Date: Sun, 12 May 2013 12:24:29 +0100
From: Steve Hunt <steve@karinya.net>
Cc: "towertalk@contesting.com" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Force 12 B-1/B-1S Baluns
Even with a perfect 50 Ohm antenna, there can be significant coax braid
current and stress on any balun that is installed!
With a perfect 50 Ohm antenna, at the 1500W level the differential-mode
feedpoint voltage would be 274v. If the antenna was perfectly balanced
with respect to ground, that could be a CM voltage of 137v across the
balun (assuming, worst case, the coax CM path impedance is low). If the
balun CM impedance was 250 Ohms resistive - meeting the often-quoted "5
times load impedance" recommendation - that would mean a dissipation of 75W!
As you can see, a perfect 50 Ohm, balanced, antenna does *not* prevent
CM current or balun stress. There is no substitute for high CM balun
impedance - preferably resistive.
73,
Steve G3TXQ
## why do they have to be resistive ?? Current squared x resistive ohms =
lotsa heat.
Current squared x XL = no heat, or minimal heat.
## If the balun is mainly resistive, it will work, but the Z has to be sky
high, so you don’t cook the
balun.
## The F-12 balun B1 baluns and also B1-S use the tiny type 43 beads..slid
over 303 teflon coax.
Never seen a B1 overheat with 1.5 kw rtty.
## the cal-av baluns use the larger type 43 beads. 1.125” long x .515” ID
x 1” OD. 17” overall
long, with a 7-16 DIN, everything is silver soldered. 10 ga teflon wire used
at balanced end. Pumped
full of silicone varnish glop in a vac chamber. Beads are slid over 393 teflon
coax.
Nobody ever over heated one of em with 10 kw rtty.
## another method is to just use a physical 1/4 wave long ( –2%) piece of
coax from top of tower
to feedpoint of yagi, with braid grounded at top of tower. If a 1/4 wave won’t
reach, the next one is
3/4 wave long. Then you end up with a sky high Z at the feedpoint....then no
balun required. But that
is a monoband device, but would work on 40m + 15m.
## another size bead that works superb with RG-393 teflon coax is made by
fair rite, but only comes
in type 43 material. .75” OD x .4” ID x 2” long. 393 coax has a .391
OD. These beads fit like a glove.
However they will NOT slide over RG-213 U..which has a .405” OD .
## another method is to wrap 4 turns of 393 teflon coax through 4-8 x 2.4”
OD type 43 or type 31 cores,
then put the entire mess inside a nema box.
Jim VE7RF
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