Is this the same general design used by the late W9INN ?
Alan, N3BJ
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <towertalk@contesting.com>
Sent: Tuesday, December 14, 2010 10:56 PM
Subject: Re: [TowerTalk] 80/40 coax trap dipole design
> 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 that....so 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
> here.
>
> 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 http://audiosystemsgroup.com/publish.htm 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
> coils.
>
> 73, Jim K9YC
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