Stryped wrote:
'"How would you go about tunibg a ground plane antenna?"

Ed Laport explains how to tune a ground plane with (4) equally-spaced
too-short radials. He connected one radial with its partner sharing the
same alignment to make a resonant 1/2-wavelength. He determined
resonance as maximum current in the loading coils. He writes on page 140
of "Radio Antenna Engineering":
"Tuning (the radial`s loading coil) is done by maximum current in the
two (radial) wires using a center-connected ammeter or wavemeter near
one of the (lloading) coils away from the oscillator to avoid direct
pickup from the oscillator."

Lacking loading coils, one can determine maximum current at resonance in
the radial wire itself.

Best regards, Richard Harrison, KB5WZI