I have tried mount ham sticks on SUV and a convertible use trunk lid mount, especially on 40m on the convertible I notice the SWR changes from opening the door and close the door. Now I am quite interested to find out how much radials a car would present to the antenna. Are there any ways to measure or give a somewhat quantified indication of how much radials an HF vertical is lacking? Something that indicates the "unblancy"? Maybe on a network analyzer?
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An "analyzer" would indicate the impedance at the feedpoint. You could model the antenna and some radials in 4NEC2 or another program to learn what radials produce the equivalent feedpoint impedance.