Sorry John, I didn't realise you are a novice.

To answer your question about how to calculate the 1/4-wave resonant
frequency of a vertical antenna, use the following formula -

Fres = 75 / Height * Velocity Factor

As given in the simple example in my first posting, Height = 1.5 metres and
Velocity Factor = 0.0701

And so the resonant frequency calculates to Fres = 3.5 MHz.

If you are unfamiliar with "velocity factor" then I suggest you do a Google
on it.
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Reg