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The VSWR reading is not the same as you typically get with a proper inline VSWR meter due to the measurement location in the transmit line - it is quite possible to get very low readings at the point of measurement due to phase relationships of the reflected signal - and that is frequency dependent.
Changing your frequency changes that relationship and the results can be dramatically different. So I suspect you have a open or short circuit somewhere along the line - that will do it and the effect will depend on the frequency and length of cable to the point the problem lies.
The antenna fault you get if the VSWR measurement goes very high so you definitely have something that reacts strongly to the tuned frequency (and therefor wavelength) in combination with a tuned stub - which is the length of your cable to the fault.
If I choose 5W or 10W for transmission on the ground and engine not running (12.5V)
all is fine.
In the air with engine running and now with 14 V, I'm good transmitting at 5W but at 10W
I get the RED ANTENNA message..
After trying a few different length of coax trying to get a perfect fit
For my range of frequency, I decided to connect the V16 to my top of fuselage
Antenna which is used to my spare Icom 200(old radio).
Bang!! ……. No SWR at either 5W or 10W, I even noticed that it is putting out +-11.5W when flying with 14.2Volts.
So.. My under the belly Antenna location is reacting to something!!!!!! That creates bad SWR.
