On Tue,12/13/2016 11:06 AM, Matt wrote:
I just don't know if the tuning stubs will change the resonance
of the antenna itself? Does this sound like a valid methodology?
You're asking a good question, but finding the answer is not easy. :)
The transmission line does, indeed, affect the resonance of the antenna,
including both the stub and whatever is connected to it in the shack. If
you've designed the stub matching, you probably have done so by
measuring the complex Z somewhere, plugged that data into Smith Chart
software, and done the design. To answer your question, I would
disconnect the coax from the antenna, measure the coax at the antenna
with the shack end connected to your receiver, then plug that value of
impedance into your NEC model.
Then add the RX antenna to the model and look at its pattern with and
without that measured impedance in the model.
BTW -- there's an excellent paper by N6LF showing that an antenna
doesn't have to be very close to resonance to disturb the pattern of a
nearby antenna. I have a 45 ft tower with a 15 ft mast holding up 3-el
20M and 4-el 15M monobanders about 60-80 ft from a Tee vertical for
160M. A quick NEC model shows it resonant around 2.4 MHz (those Yagis
with elements connected to their booms do a lot of top-loading), and the
model shows a bump of several dB in the pattern for the Tee vertical.
Google on N6LF to find Rudy's website.
73, Jim K9YC
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