[TowerTalk] 40 meter rotating dipole question

[Chris]

Don't reinvent the wheel...use this recipe:
http://www.k6if.com/40dip/40dip.html

Chris
KF7P






On Oct 1, 2013, at 1:24 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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