Hi,
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 (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

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 agree with most of G4FGQ`s response. You can expect the antenna`s
environment to affect its performance and impedance. I suggest the
transmission line which is a minimum integral number of 1/2-wavelengths
as required to connect your bridge to the antenna as an alternative to
Reg`s ladder. A 1/2-wave line repeats the impedance connected to its
end.

Best regards, Richard Harrison, KB5WZI

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

The input impedance can be measured reasonably well at ground level.

Align the antenna so that the reflector is 'down' and the last director
is 'up'. Ground effects are minimized due to the F/B of the antenna. The
antenna is radiating straight 'up'.

Next take a 1 wavelength, allowing for velocity factor, coax line and
connect it to the antenna feedpoint.

Finally, beg, borrow, requisition, pilfer, rustle, etc., an antenna
analyzer similar to the MFJ 259B. Connect it to the other end of the 1
wavelength coax.

Select the function to read impedance. Dial in your frequency and read
the impedance.

A one minute job once the antenna, coax and meter are at hand.

+ + +

Paul (Home) News wrote:

Hi,
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

Hi Paul

Gordon VK2ZAB (and others) published some time ago a complex Z bridge
thing for VHF/UHF. It uses transmission lines for tuned elements and is
band specific.

Try http://www.vhfdx.oz-hams.org/measurements.html

And do other google searches using Gordons callsign

Cheers Bob VK2YQA

Paul (Home) News wrote:
Hi,
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

Thanks Bob (Bob)

I'm going to make one of those impedance "meters". I sure appreciate
having guys like you do all the research work for me. Thanks again.

Jerry




"Bob Bob" wrote in message
...
Hi Paul

Gordon VK2ZAB (and others) published some time ago a complex Z bridge
thing for VHF/UHF. It uses transmission lines for tuned elements and is
band specific.

Try http://www.vhfdx.oz-hams.org/measurements.html

And do other google searches using Gordons callsign

Cheers Bob VK2YQA

Paul (Home) News wrote:
Hi,
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

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

They can be wonderful engineers, however.....
My 999 story: A major automobile manufacturer tasked their German
branch to design a new transmission for a "sporty" car. Prototype
arrived at the proving grounds and looked anemic. Transmission was
placed into prototype car. Everyone went to see the first use. Driver
wound up the engine to red line, and loud 9 9 9 was heard as clutch was
engaged and shrapnel was produced. US engineers turned to German
engineers and said: "We told you how it would be used - now believe
us."
An antenna system was used to send data back for analysis. 73 Mac
N8TT

--
J. Mc Laughlin - Michigan USA
Home:

"Richard Harrison" wrote in message
...
Cecil, W5DXP wrote:
snip

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