[TowerTalk] 1/4w matching, Common mode chokes and BAL-UN

[Jim Thomson]

Date: Fri, 31 Dec 2010 20:05:57 +0000
From: Steve Hunt <steve at karinya.net>
Subject: Re: [TowerTalk] 1/4w matching, Common mode chokes and BAL-UN

I have some modeled examples on my web site which illustrate the effect 
Jim [K9YC] is talking about. Take a look at the section "Why reactive chokes 
are undesirable" on this page:
http://www.karinya.net/g3txq/chokes/

The section finishes: "We conclude that a high value Resistive choke is 
the safe option for all scenarios. We also conclude that Rules-of-Thumb 
which equate the required choke impedance to some multiple of the 
differential-mode load impedance are unsound."

73,
Steve G3TXQ

##  excellent info. My conclusion is... for a monoband yagi set up, use a 1/4  or 
3/4 wave of coax  from DE  to tower... IF possible.. or close to it.  Grnd braid
at top of tower or on mast.  Or grnd braid  via the remote switch box, if remote switch
box located at top of  tower, or on mast.  OK, then use a balun at the DE....that is  mostly resistive. 
IF the balun is mostly resistive, the combo of a phys 1/4 wave  coax [x .98]  + resistive balun
= excellent combo.  I can pull that off on 20-17-15m.  Can't be done on 80-40m. Since my
remote switch box is 1/2 way up the mast, a line isolator will be located at the Input of the remote
switch box.  The line isolator is just 393 wrapped around some type 31 cores...with connector's 
on both ends.  It's job is to ...'clean up'  any residual CM currents, b4 the single coax heads down the
tower.  Braid of single coax is  bonded to  base of tower /gnd sytem  via lightning arrestor. 

## Coax then routed 75'  along the grnd to just inside of  basement..and terminates on a 2nd
lightning arrestor..which is bolted to the SPG [large sheet of 1/4" thick AL plate].  More coax to LP
filter [also bolted to SPG plate], then more coax to a 2nd line isolator.... then more coax to
wattmeter coupler/amp etc. Both line isolators are inside plastic nema box's, with type 31 cores
suspended in mid air inside  the nema  box. [to minimize stray C] .   If all that doesn't do it... I give up. 

later... Jim  VE7RF