On Sat, 23 Feb 2008 13:29:58 -0500 (EST), donovanf@starpower.net
wrote:
>Unfortunately, you're likely to have parasitic effects between
any
>vertical and your two towers, both of which may be resonant near
>160 meters.
Good point -- that can certainly happen, but to do much, it
requires more than 130 ft of vertical or near vertical conductor
(or maybe 100-110 ft with the top loading of a beam). There's a
much greater liklihood of resonance with an 80M/75M vertical.
I agree with Frank -- if the dimensions are in the ballpark, you
should do some modeling. Ah, you say -- the NEC software costs
money. Yeah, it does. But how much would you spend on the
antennas? How much is your time to build them worth? I have
W7EL's EZNEC5 Plus package, and I've learned a lot from it.
Another point -- the feedlines to any other wires (like that
inverted vee you mentioned) should also be considered as parastic
elements. Luckily, there's a VERY effective way to take them out
of the game. Simply add some good coax chokes as "egg insulators"
in the feedlines to break up the resonances. See my Coax Choke"
tutorial and Power Point for details. The PPT has few screen
prints from an NEC model showing the feedline interaction, and how
the chokes can kill it.
http://audiosystemsgroup.com/RFI-Ham.pdf
http://audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf
73,
Jim K9YC
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