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I was at Anaheim HRO and few guys there were talking about an MFJ all band vertical antenna that does not need radials. I forget what model number it is but it is one of those "wife pleasing antennae" looks not much more from a flag pole but one of the points a guy mentioned caught my attention.

You have to run at least some coax on the ground, it is radialess and it uses the shield of coax as a counterpoise.

I don't think that is right. If that is the real case for that antenna then I don't think it is good design. I did a test on an end fed long wire type antenna I have up today. Not much measurable change between run the coax on the ground and hang on the wall.

And now think about, if it does have significant changes, is that some sort of RF in the shack problem? As there will be current running on the shield of coax?

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    $\begingroup$ It would be a big help if you could research the model number so we could review the manufacturer's description of the antenna. $\endgroup$
    – Brian K1LI
    May 27, 2019 at 20:35
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    $\begingroup$ MFJ isn't exactly known for good design. $\endgroup$
    – Phil Frost - W8II
    May 28, 2019 at 1:22

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Your reasoning is correct. If the antenna is affected by the positioning of the feed line, more than it would be by any other similar metallic object, then the feed line is radiating; one could say, "is part of the antenna".

This does not necessarily mean you have an "RF in the shack problem". For example, if you install a balun (more precisely, an unun) in the feed line and/or ground the coax (connect the shield to earth), at any point between the transmitter and the base of the antenna, then you will reduce (not perfectly eliminate) the common-mode signal traveling back, quite possibly low enough that it's no stronger than just the antenna's radiation through the air.

So you can certainly use such an antenna safely and effectively, if you choose to. But when thinking about, simulating, or testing how it will perform, you have to consider the coax as part of the antenna design.

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