Roy Lewallen wrote:
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.


Most of the new analyzers that do "fixture cal" actually measure the
line, so you're not doing any estimating.

Somewhere in the future, as hams get more familiar with this kind of
thing, I see folks installing a little relay box at the feedpoint of the
antenna that has a short, open, and load, as well as the antenna.

I've done this with an RCS-8V, modifying it to add a noninductive 50 ohm
load and to short one of the relays. I then use my VNA to calibrate the
relay box (or, more accurately, to just confirm that the load and short
is good enough) with a short test cable.

After I went through the hassle of modifying, I realized that I would
have actually been better off to just get 50 ohm loads with PL-259s on
them and PL-259 shorts. oh well, it's done now.

So anyway, the RCS-8V has 5 ports and a common. I hook the the feedline
on port 1, the antenna on port 5, the short on port 2 and the load on
port 3. (Sometime in the future, I'm going to hook a phase reversing
transformer on port 4). By turning on and off the various ports, I can
connect loads and shorts to antennas and/or feedlines or leave them open.

In a real fancy application, I'd put a remote controlled antenna tuner
with a few extra relays in it at the feedpoint. Hmm.. another project to
languish half completed in my garage for years.