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Re: [TowerTalk] Combined Dipoles

To: "Tower Talk List" <>
Subject: Re: [TowerTalk] Combined Dipoles
From: "Jim Brown" <>
Date: Wed, 16 Apr 2008 08:53:45 -0700
List-post: <">>
On Wed, 16 Apr 2008 02:36:39 -0700, Bill Turner wrote:

>Yes, they will work very well indeed. My low-band antennas here 
>three inverted vees connected together just as you describe. 

Yes. I have two 80/40 fan dipoles at right angles, and two 
20/15/10 dipoles at right angles. I've used antennas like this for 
five years at three different QTHs and on Field Day. They work 
VERY well.

Some things I've learned:

1) The longest dipole of the fan will exhibit its "normal" SWR 
bandwidth. Shorter dipoles in the fan will have their SWR 
bandwidth reduced by about half. With a good tuner (I use TenTec 
229s and 238s) I can run these fans over the bottom 3/4 of 80M, 
all of 40, 20, and 15, and the lower half of 10M. I could probably 
use the 20/15/10 at the top of 10 too, but I've never tried to 
operate above 28.55.  

2) A 3-wire 20/15/10 fan will look like 50 ohms even if it is 
quite high. Ordinary dipoles will be pretty close to 75 ohms if 
they're quite high. 

3) Some references say that one dipole will be detuned by some 
percentage by the others. I've never been able to measure that. 

4) I would NOT try to put dipoles in a fan that are "in between 
harmonics" of others in the fan. That is, I would not put 30M in a 
fan with 40, because it is more likely to interact with the other 
dipoles. On the other hand, my 80/40 fans load quite nicely on 
30M, and they play very well. They also load and work well on 17M 
and 12M! 

5) If you have limited space, don't rule out loaded dipoles like 
those sold by HyPower Antenna Company. I've used them a lot, and 
they work quite well. NEC modeling shows that their efficiency is 
only a fractional dB below a full size dipole. The compromise is 
reduced SWR bandwidth, but you can still use them effectively over 
an entire band at full power with a good tuner. 


Jim Brown K9YC


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