitations.

This article explores using a nanovna to measure line loss in a similar scenario. We will use 6m of Belden 8359 (RG58A/U) @ 3.6MHz.

The same technique could be used with a quality antenna analyser.

Expectation

A short digression, what is the specification Matched Line Loss (MLL) at 3.6MHz? Using TLLC we get 0.171dB, that is our expectation.

Return Loss of SC section

A common method proposed is to measure Return Loss (RL) of a section with load end RL=0dB and halve it. Many experts advise that the section should be terminated in a short circuit (S) because short circuits are more reliable than open circuits. So let’s get cracking.

Above is measured |s11| using a nanovna with recent OSL calibration from 1-30MHz. |s11| @ 3.6MHz is by eye -0.651dB, RL=-|S11|, so RL/2=0.651/2=0.325dB.

Hmmm, that is nearly double the expected 0.171dB, time to chuck it?

No not yet, let’s do an open circuit sweep.

Return Loss of OC section

Above is measured |s11| using a nanovna with recent OSL calibration from 1-30MHz. |s11| @ 3.6MHz is by eye -0.138dB, RL=-|S11|, so RL/2=0.138/2=0.069dB.

Hmmm, that is less than half the expected 0.171dB, a quarter of the SC measurement, what is going on?

The problem

The problem is that the nanovna was calibrated using a 50+j0Ω load, and its reference impedance is now 50+j0Ω meaning that s11 and therefore RL is wrt 50+j0Ω, and since Zo is more like 51.42-j1.33Ω there is error in the two results that should theoretically agree with each other.

An approximate solution

A good approximation when the departure of actual Zo from the reference impedance is small is that \(MLL\approx\frac{RL_{sc}+RL_{oc}}4dB\).

So \(MLL\approx\frac{0.651+0.138}4=0.197dB\), not a lot worse than spec at 0.171dB, and given measurement uncertainties, we could not say if fails spec.

This is discussed in more detail at On Witt’s calculation of Matched Line Loss from Return Loss .