Mark,
Look here...
http://www.k7mem.com/Electronic_Notebook/antennas/shortant.html
or here...
http://www.k7mem.com/Electronic_Notebook/antennas/shorty_40.html
73
Doug - K1ZO
----- Original Message -----
From: "Mark, K5ER" <k5er@arrl.net>
To: <towertalk@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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