@lex_ph2lb nope it’s not the ground plane. We measure and trim our antenna’s always with a ground plane. The reason that a quarter wavelength antenna is shorter than the theoretical wavelength is the velocity factor, or antenna shortening factor.
The theoretical wavelength is calculated in free space, with the electromagnetic waves travelling at the speed of light. In any other medium, like copper wire, they move slower, hence the quarter wavelength in the wire is shorter than in free space, requiring a somewhat shorter antenna.
The order of magnitude of the delta in antenna length is around 5% so indeed noticeable!
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