RadioBanter - View Single Post - Using Tuner to Determine Line Input Impedance

Owen Duffy · June 4th 09, 10:33 PM posted to rec.radio.amateur.antenna

dykesc wrote in news:0dec42e0-48ab-44f0-8c1a-
:

....
this one requires both source and load impedance as input. I am
looking for one that will allow you to specify the source impedance
(50+j0), fill in the capacitor and inductor values, and then tell you
what the matched load side impedance is.

You want to specify the ATU input impedance which is independent of the
source, not the source impedance (which you don't know or care about).
Hams often confuse the source impedance of a transmitter and its required
load impedance.

Do you intend treating the ATU as a simple two or three component network
using ideal components (no loss, no stray capacitance)? Will that
approach give accuracy either adequate or comparable with the many other
techniques used?

I have thought about this in the past, mainly the prospect of a relay
switched autotuner that reported a calculated load Z based on the found
matching solution, but I concluded that it was not likely to be of
reasonable accuracy over the tuner range.

If you want to measure Z using an instrument based on a broadband
detector, you need to use sufficient power that the broadband detector is
dominated by the test signal. There is at least one instrument for the
ham market that purports to make such measurements at normal transmitter
power. IIRC, R&S used make a commercial product, but it wouldn't have
come cheap.

There is at least one other low level antenna analyser that represents
that it is less affected by interference than the '259B.

If I were looking for an instrument, I would desire one that could refer
the measurements to a point closer to the antenna (eg the feedpoint)
given the characteristics of the transmission line between the
measurement point and the desired reference plane.

VNAs will peform this calculation, but most low cost VNAs use a broadband
detector and low power, so they are susceptible to interference when used
on antenna systems.

Owen