[TowerTalk] Breaking boom resonance

[Jim Thomson]

Date: Wed, 29 Mar 2017 06:24:37 -0400
From: "john at kk9a.com" <john at kk9a.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Breaking boom resonance


##  What IF his 34 ft boom  is trussed with steel wire ?   That would
change the resonance.  Story I got was you want to avoid floating half
wave booms, or grounded quarter wave booms.   IF the 34 ft boom is
no doubt bonded to the tower, then you end up with 2 x quarter wave long, grounded
booms, back to back.....= bad news.  By adding the overhead wire truss to support the
34 ft boom, the resonance should change.   You could also put an insulator between truss and mast,
then wire a variable cap across the insulator, to change the boom resonance..but that would require the
use of 2 x insulators and 2 x variable caps. 

##  as john noted below, you can also end up with more issues when the ants can be independently 
rotated.  On a side note, looking at the f12 book, they depicted a case with a 2 el 80m loaded yagi 
mounted above a F12  620-340. The 80m yagi would resonate on 20m.   This is with both yagis
pointed in the same direction, mounted on the same mast.  Their fix was to use vac relays on each 80m ele,
to open off the center, when using 20m. 

Jim  VE7RF      



I had a similar issue with my 20m boom being resonant on 40m. In my case I
made it so the antennas could not be turned separately.

For for a fixed wire beam I think that the easiest solution would be to
use a fiberglass boom.

John KK9A


To: towertalk at contesting.com
Subject: [TowerTalk] Breaking boom resonance
From: Steve London <n2icarrl at gmail.com>
Reply-to: n2ic at arrl.net
Date: Tue, 28 Mar 2017 09:57:35 -0600


I have a 34 foot boom supporting an 80 meter wire beam. The elements of
the wire beam are completely insulated from the boom. 30 feet below that
boom is a 20 meter beam. Yes, there is definitely interaction.

I am looking at techniques for detuning the boom using the W8JI tower
detuning method. Anyone want to make a guess at how long the detuning
section should be, and what value of capacitor would get me in the
ballpark for resonance at 14 MHz ?

73,
Steve, N2IC