Ian Wade G3NRW wrote:

Mike,

With the 4170 I can calibrate the instrument to compensate for the
feeder impedance. After calibration, the indicated impedance at the TX
end of the feeder is actually the antenna feedpoint impedance. This
makes life a *lot* easier.

Easy, yes. But If you're not careful, this can be a great example of
garbage in, garbage out.

I frequently calculate out the feedline transformation when making
antenna measurements. But it's essential that you realize a small error
in estimating the feedline loss(*) or length can sometimes result in a
very large error in calculated impedance. This is particularly true if
there's a large impedance mismatch between the line and antenna.
Transmission line impedance, which can vary a lot from the specified
nominal value (I've seen +/-20% with coax, more with ladder line), also
has an effect on the result. So whenever I need accurate results or
whenever the line Z0 is quite different from the antenna impedance, I
start by carefully measuring the properties of the actual transmission
line I'll be using.

If you're not convinced, spend a few minutes playing with something like
N6BV's TLW calculator that comes with the ARRL Antenna Book.

(*) Some simplified techniques ignore transmission line loss altogether.
This can lead to very inaccurate results in some situations. And loss is
often quite different than the specified value, so it really has to be
measured if it makes a significant difference.

Roy Lewallen, W7EL