You can improve the dipole by doing the following:
1. Length can be increased as necessary by allowing an additional
10 to 15 feet of wire to hang down vertically at each end.
100-ft is not a bad length but you could increase it another 30
feet or so by allowing the ends to hang down. The "hanging ends"
formation does not materially affect performance.
2. For feeding and matching I would bring the balanced line (I am
assuming it is the 450-ohm type) down to your house outside your
shack window, and at that point attach it to a 4:1 balun. Run
coax into the shack, via an antenna tuner.
This should give you good all-band performance from 80-10 meters.
You could fiddle with the antenna length by a combination of
antenna modeling and cutting-and-trying as far as getting the
best match on your favorite bands is concerned. For someone with
restricted space you are fortunate to have an approximate 100-ft
clear span and good height.
The sag is about normal. Unless you have an easy way to support
the middle then you just need to forget about it.
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com]On Behalf Of Art Boyars
Sent: Monday, June 27, 2005 8:34 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Improving my dipole
On the CQ-Contest Reflector K7QQ said: "You have to keep in mind
that the best antenna doesn't exist. You can only get up what you
can and it is never enough." Well that's where I am, but I'd
like advice from you experts on how to improve my antenna, within
my personal constraints.
My antenna is a dipole, about 99-ft long, strung between maple
trees of the neighbors on either side. (It's already much higher
than the peak of my roof. No other supports available.) The
feed line is mostly window-pane twin lead, with several feet of
coax to get into the house and reach the Transmatch. Yeah, I
know it's not balanced; but should I really care, as long as the
RF goes somewhere other than losses? (And I know that the high
SWR might cause the coax to arc over or have noticeable resistive
loss, but that's one of the constraints -- for now.) My main
operating interest is SS CW, with a little other casual
contesting (work the Club members and the big guns) and a bit of
40M CW DXing (if I can get the antenna to work better).
I've pulled the ends of the dipole as far as I can into the trees
(maybe too far), but it still sags quite a bit -- 5 to 10 feet,
by eye. Some of the sag is probably from the weight of the
antenna wire and the feedline, but I think a lot of it is simply
from the dipole's being longer than the span between the trees.
I have two seat-of-the-pants concerns. First, I may be getting
losses from having the ends of the dipole in the foliage.
Second, the vertical-V might be more NVIS-like than Iis good for
me. Obvious solution is to shorten the dipole a bit, but that
would make it even shorter than the recommended 100-ft minimum.
So, first question for the experts: Do you think I'll do better
to shorten it, or should I leave the ends in the trees?
Second-order solution and question: If it turns out that the
feed line is weighing down the center, would I be better off to
let it sag than to change to something more like TV twin-lead?
(I recall some discussion here about Radio Shack selling a
twin-lead with heavier-than-typical conductors.)
Thanks for your help.
73, Art K3KU
_______________________________________________
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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_______________________________________________
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.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
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