[TowerTalk] Baluns and SWR

[Tom Rauch]

> Tom, if you are bringing your coax cables down to a coax switch mounted
> near the bottom of the tower, would you still recommend bonding the shield
> of the coax? If so, do you bond it as close to the balun as possible - the
> mast of the beam, or somewhere on the tower, or does the grounded coax
> switch take care of this? I am planning on using the Force 12 B-1 baluns.

The coax should always be grounded at the top of the tower when 
a balun is used. That insures the cable shield is at tower potential 
for both lightning and RF. 

I had a bad case of TVI once because a barrel connector was just 
barely away from the tower leg (held away the distance of a few 
layers of tape), it would arc to the tower leg. Just the smallest arc 
100 feet in the air will really wipe out TV sets! I had TVI for a mile 
on fringe channels.

I also had a case where a feedline was arcing to the tower leg 
through the jacket of some hardline through a steel hanger 1/4 wl 
down from a dipole's feedpoint! Grounding the cable at the top fixed 
both problems, so I now make it a point to ground at each end of 
the tower. Usually I have an antenna switch at the bottom. 

It is important for both RF and lightning (which has high levels of 
RF energy) that the shield is grounded.

I ground the cable as close to the balun as I can, but only on the 
shack side of the balun. You certainly do not want to ground the 
shield around 1/8 to 3/8th wave away from the feedpoint if you are 
using a choke balun, because the balun works best when the 
common mode cable impedance is low.

It's a good idea to have a good connection between the mast and 
the tower, if you have a rotor. 


73, Tom W8JI
W8JI@contesting.com 

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