"Jim Lux" wrote in message
...
Jerry wrote:
"dykesc" wrote in message
...
On Jun 4, 2:10 am, Ian Wade G3NRW wrote:

Is this any help?

http://www.wy2u.com/

Scroll down to "Electrical / Electronic Engineering Links" and click on
"L/C Impedance Matching Design Tool"

--
73
Ian, G3NRW

Thanks Ian. That is the type of calculator I am looking for. However,
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.

Hi dykesc

I may be missing something. But, if the objective it to learn if the
local 105 MHz signal is actually introducing error into your impedance
measurement, only a few Smith Chart Polts are needed. You know the path
(on the Chart) the shunt reactance will have taken while being adjusted
to make a "match". You also know the path the series reactance took.
Start from the Chart center and move the impedance along the circles of
constant resistance for the series reactor. Move along the circles of
constant admittance for the shunt reactance. When the Xc and Xl are
both known, and you know which is closest to the "transmitter", it seems
that a "program" is unnecessary. What am I mising?


Fine, for doing a couple or three. Now do it for a dozen measurements at
different frequencies, especially if you have to convert L and C into Z
for each measurement. A program or spreadsheet is nice to have, because it
automates the tedious calculation.

Heck, if you have a RS-232 interface to the antenna tuner, you can
automate the whole process. Quod fecit.

By the way, the assumption that the run of the mill ham rig has a 50 ohm
resistive output impedance is not necessarily valid. In practice, nobody
cares.. they just adjust until the reflected power is minimized. But if
you're trying to use the tuner as a measuring instrument (essentially, the
variable part of an impedance bridge), it's important.

Hi Jim

As you can see, this plan to 'double check" antenna impedance by relating
the impedance to that which the tuner produces, is very limited in its
accuracy. One might say *why bother*. My thought was/is, the original
poster asked for an online calculator. When that calculator was given, and
he still asked for more, I thought it would be helpful to point out how easy
it is to use the Smith Chart to get some insight into evaluating his
impedance meter. It is my opinion that extensive calculations with a
computer program are not able to provide any exact answer to the question
about actual antenna impedance when only the tuner Xl and Xc are known. The
Smith Chart is a great tool for estimating how impedance matching can be
realized. I assummed that the OP could possibly benefit from knowing how
easy it is to estimate antenna impedance using the Smith Chart if it can be
assummed that the LC circuit is known and that the resultant impedance is 50
ohms. Shunt reactances move the impedance along the circles of constant
1/R and series reactances along the circles of constant R.

Jerry KD6JDJ

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

On Thu, 04 Jun 2009 21:33:41 GMT, Owen Duffy wrote:

Hams often confuse the source impedance of a transmitter and its required
load impedance.

sigh....

On Jun 3, 10:57*am, dykesc wrote:
I am trying to validate impedance values I am measuring with my
MFJ-259B. I want to do this by using my MFJ-993B auto tuner. The tuner
uses a simple L network to create the conjugate match. I want to take
the final inductance, capacitance and swr values from the auto tuner
digital display after matching is completed and back calculate the
impedance that the tuner is seeing. Is there an online calculator that
will do this?

Many thanks for replys.

In a quick scan of the replies, one thing I didn't see mentioned is
that if the indicated SWR is not 1:1, then there's an uncertainty in
your answer. For example, a 1.1:1 indicated SWR, taken with a meter
calibrated accurately to 50 ohms, could represent 55 ohms or 45.5
ohms, or any other impedance with the same magnitude of reflection
coefficient relative to 50 ohms. If you start at 55 ohms, you'll get
a different answer for a particular L and C than if you start at 45.5
ohms. Of course, each of those will be different than the answers for
other phase angles of the same reflection coefficient. You can get
rid of that particular uncertainty by adjusting to 1:1 SWR. Of
course, you're still left with other uncertainties, but since you have
control over that particular one (presumably), at least get rid of it.

I'd use WinSmith to do the calc you mention, or hack a simple
spreadsheet if I had several to do, or put the values into Matlab or
Scilab.

Cheers,
Tom

On Jun 4, 12:06*pm, Richard Clark wrote:

The source code is at:http://www.hoflink.com/~mkozma/match19cb.html
which, if you can follow the design and make your own changes, should
be ammenable to your needs.

73's
Richard Clark, KB7QHC

Thanks Richard. I found the source code. I have done a little
programing long ago, but modifying the code there would be a little
much for me I'm afraid.

73
Dykes AD5VS

On Jun 4, 12:29*pm, "Jerry" wrote:

* I may be missing something. *But, if the objective it to learn if the
local 105 MHz signal is actually introducing error into your impedance
measurement, only a few Smith Chart Polts are needed. *You know the path (on
the Chart) the shunt reactance will have taken while being adjusted to make
a "match". * You also know the path the series reactance took. * Start from
the Chart center and move the impedance along the circles of constant
resistance for the series reactor. *Move along the circles of constant
admittance for the shunt reactance. * When the Xc and Xl are both known, and
you know which is closest to the "transmitter", it seems that a "program" is
unnecessary. *What am I mising?

* * * * * * * *Jerry * KD6JDJ

Thanks Jerry. No you aren't missing anything other than the fact that
my familiarity with Smith Chart analysis is limited to working through
a few exercises in the last chapter of the ARRL Antenna book. I will
be looking for more Smith Chart tutorial info on the web and am
certain I can get myself up to speed enough to start working with
conductance, suseptance, admittance, etc.

Thanks again. From your post it appears it will be a straight forward
exercise once I get my head around it.

73
Dykes AD5VS

On Jun 4, 2:13*pm, dave wrote:

RF Cafe has one that uses Excel. *www.rfcafe.com

Thanks Dave. I just downloaded it.

On Jun 4, 4:33*pm, Owen Duffy wrote:

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.

Probably would be as accurate as any other gear we can afford Owen.
I'm going to suggest that MFJ make that a feature on an auto tuner. I
don't live to far from their headquarters in Starkville, MS. I have a
son going to Mississipi State there and have never been to their
facility. This will make a good topic of conversation.


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.

I looked on the R&S site. The only thing I found was a "Field Fox"
analyzer selling for $7,599.00. It is the cat's meow, but guess I'll
have to stick with my 259B.


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

Which one Owen.

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.

Now that would be a great feature. Input your line type and physical
length and the analyzer does the transform. Lots of programs available
on the web for that, but would be nice to have at your fingertips.

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.

That has become very obvious to me.

On Jun 4, 7:24*pm, K7ITM wrote:

In a quick scan of the replies, one thing I didn't see mentioned is
that if the indicated SWR is not 1:1, then there's an uncertainty in
your answer. *For example, a 1.1:1 indicated SWR, taken with a meter
calibrated accurately to 50 ohms, could represent 55 ohms or 45.5
ohms, or any other impedance with the same magnitude of reflection
coefficient relative to 50 ohms. *If you start at 55 ohms, you'll get
a different answer for a particular L and C than if you start at 45.5
ohms. *Of course, each of those will be different than the answers for
other phase angles of the same reflection coefficient. *You can get
rid of that particular uncertainty by adjusting to 1:1 SWR. *Of
course, you're still left with other uncertainties, but since you have
control over that particular one (presumably), at least get rid of it.

Yes, I thought about that Tom. The premise of my undertaking is that I
can achieve a 1:1 match. At times I'll bet my tuner won't make it.


I'd use WinSmith to do the calc you mention, or hack a simple
spreadsheet if I had several to do, or put the values into Matlab or
Scilab.

I'll check them out.

Thanks for taking the time to help me out.

73

Dykes AD5VS

"dykesc" wrote in message
...
On Jun 4, 4:33 pm, Owen Duffy wrote:

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.

Probably would be as accurate as any other gear we can afford Owen.
I'm going to suggest that MFJ make that a feature on an auto tuner. I
don't live to far from their headquarters in Starkville, MS. I have a
son going to Mississipi State there and have never been to their
facility. This will make a good topic of conversation.

Unless they can find someone elses to copy, it won't happen.

Dale W4OP