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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