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Recently purchased my first VNA (Nano VNA) and I'm trying to learn the basics.

I have a 150MHz 1/4 wave whip antenna that I need to install on a large metal object so I was hoping to use a VNA to help me understand the performance of the metal object as a ground plane.

When I attach the antenna directly to the VNA I get results that seem to match the antenna specs. Optimum frequency is around 150MHz, SWR close to 1. enter image description here

When I connect a length of coax cable (<1m) between the VNA and antenna everything shifts down to ~130MHz. enter image description here

Is this telling me that the coax cable is detuning the system down to 130MHz or am I using the VNA incorrectly? Do I need to calibrate the VNA with my coax attached first? What is recommended for testing an antenna/ground plane in its final configuration?

Thanks

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    $\begingroup$ Not an answer because I don't have the expertise beyond my own small amount of research, but you definitely want to test the entire antenna system, ideally in situ, when trying to determine SWR. This includes the feedline. Calibrate as documented for the band you are interested in, but you are measuring the whole thing where it lives (in an ideal world). $\endgroup$
    – user21789
    Apr 21 at 13:56
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    $\begingroup$ PJ, welcome to the site! $\endgroup$
    – webmarc
    Apr 21 at 16:10
  • $\begingroup$ Thanks @webmarc $\endgroup$
    – P_J
    Apr 22 at 2:01
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    $\begingroup$ Welcome @P_J and fun question. This isn't a full answer by any means, but you might find youtu.be/9thbTC8-JtA?t=266 and other videos from that channel to be interesting :-) $\endgroup$
    – natevw - AF7TB
    Apr 29 at 23:06

2 Answers 2

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You want to use the VNA against the final system configuration... or at least as much as practical. Why? The antenna's environment and transmission line will impact its characteristics.

It's pretty trivial to place and retrieve an antenna on a magmount on the roof of the car. So in this case, you simply plug the VNA in to the antenna in place of the radio. If you find you need to adjust placement of the magmount or length of the antenna, just grab it down and make the change. It's more challenging to do the same with a tower mounted yagi, so compromises will be made in testing configurations.

I suspect what you're finding is the need to place an appropriate choke at the antenna feed point. Dropping frequency by adding transmission line may indicate that the outside of the shield is a meaningful component of the system, which it should not be. Adding a choke at the feed point will help address the situation if that is indeed the cause.

When it comes to calibration, you'll want to do so with everything in the system attached to the VNA, replacing the antenna with the open/short/50Ω slugs that came with your VNA.

Happy engineering!!

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  • $\begingroup$ I was confused by your last sentence. It reads a little like you are advising to calibrate with the antenna attached to the NanoVNA. $\endgroup$
    – user21417
    Apr 22 at 0:10
  • $\begingroup$ Thanks webmarc, I'm finding the final build up is having a huge impact on the characteristics so I need to do some more experimenting! $\endgroup$
    – P_J
    Apr 22 at 2:09
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    $\begingroup$ Re calibration, "everything in the system" meaning transmission line, metal plate, RF choke, balun, everything! and remove the antenna and use the open/short/50Ω slugs at the feed point. THEN you can attach the antenna after calibration. $\endgroup$
    – webmarc
    Apr 22 at 19:28
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If you want to understand what your radio "sees", which is probably the case, then you want to measure the entire system including the cabling to the antenna rather than just the antenna itself.

You calibrate the VNA for the configuration where it connects to the device under test.

So, from my point of view, the best test is to install the antenna; then measure at the coax connector that will attach to your radio. If you're connecting that directly to the VNA, then do your SOLT VNA calibration directly on the VNA port. If you're using a coax stub from the VNA, calibrate at the end of the stub.

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    $\begingroup$ Nice answer. Further to this, a technique is to calibrate for a wider bandwidth to see just where resonance happens to be, and then switch to a calibration centred around the frequencies of interest for tuning. $\endgroup$
    – user21789
    Apr 21 at 15:51
  • $\begingroup$ Thanks nhw76, when using only one port on the VNA would it still be a SOLT calibration? What configuration would be used for the "through" measurement? $\endgroup$
    – P_J
    Apr 22 at 2:08
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    $\begingroup$ Apologies - you obviously don't need a through for an S11 calibration! $\endgroup$
    – nhw76
    Apr 22 at 3:00

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