Tom Rauch wrote:
>An 80m dipole antenna near the inverted L can drastically
>affect SWR. Many low 80m dipoles combined with the feedline
>act like a T antenna and resonate on 160.
>Also, as a general rule the better the radial system (as a
>general rule, do not take this as an "always" rule) the
>lower the antenna feed impedance. It might easily be 15-20
>ohms feed impedance.
Good point Tom. I overlooked the feedline. You are absolutely correct,
the feedline combined with the 80M antenna could be resonant as a T on 160.
As a test, the feedline length could be changed to see if the SWR
changes on the L. The end of the 80M feedline could also be alternately
connected/and removed from ground to see if there is any change in SWR
on the 160M L. This would be an indication of interaction.
The original number I gave for expected R for the 160M L (35 to 45
ohms), was in error, and too high.
For an inverted L with a 70ft vertical section, and a flat top section,
I get about 21 ohms as a minimum (at resonance) with a perfect ground.
If the flat top droops, this could be less. With a real ground (average
ground 13/.005) and lots of radials, (say 128 radials, 1/4 wave) this
will add around 5 ohms, making a minimum of about 26 ohms. Most guys
won't add this many radials. If you use about 32 1/4 wave radials, this
will add about 8.5 ohms. Throw in another 1.2 ohms for #12 copper wire
and that would make 30.7 ohms a more realistic minimum number.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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