I posted this yesterday, with a screen grab of the SimSmith model, but
the list rejected it. Here it is again, but without the screen grab.
You'll have to imagine it. :)
= = = = = =
Here's the Power Point for a tutorial I presented to the Pacificon
Antenna Forum last fall. It includes a VERY slick way of broadbanding
most resonant antennas, especially dipoles. The technique is probably
older than most of us, including me -- Prof Dave Leeson, W6NL, who
teaches it in his EE courses at Stanford, says he's seen references to
it from the '40s.
http://audiosystemsgroup.com/PacificonSmithChart.pdf
Back to the 80M dipole tuned to 3800. Attached is a screen grab from a
SimSmith simulation based on an NEC model of an 80M dipole up 30 ft,
tuned to about 3590 kHz. The behavior of this antenna at 3900 kHz is
roughly comparable to the 3800 kHz antenna at 3500 kHz. The black curve
is the SWR at the antenna, the blue curve is the apparent SWR that you
would measure at the end of 100 ft of Belden RG8X, and the dashed curve
is the loss in dB. Note that for this very low antenna (only 1/8
wavelength) the feedpoint Z at resonance is only about 25 ohms. The
feedpoint Z of my very high 80/40 dipoles (about 115 ft) is on the order
of 80 ohms, so I feed them with Belden 8213, a low loss RG11.
Note the loss in the RG8X at 3900 kHz is about 3.25 dB. If we replaced
the RG8X with Belden 8237, a good RG8, the loss at 3900 kHz would be
about 1.8 dB.
And you thought a dipole was simple.
BTW -- Ward Silver, N0AX, has written some good tutorial stuff on Smith
Charts. Search the QST archives. I think I have references at the end
of my Power Point.
73, Jim K9YC
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