Well, Glenn, I would lean toward voltage-feed at the base, since you have
that insulated tower! Because of the lower current, the ground-losses
would be minimized, and the current maximum would be fairly high on the
tower, resulting in a nice low-elevation angle, omni-directional radiator
for long haul 80m DX work. Of course you would need a robust tapped
parallel LC network with a robust inductor, probably mad of copper tubing
and a really high voltage air variable capacitor, or better, a vacuum
variable and there would be some substantial switching challenges to switch
the feed between 80 and 160, that would be avoided with your trapped top
loading approach, but full-power 80m traps 90' in the air are non-trivial
components also! I LIKE the voltage-fed 1/2 wave vertical approach and have
had wonderful success with it on 40m, where I fed the vertical 1/2 wave
through a 1/4 wave 450 om line for an almost perfect match!! Great antenna.
And I used it as the driver for a wide-spaced 3-element vertical 40m yagi
for Peter 1 Island and Bouvet and it was a killer antenna for SE Asia on the
evening LP.
So, that's my $0.02! If it were me, I'd leave the top-loading alone and do
the work to voltage feed that insulated tower! Should be a great antenna
for both 80 and 160, and has the advantage that all the tinkering and
tweaking can be done at ground level!
GL and have fun!!
73,
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Glenn
Biggerstaff
Sent: Saturday, February 28, 2015 8:16 PM
To: topband@contesting.com
Subject: Topband: 160 Tower on 80
Hi all, I have a 90 foot Rohn 25 tower with an insulated base and insulated
guy wire sections for top loading ,base fed for 160 meters. It work great
,but I would like to use it on 80 meters as well.
The 3 ideas I have considered are voltage feed at the base with a resonant
LC network at the base, but I am a little worried about the voltages
present at legal limit power. Second idea,disconnect the top loading and put
a trap between the top loading and the tower to divorce the top loading on
eighty then an L network at the base for 80. Third idea, run a wire as a
sloper either a quarter wave fed against ground or a 1/2 wave dipole from
the tower.
Any thoughts or alternative ideas would be greatly appreciated.
Glenn WW4B
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