Date: Tue, 14 Dec 2010 19:56:02 -0800
From: Jim Brown <jim@audiosystemsgroup.com>
Subject: Re: [TowerTalk] 80/40 coax trap dipole design
The shortened multiband dipoles sold by Hypower Antenna Company are a
pretty good design. I've successfully used two different versions in
several different locations with very good results. Plugging the
parameters into an NEC model yields an efficiency number 0.4 dB below a
full size dipole.
Barry's designs use a loading coil in each leg rather than a trap, and
the coils are roughly at the point where the insulator would be for a
half wave on the higher band. Then the coils, and a bit more wire. The
160/80 version is roughly 160 ft long, the 80/40 version is a bit under
100 ft.
## whoa. This just amounts to a midpoint placement of a loading coil
in each leg. Nothing new here...except how do you get it to work on the
higher band?? If it's not a trap, and if it's really a loading coil, then it
will
[or should] only resonate on the lower freq.
Although I've measured Barry's coils and know their dimensions and
inductance, I won't publish that information, because it's his
engineering work, he sells a good product, and he deserves to make a
fair profit for it. But I will tell you that these are NOT traps that
resonate with their own stray C, as I had first suspected. Rather, their
self resonance is WELL above 20M! So they are really working as loading
coils.
## You can use the on line
http://www.dxzone.com/catalog/Antennas/Antenna_Calculators/
scroll down till u get to 'ant loading coil calculation'. For a shortened
dipole, enter the
1/2 length of the overall shortened dipole for the freq u want. IE: for a
100' long
80m shortened dipole, enter 50 feet. Then enter where you want the coils to
go..which can
be anywhere from the feedpoint '0 feet' all the way out each ele leg. On
a typ 100' long
80m shortened dipole, the coils would go out 25' on each side. That way the
feedpoint radiation
resistance would be the highest. If the pair of coils straddle the feedpoint,
the required uh will be
1/2 the midpoint value... BUT the feedpoint RR will be a lot lower. In
either case, a simple helical
hairpin consisting of aprx 5 turns of wire on a 3" ID and 4" long will
result in a 1:1 swr...on 80m.
## The above scheme applies to shortened ant's on any band. It appears that
by locating the coils
out even further than their midpoints...like 33' on either side [ for our
100' long 80m shortened dipole]
that the 80m loading coils some how now look either like traps, or some other
high Z decoupling scheme ?
In any event, the required UH of the coils, if placed 33' on either side
would require even MORE uh...
vs placing em 25' on either side. If you say the coils self resonate well >
20m...then what system is
divorcing the ends beyond the coils..such that they resonate on 40m..... which
just happens to be the
correct length inboard of the coils..for 40m ??
## If his scheme actually works, he has a winner. Perhaps the coils + the
17' end pieces sorta look
like linear decoupling stubs on 40m ? What's the trick here ? What are we
missing? What's the theory?
Later.... Jim VE7RF
73, Jim K9YC
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