[TowerTalk] Resonant boom

[Stan Stockton]

John,

You may be right....

I have an issue with this particular antenna since it folded my 35 year 
old gin pole when the antenna was almost at the top of the tower going 
up 18 months ago - not a fun experience.  I really don't want to take it 
down and put it back up again.

All the elements are already insulated.  Actually looking at it today 
the side/vertical trusses are fiberglass rod that go out part way but 
the straight up vertical truss is 1/4" EHS (good news) which doesn't 
show up in the picture on the website.   The only problem other than 
time may be to add a step or two to the truss support to insulate those 
vertical trusses in the center.  I believe the truss support is 
approximately 8 feet above the boom.  Kevin could perhaps reach it but 
I'm afraid his YF might not speak to me afterwards. :-)

Modeling it as best I can the boom by itself  is resonant around 7.2 
Mhz.  Insulating those guy wires in the center should do the trick.  It 
moves it down to about 5.3 Mhz.

This is what exactly what K3LR ended up doing when he had a similar 
problem with a 20m boom interacting with a 40m antenna.

73...Stan, K5GO


----- Original Message ----- 
From: <john at kk9a.com>
To: <TOWERTALK at contesting.com>
Cc: <k5go at cox.net>
Sent: Sunday, October 24, 2010 4:21 PM
Subject: Re: [TowerTalk] Resonant boom


> Stan, you just have too many long boom antennas!
>
> It should not be that difficult to model the system and then you can
> experiment and see how to fix the problem.  The 20m beam at 70' with 
> nothing
> below it should not be that difficult to take down, adjust and 
> reinstall.
> Can the boom be electrically shortened by using fiberglass on the ends 
> to
> support the outer elements?
>
> John
>
>
>
>
> To: "TexasRF at aol.com" <TexasRF at aol.com>
> Subject: Re: [TowerTalk] Resonant boom
> From: Stan Stockton <k5go at cox.net>
> Date: Sun, 24 Oct 2010 10:58:19 -0500
> List-post: <towertalk at contesting.com">mailto:towertalk at contesting.com>
>
> Installing dummy elements out as far as I could reach was what I was
> going to try first but dont know whether the would be far enough out
> to do the trick.  Yesterday I looked to see where the nearest element
> is and it is probably out of reach.  I may try tuning the thing and if
> no luck put up another tower (temporarily or not) 30 feet to Northwest
> of it, rotate the antenna to extend the boom on either end, and then
> (if permanent) take a section off the new tower and put up an H frame
> of 10m antennas fixed on Europe using both towers.
>
> Not sure I even want to know the real impact of this resonance on the
> pattern of the 40.  I can say that I haven't had the "feeling" that
> the 40m antenna has worked as well as it did a few years ago.
> Unfortunately, about 75% of the time the 20 needs to be pointed in a
> direction that makes it the worst.
>
> Thanks to you and others for the thoughtful suggestions. I'll report
> what I end up eventually doing.
>
> Stan, K5GO
>
>
>
> Sent from Stan's IPhone
>
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