In reply to G8TMV:
Hi Colin,
it is even a bit more complicated - impedance of the output stage of the transmitter varies with supply voltage (battery powered) - and can be different for each band and another rf power value setting.
I measured it using a directional coupler, differences related to real 50ohm are not negligible - even at a factory tcvr!
I think very simple and reliable is then also always good to find the maximum antenna current.
Karel OK2BWB