A SWR meter will vary in swr with coax length if unbalanced as your coax length is around 1/2 wave length multiple.

Does a VNA ( nanoVna ) in this case experience the same effect, (obviously assuming calibration is done for varying lengths) will a certain length coax lead to a incorrectly low swr ??


The NanoVNA, and any VNA, when calibrated with open, short and load standards, can correctly measure through an arbitrary two port network, like a lossy, imperfect cable.

Any combination of reflections and losses, like a series of cables, adapters, etc, can be reduced to a single reflection and transmission device, and fully calibrated. Calibration is done at each frequency independently, and the exact length of the cable doesn't matter.

Here is a classic application note from Hewlett Packard about network analyser calibration

But your statement about a regular SWR meter isn't correct. A working SWR meter should read the same for modest changes in cable length, because the standing wave ratio or the magnitude of the reflection coefficient does not change with cable length, only the phase does. The forward and reflected power are the same at any position in the cable. One caveat - if the cable is long enough that the loss starts to become significant, then the SWR will appear lower than it is at the antenna.

A few ways you might see the SWR change with cable length, apart from a defective SWR meter:

  • If there's a secondary reflection, perhaps from a bad adapter in the middle of the cable, then there will be continuous changes to the SWR over frequency, but these are real from the point of view of the SWR meter, so correctly measured.

  • If the antenna is using the coax as part of the antenna, like an "end fed" or dipole without a balun, then changing the cable length will affect the actual antenna behavior, which will change the SWR. This will affect a VNA measurement too. In fact, one way of judging the amount of current on the cable, is to run your hand down the cable and watch the SWR on the VNA.

  • Very long cables can be a problem for a VNA as the source and hence receiver has changed by the time the reflection arrives, this can sometimes be managed by selecting a slower sweep or increasing the dwell time per point. This usually only happens over 30 metres, or more.

  • Long lossy cables also reduce the dynamic range of the calibrated instrument, making the measurements more noisy. This also makea it much more sensitive to imperfect, time-varying RF connectors at the instrument end of the cable.

  • $\begingroup$ Thanks for your help. The situation where I read swr can change with length is when you have a mismatch, I am dealing with an end fed/ (meant to be) ground independent dual band whip, that im trying to match. I do seem to experience swr meter difference with different cable length or frequency. And just trying to see if a nanovna (which i also have) can be 'tricked' similarly, oh and i do get small shift if i put my hand onn the vna or cable. $\endgroup$ Mar 9 at 21:57
  • $\begingroup$ ad5gg.com/2017/06/11/coaxial-cable-length-does-not-change-swr "Cutting the coaxial able to such a length where the end which plugs into your SWR meter or radio is at a point where the incident and reflected waves arrive out of phase with each other will fool your meter or radio (and you) into thinking you have a well tuned antenna system. " $\endgroup$ Mar 9 at 21:59
  • $\begingroup$ a point where the incident and reflected waves arrive out of phase with each other is a high (or low?) impedance point. SWR is unchanged. Maybe that suits your radio better. But an SWR meter, to the extent that it is working correctly, should read the same at all points in the cable. $\endgroup$
    – tomnexus
    Mar 10 at 22:20
  • $\begingroup$ Im talking about when they meet at half wave length apart and subtract from each other. $\endgroup$ Mar 15 at 5:18

A SWR meter will vary in swr with coax length if unbalanced as your coax length is around 1/2 wave length multiple.

I have never seen an SWR measuring device that was designed to be used with a balanced load. This could certainly be done, but such devices are usually intended to measure SWR on a coaxial, i.e., unbalanced, line.

For the purposes of SWR measurement, the feedline is part of the "load." The load attached to the SWR meter includes the feedline and everything attached to it; e.g., antennas, splitters, stubs, etc. A properly designed and operated SWR meter will correctly measure SWR regardless of the load.

In general, the SWR at the shack end of the feedline will not be the same as the SWR at the antenna. Feedline loss attenuates the driving signal as it travels forward along the line, so the signal available to be reflected at any point is reduced. Similarly, feedline loss attenuates the reflected signal as it travels back along the line, reducing the signal available to be detected by the SWR meter. In general, feedline loss reduces the SWR observed at the shack end of the feedline.

In addition to loss, the transmission line changes the phase of the incident signal traveling forward along the line and the phase of the reflected signal as it travels back. The incident and reflected signals are superimposed on (i.e., add to) each other at every point along the line. At some points, the incident and reflected signals will tend to reinforce each other, while at others they will tend to cancel each other. These reinforcing and canceling effects produce the standing waves we measure.

The effects of loss and phase can be seen by plotting the impedance along a transmission line on a Smith chart:

![enter image description here

The SWR at the 1K$\Omega$ resistor is 20. Traveling 60$^o$ toward the generator on RG58/U coax, the impedance changes dramatically to 5.9-j31 $\Omega$ and the SWR is about 11.7; 45$^o$ further toward the generator, the impedance changes to 9.4+j13 $\Omega$ and the SWR is about 6.2; completing the half-wavelength journey toward the generator, the impedance changes to 257-j0.6 $\Omega$ and the SWR is about 5.1. (Note: the small differences in distances along the transmission lines are a consequence of the parametrized model used.)

This tool also shows that, of the 1.00-W incident on T3, only 0.247-W arrives at the 1k$\Omega$ resistor, representing about 6.1-dB of loss. By comparing this loss to what would be lost on a matched line (resistor = 50-$\Omega$), we see that more that more than 77% of the loss is a result of the large mismatch.

  • $\begingroup$ Thanks for you answer, It seem to confirm that measured SWR can vary at different line lengths, my main question however is does this same effect play out with a VNA instead of SWR meter. $\endgroup$ Mar 10 at 2:02
  • $\begingroup$ @HaydenThring Yes. A VNA and an SWR meter use the same "information", they just "process" it differently. $\endgroup$
    – Brian K1LI
    Mar 10 at 12:07
  • $\begingroup$ I was about to say RG-58 isn't that lossy, as I've many times had it go 5 times around the Smith chart without spiraling in. But at 1 MHz, a half wave of coax is really lossy. Loss per wavelength of coax goes up as frequency goes down! (consider DC, the limit). Interesting. Still, this shows the expected steady decrease in SWR as the coax gets longer, only due to cable loss. There is no magic length where the SWR is lower than other lengths. $\endgroup$
    – tomnexus
    Mar 12 at 6:22
  • $\begingroup$ Im not talking about coax loss at all, but the forward and reflected wave subtracting each other to reduce the perceived swr. $\endgroup$ Mar 15 at 5:17

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