[TowerTalk] : 40 meter rotating dipole question

[David Blake]

>From personal experience I have found the shortened
Mosley trap 40 meter elements effective.  I had the
Mosley TA40KR 40 meter add on kit on an old TA33
and worked over 200 DXCC on 40M with it at less than
50 feet.  I then moved from Virginia to Ohio and put up
a Mosley PRO67B and then worked over 320 DXCC on
40 meters with it.  I know Mosley has been beaten up 

pretty bad on this reflector, but with mine I have Honor
Roll Mixed, Phone and CW and #1 Honor Roll.

BTW my PRO-67B was only at 50 feet.  It was designed
to work on 6 bands, and I managed over 300 countries on
all 6 bands.  On 40M I had a choice to set for the low end
of the band, center or top end.  I chose low end but still
had an SWR less than 2.0 to one up to about 7.230Mhz.

YRMV  73

Dave -N4DB-



________________________________
 From: "john at kk9a.com" <john at kk9a.com>
To: TOWERTALK at contesting.com 
Cc: k5er at arrl.net 
Sent: Tuesday, October 1, 2013 9:43 PM
Subject: Re: [TowerTalk] : 40 meter rotating dipole question
 

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 at contesting.com
Subject:[TowerTalk] 40 meter rotating dipole question
From:"Mark, K5ER" <k5er at 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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