[TenTec] Tuning Amp on 17 meters

[Jim Brown]

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