[TowerTalk] Force 12 B-1/B-1S Baluns

[Jim Thomson]

Date: Sun, 12 May 2013 12:24:29 +0100
From: Steve Hunt <steve at karinya.net>
Cc: "towertalk at contesting.com" <towertalk at 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