With sufficient accuracy, knowing its maximum value, it is possible to
estimate the value of an air-spaced variable capacitor setting just by
looking at it.

Similarly, the value of a coil can be calculated by counting the
number of turns and guessing its length and diameter. The calculation
is simple enough.

I've done it dozens of times. It gets you into the right ballpark.
Which is quite accurate enough for amateur radio purposes. There's no
need for rocket science.

I can't understand the abysmal state of education in simple arithmetic
in our primary schools. We should import the kids who inhabit the
sewers of Rio de Janerio and other American cities and employ them as
arithmetic teachers. It would be more economic than culling by armed
police.
----
Reg, G4FGQ

"Reg Edwards" wrote in message
...

With sufficient accuracy, knowing its maximum value, it is possible to
estimate the value of an air-spaced variable capacitor setting just by
looking at it.

Similarly, the value of a coil can be calculated by counting the
number of turns and guessing its length and diameter. The calculation
is simple enough.

I've done it dozens of times. It gets you into the right ballpark.
Which is quite accurate enough for amateur radio purposes. There's no
need for rocket science.

I can't understand the abysmal state of education in simple arithmetic
in our primary schools. We should import the kids who inhabit the
sewers of Rio de Janerio and other American cities and employ them as
arithmetic teachers. It would be more economic than culling by armed
police.
----
Reg, G4FGQ


I credit the California and Oklahoma Public Schools for my mathematical
illiteracy. Went to school in Bakersfield in 1947, got straight A cause the
math they taught in the 9th grade was the math I learned in Oklahoma in the
4th grade. Returned to Oklahoma in the middle of the school year and was
placed in a 2nd semester
Algebra class. Teacher was a coach who spent most of his class time working
with (on?) a female student whom he later married. Even if I had been truly
interested I'd never have been able to catch up. I was passed on even though
I was totally incompetent in Algebra.( this was before social passing became
common in the public schools)They should have adjusted my schedule and had
me start at the beginning Algebra semester even if it required some class
juggling the following year. Perhaps they thought they were doing me a
favor, or maybe they just didn't realize how retro the California school
math was.

Harold
KD5SAK

On Mon, 01 Aug 2005 21:09:16 GMT, (Robert
Lay) wrote:

On Mon, 01 Aug 2005 12:21:59 GMT, chuck wrote:

If he has an RF bridge, couldn't he simply terminate the tuner's input
with 50 ohms resistive and measure the impedance at the tuner's output?
Well, the conjugate, anyway. I'm trying to imagine the benefit of doing
it that way, rather than simply measuring the impedance at the
transmission line directly.

Chuck,
NT3G

I will break my own rule of not responding to responders and answer
your question - otherwise you would assume that I was ignoring you.

If I might answer a question with a question - why do you think that a
tuner would give up its settings so easily? It's a an interesting
speculation. First, let me clarify what I mean by a Tuner's input
terminal and its output terminal. Considering that a tuner handles
transmitted power in one direction only, we should refer to its
"input" side as the port that connects to the transmitter and its
"output" port as the one connecting to the antenna or feedline.

That agrees with what I interpret your post as suggesting. Therefore,
a 50 ohm termination at the "input" side "should" produce a measured
impedance at the "output" port that is, as you say, the complex
conjugate of the impedance seen looking into the transmission line. I
haven't done a rigorous analysis of that configuration, but it
certainly seems reasonable and correct. It would, indeed be an optimum
way of inferring the impedance that the original poster is trying to
measure.

I see nothing wrong with your suggestion and would recommend it.

Let's pretend that the antenna (load) Z = 49 +j0. Let's also pretend
that the "tuner" consists of a series 1 ohm resistor.

Looking into the input of the "tuner" we measure 50 +j0. Pretty good,
huh?

Terminating the input of the tuner with 50 +j0 and looking from the
other end we measure 51 +j0. Whoops.

Best have a lossless tuner to play this game.



I also see no reason to do any of that as it would be just as easy to
measure the transmission line input impedance directly - as you said.

It's amazing how much good stuff comes out of the woodwork when people
are interested in their hobby and interested in learning.

Thanks,

Bob, W9DMK, Dahlgren, VA
Replace "nobody" with my callsign for e-mail
w9dmkatcrosslinkdotnet
http://www.qsl.net/w9dmk
http://zaffora/f2o.org/W9DMK/W9dmk.html

Good point, Wes. A reminder that a 1:1 swr at the input of a lossy tuner
is not proof of a perfect match at the output!

Chuck
NT3G

Wes Stewart wrote:



Let's pretend that the antenna (load) Z = 49 +j0. Let's also pretend
that the "tuner" consists of a series 1 ohm resistor.

Looking into the input of the "tuner" we measure 50 +j0. Pretty good,
huh?

Terminating the input of the tuner with 50 +j0 and looking from the
other end we measure 51 +j0. Whoops.

Best have a lossless tuner to play this game.

wrote:
I have a pi-network, or at least I believe it is a pi-network in my
antenna tuner. It has two caps to ground and one in series inductor.

If I tune my SWR for 1:1 and go back and measure each value of the
capacitors and inductor, is there a equation or computer program I can
use these measured values to obtain my complex impedance at the input
of the coax cable?

I went through this exercise about 15 years ago using my MFJ-949E.
I assumed linearity for the capacitor setting numbers on the front
panel, i.e. setting 10 = 208 uf. I don't remember what I assumed
for the minimum capacitance zero setting, maybe 10%?. Using a dummy
load and setting the caps near half scale, I then calculated the
different values of inductance for the various switch settings.

For instance, with C1 set to 3.1, C2 set to 3.2, a match was
achieved on 3.5 MHz with the coil set at J. A match was
achieved on 4.0 MHz with the coil set at I. I calculated
52uH for position J and 40 uH for position I.

I wrote a BASIC program that took the switch settings as inputs
and output the estimated impedance looking into the feedline.
I don't know how accurate it was, but it gave me a useful
ballpark value. Ballpark values are often good enough to
indicate what needs to be done next. Shortly after the above
exercise, I bought an MFJ antenna analyzer.

What I found to be a more useful technique is to adjust the length
of the feedline until a purely resistive value is achieved. That
happens at the current minimum points on the SWR circle. An MFJ
antenna analyzer will read that resistive value. Knowing that
resistive value, the Z0 and loss characteristics of the feedline,
and the length of the feedline allows a fairly accurate calculation
of the antenna feedpoint impedance which, at my QTH, agrees closely
with the feedpoint impedance predicted by EZNEC.
--
73, Cecil http://www.qsl.net/w5dxp

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KJ4UO asked:

If I tune my SWR for 1:1 and go back and measure each value of the
capacitors and inductor, is there a equation or computer program I can
use these measured values to obtain my complex impedance at the input
of the coax cable?

Jim Tonne, WB6BLD, wrote a program in Visual Basic that does
precisely that for all common tuner configurations. It is
called Revload and can be downloaded for free from:

http://www.tonnesoftware.com/revload.html

You may want to check out his other ham-radio related programs,
listed on his main page:

http://www.tonnesoftware.com/

As others have pointed out, this is not a particularly accurate
way of measuring impedance, but it should be adequate for "ballpark"
estimates.

Jim, K7JEB