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Re: [TowerTalk] 80/40 coax trap dipole design

Subject: Re: [TowerTalk] 80/40 coax trap dipole design
From: Jim Brown <>
Date: Tue, 14 Dec 2010 19:56:02 -0800
List-post: <">>
On 12/14/2010 6:16 PM, WW3S wrote:
> Anyone have a good design for one of these? I got the 40m length ok, on a
> fence about the traps...some designs say to make them resonant, W8JI
> recommends again if they aren't resonant on 40m, what freq do you
> make them resonant for? And does that affect the length of the 80m leg?

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. I've run legal power to them in heavy duty contesting for 
years.  I've added a 40M parallel dipole to the 160/80 version, and it's 
my primary 80/40 antenna for EU and VK/ZL.  People tell me it works. :)  
I've also used it on 160, but once I had an 86 ft Tee vertical with a 
lot of radials, I found that the vertical usually (but not always) was 
better. Although the loaded dipole is up about 110 ft, that isn't very 
high for 160 (less than a quarter wave).  The 80/40 version was my main 
antenna for those bands on my city lot in Chicago, and it worked well 
enough that I carefully took it down and brought it to CA when I moved 

Another thought on this.  If you don't have enough space for a half wave 
on 80, by all means do one of these loaded dipoles, but if you have room 
to rig a half wave for 80, by all means use a FAN dipole (that is, with 
a parallel element for 40M) rather than a trap.  The FAN will have much 
better SWR bandwidth on 80, and enough bandwidth for 40M.  For detailed 
building ideas, go to  and 
select the two pieces (text and Power Point) on Antennas For Limited Space.

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 

73, Jim K9YC

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