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Re: [TowerTalk] 40/80m dipole

To: "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] 40/80m dipole
From: "Jim Brown" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Thu, 25 Mar 2010 08:40:05 -0700
List-post: <">>
On Thu, 25 Mar 2010 07:39:31 -0700, Dan wrote:

>I should have mentioned that the 40/80 antenna for my son will be almost 
>vertical. He has a single pulley in a tall tree to suspend the antenna. 
>I agree that the fan dipole or bent full size dipoles would be good 
>solutions but a single 2 band dipole is better suited to his needs.
>Thanks for all the comments. I was hoping to get an inductance value and 
>location in a 90 ft dipole. Antennas of this type are widely available 
>from suppliers but it is more fun to build it and he will be visiting in 
>a week or so. It makes a nice educational project.  I do have an 
>instrument to measure inductance at those frequencies.

Several thoughts. First, a vertical dipole is not a very good antenna. I 
say that from direct experience -- I have a 40M vertical dipole with the 
top suspended at about 100 ft from a tall redwood. It would work better 
as a sloping dipole, coming away from the tree as far as possible. That 
would also allow it to be longer. 

Second, you might consider a vertical long wires, loaded against radials. 

Third, if you still want to wind a coil, look up an article in QST from 
about four years ago that shows how to design shortened dipoles using 
loading coils. Or, even better, use the $80 or so you would have spent on 
a pre-built antenna on a copy of EZNEC from W7EL's website. You'll still 
have to design it, but you'll be able to show him how various design 
changes affect antenna performance. 

In my professional life, I design sound systems for large public spaces, 
and I use acoustic modeling software extensively. It is a WONDERFUL 
LEARNING and TEACHING tool, because it graphically shows results as you 
make changes to the design. So is EZNEC, and for the same reasons. 

EZNEC is also a very good predictor of actual antenna behavior (provided 
that you actually build what you model (or accurately model what you have 
built). The mediocre performance of that vertical dipole that I clearly 
observed on the air (by switching it between my horizontal dipoles) was 
clearly predicted by EZNEC. 


Jim Brown K9YC


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