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

[Patrick Greenlee]

With a heat source fed a constant quantity of energy (more or less a balun 
under test if not actual use) its temp will rise until the temperature is 
sufficiently above ambient for the delta temp to drive a loss to the 
environment equal to the balun's heat input.  Put the balun inside an 
enclosure and the additional thermal ohms imposed on the heat's path leading 
to the environment will raise the steady state temperature of the balun. 
How much the temp is raised depends on the enclosure.  If it were truly 
large (say 100s of feet on a side) the difference in the balun's temp would 
be negligible.  If the box had significant insulating value and was not very 
large the balun temp would rise significantly.

I suppose if the enclosure were properly constructed and vented a "chimney" 
effect could be generated with the ambient air being drawn through and or 
over the box such as to cool the balun better than having it in the open, 
sans box. Similarly, if the box had "fins" ala a heat sink and the chimney 
effect it might keep the balun cooler than the no-box example.

If anyone (or a group/consortium) should desire to fund me in researching 
designs of finned chimney "cooler boxes" for baluns just contact me off-list 
with your offers. ;) ;)

73,

Patrick NJ5G

-----Original Message----- 
From: Jim Thomson
Sent: Tuesday, November 04, 2014 5:08 AM
To: towertalk at contesting.com
Subject: [TowerTalk] Terminate braid at top of tower...or

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
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