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[TowerTalk] 80/40 coax trap dipole design

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Subject: [TowerTalk] 80/40 coax trap dipole design
From: "Jim Thomson" <>
Date: Wed, 15 Dec 2010 06:15:14 -0800
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Date: Tue, 14 Dec 2010 19:56:02 -0800
From: Jim Brown <>
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 
[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 

##  You can use the on line
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 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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