[TowerTalk] Resonant boom

[Jim Thomson]

Date: Sat, 23 Oct 2010 20:04:03 -0500
From: Stan Stockton <k5go at cox.net>
Subject: [TowerTalk] Resonant boom

Anyone by chance have an easier to accomplish idea other than one I  
remember being suggested to someone else with a similar problem -  
fishing a wire through a hole near the center of the boom and letting  
it hang out the end.  I can watch the SWR on my 40 meter antenna go  
from 1.2/1 to 2.0/1 just by turning the rotatable 40m Yagi or the 20m  
Yagi which is about 175 feet away and 65 feet lower.  Boom is 65 feet  
and elements are insulated.

Lesson learned on boom lengths...

##  the 65' boom is really  2 x 1/4 wave grnded booms....back to back. 
And you only require just one side of the boom [32.5'] to screw things
up.   OK, what are you using for a truss line on the 20m boom ?? 
If the 20m truss is steel cable, like ehs/winch cable, and terminates to the
mast, or it's own riser.....then you could install one egg insulator 
on at least one of the 2 x truss's... on one side,.. which  would
move the resonance point way off.   k3LR  had a similar  problem, and opted
instead to use a garolite insulator on the mast, to terminate all the truss lines. 

## either way, if say an egg is used in each half of the truss.... you also have the
option of bonding the 2 x 'hot sides' [ outboard of each egg].... and make the boom
into one big delta loop... like a metal coat hangar,which probably won't be required. 
You could also use a single  cap /coil instead of  bonding as above.  You could also 
place a cap /coil from the hot side of one/both eggs..to the mast. 

##  Now if ur truss is  phillystran, then u really have a problem.  IF  you have to  pull
down the 20m yagi,[and still want to use philly]  one end of the boom could have a short
ele  bonded to the boom, in front of the REF, which would  make for a F-12 style 'T-bar
capacity hat', which will LOWER the resonant freq of the boom.  Plan B  would be to
install a single ele into the end of the boom [past the REF].... electrically extending
the boom,and again lowering the resonance. Plan B  would upset the torque balance though.

##  In any event,  you would have to shift the resonance down a whole bunch..so the  65' long 
20m boom.. doesn't screw up the 40m yagi...[ or any other yagi via boom resonance on it's odd
harmonics 3/5. ]

Later... Jim   VE7RF   
  





I would prefer to fix it with the least amount of work.  I can  
probably do the wire thing on one side but not the other due to an  
weight-balance obstruction.  Hate to have about 15 feet of wire  
hanging out if the back of the boom.

Thanks...Stan, K5GO