[TowerTalk] 40 meter rotating dipole question

[Doug Scribner]

Also here...

http://home.comcast.net/~nwilson343/40meterdipole.html

Doug - K1ZO

----- Original Message ----- 
From: "Mark, K5ER" <k5er at arrl.net>
To: <towertalk at contesting.com>
Sent: Tuesday, October 01, 2013 3:24 PM
Subject: [TowerTalk] 40 meter rotating dipole question


> 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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