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Re: [TowerTalk] : 40 meter rotating dipole question

To: <TOWERTALK@contesting.com>
Subject: Re: [TowerTalk] : 40 meter rotating dipole question
From: <john@kk9a.com>
Date: Wed, 2 Oct 2013 01:43:01 -0000
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
The price of a full size 40m element is considerably more.  Not only do you 
need more aluminum and a stronger center piece, you also need a stronger 
tower and rotator.  I have had full size 40m beams and they are great.  You 
will get better bandwidth and higher efficiency. If you do wish to shorten 
it, split the element and use a Hi-Q coil or some type of linear loading. 
As far as designing and building one that requires no adjustment, I think 
that will be a challenge, especially of you load it. I model all of my yagis 
and have always needed to make an adjustment to the DE.

John KK9A



To:towertalk@contesting.com
Subject:[TowerTalk] 40 meter rotating dipole question
From:"Mark, K5ER" <k5er@arrl.net>
Date:Tue, 01 Oct 2013 14:24:58 -0500

Hi Guys, I have checked the archives, but not found specific answers, so 
forgive me if this has already been covered and I am asking the wrong 
questions. To help a friend, I am building a 40 meter rotatable dipole, to 
live at 90' on his mast. It will be about 12' above his 36' boom 20M OWA. I 
will fabricate a heavy duty mount/center insulator, and incorporate 2 truss 
cables, either dacron or kevlar. He delivered to me two (used) elements from 
a previous unidentified antenna. Each element is abt 28' long, with the 
taper schedule going from 1 1/4 down to 5/8. I have spare material in stock, 
so I could easily add a 1/2" x 5' tip to each and end up with a full size 
dipole.

Question one - The price of material isn't much more, so why does it seem 
that "nobody" builds rotatable dipoles to the full 66-67 feet?

Question two - IF a shortened antenna is better, is there a "better" length? 
I see many in the range from 38 to 55 feet.

Question three - If shortened, I know the ant will be capacitive, and 
therefore can be matched by adding an inductor across the feed. I find many 
pages of formula with lots of Latin characters, but missed that day in math 
class. If someone had a hint of a starting point, ie> 4" long, 5 turns on 2" 
diam. , or 4 turns on 3" diam, etc. I could then work backwards and make 
this work for him.

I have EZnec 5+, but have only used it to build OWA antenna so far, which 
makes a beautiful direct feed, so I have no experience with adding a load. I 
plan to input the taper schedule into EZnec, but if a given length is known 
to be superior, I am not too proud to ask for it, rather than spend hours 
building model after model working my way through the possibilities.

I understand that height can affect performance and feed point impedance, 
however, neither the owner nor I climb. I need to model and build this 
antenna, and then expect it to work as modeled when the climber puts it in 
place, so any ideas would be appreciated.
Thanks,
Mark, K5ER 

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