[TowerTalk] Terminate braid at top of tower...or

[Jim Thomson]

Date: Sat, 01 Nov 2014 22:04:41 -0700
From: Jim Brown <jim at audiosystemsgroup.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Fwd:  Terminate braid at top of tower...or
not?

On 11/1/2014 6:01 AM, Hans Hammarquist via TowerTalk wrote:
> Using a metal box to contain your current choke is OK

I disagree, because the box contains any heat produced in the choke, 
reducing the dissipation of that heat to the outside air, thus reducing 
the power handling of the choke.

The reason for bonding coax at top and bottom of a conventional tower 
has nothing to do with the shack, but rather is to keep the shield of 
the coax at the same potential as the tower, with the intent of reducing 
the likelihood of arcing between the coax shield and the tower in the 
event of lightning. In other words, it is to protect the coax.

73, Jim K9YC

## IF a hairpin coil is used at the feedpoint of the yagi /rotary dipole, the center
conductor is dc grounded to the braid, via the hairpin. If the braid is bonded to top
of tower, the center conductor is also dc grounded to top of tower. Dunno if a 
lightning arrestor is even required at all ? 

### My problem with the K9YC balun winding technique, as described
in his tutorial, is the winding technique itself.  He depicts these huge
loops of coax on the outside of the cores.  How is the completed assy
mounted to the feedpoint of the typ yagi ?  IMO, it would have to hang
well below the boom.  You don’t want those loops getting anywhere near 
the boom.  The stray C from the loops to the boom is the issue, they cant
touch the boom. You also have to be concerned with keeping the  
wide spaced loops separated from each other in the wind, etc.  The loops
would have to be away from the ele halves as well. 

## Ok, on something like a 80 or 40m rotary dipole, it would be next to impossible
to keep the loops away from the mast itself.   How close to the feed point does the
balun have to be?  Can it be .5-2 foot downstream ? 

##  metal boxes will work..barely.  As noted, the input and output connector shields
would have to be isolated from the metal box. Metal boxes are usually wet inside from
condensation, even if a weep hole is used.  Plastic, gasketed  nema boxes work better.
Seal em up tight, then puts some silica gel inside em.  At least inside a nema box, the
stray C to mast or boom can be controlled.  Baluns inside metal boxes means the 
boxes have to be oversized to minimize stray C  from coax windings to box. 

##  another similar winding technique is to use smaller diam loops on the outside of the
cores, and barely any spacing between the coax turns..external to the cores.  No doubt,
this would result in more C between turns, and higher uh overall, from the compressed
external turns.   Flip side is, with smaller diam loops, the uh may well decrease..so
it might come out a wash.  An extra core, or more turns could be used to tweak the assy. 

##  test gear on the bench is one thing.  Trying to get the desired CM at the freqs you want,
when installed, is another issue.  Im not talking about wire dipoles here. Im referring  to
yagis and rotary dipoles.  IMO, the real test would be to use a clamp on RF ammeter, and
run it up /down the coax, and measure the CM current with say 1 kw cxr applied at XXX
mhz.  Use that as your benchmark.  Then you can  directly compare different configs and baluns.   

Jim  VE7RF