J.B. wrote:
"As soon as the load gets reactive, the phase between the voltage and
current are different and the length of the coax and its velocity factor
now contribute unless it is really short compared with wavelength."

The Bird wattmeter won`t affect tuning and loading of an antenna if it
is made part of a half-wavelength of line inserted for the measurement.
This may be important at VHF and higher frequencies. The inserted line
and its meter need to be specially tailored to make up a half
wavelength, if it is to be inserted in nonflat lines. For several
different frequencies, several line measurement assemblies are required.

Best regards, Richard Harrison, KB5WZI