If you have the materials, why not get the easiest to model and best
performance antenna? Should be great at 90'!
Since survivability is one reason to avoid full size, do a mechanical
analysis. If you don't have a mechanical stress analysis program,
consider Yagi-Mech from DX engineering. It's easy to use and will tell
you the weak spots that can be reinforced with internal sleeves. Then
add two Phillystran guys to a cross arm above on the mast about half way
out the antenna.
I have a center loaded 86' long 80m rotatable dipole with SECO Tornado
drive pair of center loading inductors and the EZNEC modeling was right
on (albeit inductors adjustable from the ground kind of makes "right on"
a dubious claim). I did the Yagi-Mech work and added a couple of
sleeves each side and the Phillystran.
Actually there are some commercial 80m shorty rotatable dipoles about
67' long, using a Tornado, so add a DPST shorting relay you could have a
80 & 40 rotatable dipole.
Grant KZ1W
Redmond, WA
On 10/1/2013 12:24 PM, Mark, K5ER wrote:
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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