Richard Harrison wrote:
Paul, VK3DIP wrote:
"Is there a better way (which doesn`t involve large sums of money) to
measure antenna impedance at say 146 MHz?"

Use a line of any number of 1/2-wavelengths to connect the antenna to a
VHF admittance or impedance bridge complete with signal source and
bridge detector (VHF receiver). Measure away and record your results.

I've been out of town and not following this thread. Here's what I do
for HF - knowing the length, VF, and attenuation factor of ladder-line.
Trim the laddder-line until the impedance looking into the ladder-line
is purely resistive. Draw the corresponding SWR circle on a Smith Chart.
Using the line-attenuation factor, draw an SWR circle outside of that
one. The antenna feedpoint impedance lies on that outside SWR circle.
Calculate the exact electrical length of the length of ladder-line
being used and use the Smith Chart to track from the purely resistive
feedpoint impedance back to the antenna feedpoint impedance on the
largest SWR circle.

Of course, the accuracy of the final indirect measurement depends upon
the accuracy of all the parameters used in the calculation. My accuracy
has always been good enough for what I needed.

I've never done it with coax but I assume the same principles apply.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil, W5DXP wrote:
"Trim the ladder-line until the impedance looking into the ladder-line
is purely resistive."

Sure. The line is purely resistive at resonant lengths where the power
factor is one. No reactance. A 1/2-wave is a resonant length.

Charlie Wright, an A.D. Ring and Accociates engineer used to drive our
German engineers crazy, telling them that slopes on the autobahn used
coble stones because they didn`t know how to pour concrete on an
incline.

Charlie also got to a group using an RCA WM-30A phase monitor to tune
parasiitic elements in a curtain array. Most medium-wave directional
stations at the time used a WM-30A as a phase monitor, just as shortwave
stations used them for tune-up.

Charlie had used the monitor for years and knew it had an underated
resistor which sometimes failed. The group had upended the chassis and
Charlie offered to help troubleshoot. The Germans acquiesced.

Charlie asked for voltage measurements from unrelated parts of the
circuit, took out his slide rule and feigned a few calculations. Then,
Charlie pointed to the defective resistor and said: "Change that one."

The crowd shook its collective heads but complied. The monitor
miraculously sprang to life again. Charlie chuckled to himself as he
left the incredulous crowd.

Best regards, Richard Harrison, KB5WZI