On Thu, Nov 19, 2020 at 10:42 AM, Arie Kleingeld PA3A wrote:
Yes, the grounding effect is definitely there.
Remember that anything connected to the antenna system becomes part of it.
For example if you disconnect the radio in order to connect the NanoVNA, you change the antenna system. If you change a cable length, you change the antenna system. If you connect a mains power supply, you change the antenna system, and so on.
When you connect the NanoVNA you need to keep everything as close to the exact operating configuration as possible. If you disconnect the radio, then try to make as short a connection as possible between the screen of the coaxial cable where it connects to the NanoVNA and the chassis of the radio. If you connect your NanoNVA directly to an antenna, then make sure the screen of the coax still makes contact with the screen of the NanoVNA connector.
If the NanoVNA is in turn connected to a computer, that also forms part of the antenna system, so it is always best to use a 'floating' laptop (or Bluetooth adaptor) in order to minimise such problems. You can use a common mode choke, but..... yes you guessed it, that also becomes part of the antenna system :-)
Antennas such as the end fed half wave and variants are particularly problematic, as the feed point is at a very high impedance node, and the feed line which forms part of the cannot easily be choked off. So anything that is connected to the feed line, or even comes within close proximity, can interact quite dramatically and change its electrical characteristics.
Budget test equipment such as the NanoVNA is now allowing folks to see what is really going on with their antenna systems, and this can often lead to surprising revelations :-)
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Regards,
Martin - G8JNJ
