Jim,
You are 100% correct but in a case like James has, I think you are kind of
splitting hairs.
"If" the antenna is a monoband and fairly close to being 50 Ohms resistive,
there won't be much transformation due to the transmission line, and the
resonance frequency measured will be very close to accurate.
Specifically, in the case of the RigExpert analyzers (which James also has),
I set mine to Z=R||+jX, find the two points where the Xp changes directions
and take the median. This is very close to the true resonance frequency of
the antenna. It is, as measured on 4 different antennas here, usually 100
to 300 kHz away from the point of minimum SWR.
If I had a significant mismatch, then the transmission line would indeed
induce a significant transformation, but based on dipoles cut and trimmed to
the theoretical calculated length, (or James' vertical), these aren't going
to be very far off, are they?
My basic point to James was on his monoband vertical, measuring in another
mode would give him a more accurate indicate of resonance than SWR.
Hw?
Rick, DJ0IP
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Thursday, April 18, 2013 6:13 PM
To: tentec@contesting.com
Subject: Re: [TenTec] Tuning Amp on 17 meters
On 4/18/2013 1:51 AM, Rick - DJ0IP / NJ0IP wrote:
> It is very easy to confirm that with all of your antennas, using your
> RigExpert AA-230, but NOT in SWR mode.
> Switch it to Phase or one of the R/X/ modes. There you can find the
> true resonance frequency. Then go find the frequency of minimum SWR.
> They are almost never the same.
Not that easy unless the impedance of the antenna at resonance is the same
as the impedance of the transmission line, and many (most) antennas are not.
The impedance of the antenna is transformed by the transmission line, and
both the transformation and the math that predicts it are complex.
To find the resonant frequency, you must first be able to measure the
complex impedance (that is, R + jX) at some point on the transmission line
(could be in the shack) save that data as a standard interchange file,
measure the electrical length of the line, export the complex data to Smith
chart software, and rotate the measurement around the Smith chart the number
of electrical degrees corresponding to the length measurement that you made.
The good news is that very good Smith chart software is both FREE and easy
to use. it's called SimSmith, runs in Java, and google will find the
download site. More good news is that once you have that software you can
use it to design nice matching networks. The bad news is you'll have to
study how transmission lines work. :) But more good news is that you'll
learn stuff, which is a fundamental objective of ham radio.
73, Jim K9YC
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