Put the NanoVNA on a non conductive surface well away from where the antenna is mounted.
The reason you are getting different readings is because you are changing the ground plain of the antenna system with your body or metal surface or non metal surface. I would guess the antenna itself doesn't have a sufficient ground plain to work off of and so any small changes to the ground side of the system, IE where the coax is finding ground, will alter the measurements.
When you say it's changing, how much is the SWR changing? If by a couple points it's not a big deal. If it's changing from say 1.1 to 1 to 3 to 1 or 4 to 1 by placing the NanoVNA on the hood verses in your hand then you've got a antenna ground issue.
You can try connecting a counterpoise to the ground side of the antenna mount if you can't electrically ground it to the vehicle. That would be a quarter wave of wire or so on the band you are trying to tune.
I would say that the most accurate reading will be the one where the NanoVNA is on the insulated pelican case.
Is the radio you are using going to be chassis grounded to the vehicle or floating inside or outside the vehicle during operation? If floating you want the NanoVna to most accurately represent where the radio will be.
You can cross check this by tuning with the NanoVna, then checking the swr with the radio itself. If they are close, within .5 or so,? you'll be good to go on SWR.
73
Colin, VA6GG
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On 2025-02-02 6:04 p.m., Coyote via groups.io wrote:
When using my NanoVNA to adjust length of a 20m hamstick, the readings kept changing depending on what the NanoVNA was sitting on (truck hood, pelican case, my thigh, or in my hand).
How do I get a reliable reading that I can count on for a properly adjusted antenna?
Thanks, Coyote
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