Dave, I was replying to the original questioner. But by immediately
following your response with mine and including a comment of yours caused a
little confusion. Sorry!

I agree your method will work. The problem, a practical one, is obtaining a
COAXIAL line length exactly an integral number of 1/2-wavelengths long.
There's no way of proving it exept by climbing a ladder and disconnecting
the line from the antenna.

And it is an exact 1/2-wavelength long at ONE frequency only. But it is
required to make measurements over a whole band of frequencies. To shift to
other frequencies
involves calculations taking Zo into account. But Zo is not accurately
known. So then you have to measure line Zo. And so on.

And you have to know exactly what you are doing because the 259B does not
provide the sign of jX in R+jX.

But as I said before, all you want to know is whether or not the transmitter
is loaded with 50 ohms. To hell with SWR and antenna input impedance. ;o)
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Reg, G4FGQ