"Paul (Home) News" wrote
I have done quite a bit of numerical analysis of VHF Yagi's with usually
very good agreement between theory and performance at least for gain and
pattern. One of the things that seems more difficult to correctly predict
however is the input impedance, and thus to design some sort of a match
without some sort of subsequent trial and error. In an effort to explore
the
differences further I have been looking at ways of measuring the impedance
directly rather than indirectly by vswr with the matching in place. I have
made a number of different return loss bridges, and even tried the
technique
described in an old Ham Radio Article where you take two VSWR readings
with
and without an added series resistance. All of course making allowances
for
coax length etc. The problem I have is all of them give, in some cases
wildly, different answers even when used on the same antenna. So my
questions a

What sort of accuracy can I expect from these sorts of methods?

Is there a better way (which doesn't involve large sums of money) to
measure
antenna impedance at say 146Mhz?

Thanks

Paul VK3DIP

===============================
Paul,

Attempts to accurately determine antenna input impedance, using an
inherently ambiguous, innacurate SWR meter at the transmitter end of a line
of uncertain length and velocity, are doomed to failure.

*Never* expect to obtain numbers worthy of serious engineering application.
There are far too many uncertainties of unknown magnitudes.

The only way of obtaining an accurate measurement is to climb a ladder
taking with you an R plus or minus jX hand-held impedance bridge. Can you
borrow one ?

But why do you wish to know antenna input impedance when you are aleady
quite happy with using an inaccurate SWR meter to fiddle a 1-to-1 SWR at the
transmitter end.

The ultimate objective, of course, is just to obtain a 50-ohm load for the
transmitter regardless of what the SWR and antenna impedance might be.
----
Reg, G4FGQ