When calibrating a VNA, does the feedline length matter?

Question

We need to measure an antenna feedpoint (to record an .S1P file) that is far away from the VNA. We can calibrate the VNA to the feedline that will ultimately be attached to the antenna before it goes up.

Practically speaking, if the VNA is calibrated for that feedline (sort, open, load, through), then does the length of the feedline matter for the purposes of getting an accurate measurement? (For example, is there any frequency dependence on the length of the cable such that it should be some λ-fraction of the wavelength?)

Answer 1

In theory, performing a calibration at a port at the far end of any length of an ideal arbitrary line connected to an ideal VNA gives you the full VNA accuracy for measurements at the far port.

In practice, the deviations of both from non-ideality causes problems.

A real VNA will have a finite dynamic range. Line loss will eat into that dynamic range by twice the line loss, giving you noisier readings. The noise floor can be clawed back somewhat by taking many averages, for both cal and measurement.

A real transmission line will be unstable in length (with temperature and flexion and strain) and in impedance (tempco of dielectric and conductor dimensions). Any change of electrical parameters between calibration and measurement will give you a systematic error in the measurements, which cannot be averaged out. In the worst case that the length changes by half a wavelength, having VNA calibration would give you a worse 'corrected' measurement than just normalising for feeder loss and nothing else.

Even if the line stays totally stable, phase/frequency noise in the VNA source will also change the phase shift of the line. This is not usually a problem for most measurements, but as you say this is a 'long' feeder, source phase noise could become relevant.

Make an S11 measurement on the open feeder and observe how the phase of the reflection changes, at trace to trace timescales that can be averaged, and hour to hour ones that can't. You might want to bend and straighten the cable to see the effect of flexion, if you're going to move it during measurement. Then decide whether you have the stability to meet your measurement accuracy requirements.

As to whether multiples of some fraction of a wavelength will be better or worse ... For any reasonable 'long' length of cable, for any reasonable measurement bandwidth, the length of the cable will whizz round the Smith Chart many times, so you don't get to choose. If the cable and the VNA are reasonable approximations to 50 ohms to start with, then the effect of cascading them will not be too severe, in terms of resonances and suck-outs. Any that do occur will simply exacerbate the problem of working over a long unstable cable - suck-outs will still degrade SNR by twice their depth, 'filter-like' resonances will further amplify the effects of source phase noise and cable-length instability issues.

Answer 2

One last thing - Network analysers have a problem with long cables (or free space paths). When you get over a few 10s to 100s of metres, depending on the analyser, you need to increase the dwell time on each frequency.

Essentially there needs to be enough time for the signal to get back to the analyser before it moves on. This is not a wavelength-dependent effect, so it can bite you at 5 MHz where the cables might be very long.

The problem manifests as an increased path loss or return loss, that changes with bandwidth, sweep time or dwell time. Increase the dwell time one click beyond the point at which there is no change.

Some analysers - I recall an Anritsu display-less one, and maybe my nanoVNA - don't have the option to wait, they assume that you're measuring something on the bench, and only wait long enough for their own source and switches to stabilise. These ones simply aren't useable beyond some path/cable length.