Dave,
Thanks for the feedback. You are doing what I thought could be done, but I
was not sure how high the inductive reactance would have to be at 3.5 to 4.0
to ?terminate? the antenna at that spot for 80 meters.
I?d be most interested in your plans and yes, the extra switchable
inductance at the bottom makes sense. I had thought of making it resonate
at the low end and putting a capacitor in.
Thank you again,
Art
_____
From: DAVID CUTHBERT [mailto:telegrapher9@gmail.com]
Sent: Saturday, January 30, 2010 11:22 AM
To: atrampler@att.net
Cc: antennaware@contesting.com
Subject: Re: [Antennaware] center loading versus ground loading
A two band vertical can be built without a trap. The 80 meters vertical is
full sized. On top of that sits a coil and a top hat for 160 meters. For
proper decoupling the coil (inductance) needs to be fairly high. The
tradeoff is a reduction in 160 meter bandwidth.
I have used this method to build 80/160 and 40/80/160 meter verticals.
Automatic bandswitching, no relays needed. The downside for you is that the
antenna needs to be taken down (tilted over) to adjust the top hat coil. Or,
make the coil 10% smaller and tune with a small base coil. This is what I do
for a single band top loaded vertical. But that means for two band coverage
a relay at the base is needed or a wire you move by walking out to the
antenna. The relay does not have to be a HV relay.
I will post plans here shortly.
Dave WX7G
On Fri, Jan 29, 2010 at 9:48 PM, Art Trampler <atrampler@att.net> wrote:
I live on a fairly small lot and use a Hy-Gain AV640 for 40 through 10 and
currently have no antenna for 80 or 160.
My backyard is about 90 x 70 but has some ill-placed and ill-shaped trees
for either wire antennas or a tower.
So I am thinking of another vertical, but this one ¼ wave with a radial
field. I would like to get 80 and 160 out of it. I don?t mind having to
guy it, or even having to pour a concrete base for it.
My first thought is to use aluminum irrigation pipe as others have, and have
about 60 to 65 feet of it, an insulator and inductor and high voltage relay,
and then perhaps 15 to 20 of much smaller aluminum tubing, with a sloping
capacity hat of four wires going partially down the four top guys. I don?t
know if I could get away from the relay, and put up a trap instead but am
wary of using a true trap (coil & capacitor) rather than just a large, high
Q coil.
As you can see this idea is full of possibilities and mechanical drawbacks,
so the question is, is there that much to gain from the center-loaded design
with capacity hat, versus a switchable tuning network at the base of the
antenna?
Your input is appreciated. I am hoping to make this a summer project and
reward myself with 80 and 160 in the winter.
73,
Art
Art Trampler, KØRO
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