Hugh,

How have other people done it ?  Any ideas ?

I built a 1A antenna current meter with a current transformer.
For calibration, I put a 10 ohm resistor (actually 10 x 100 ohms
1W in parallel) in series with the meter and applied RF from
a 137 kHz TX until the oscilloscope reading was 28.3Vp-p.
The potentiometer in the meter was then adjusted for full scale
deflection. Just an example.

A small drawing is attached.

73
Johan SM6LKM