[TowerTalk] common mode chokes, baluns and multiband

[Jim Thomson]

Date: Tue, 12 Dec 2017 12:12:26 +0000
From: Steve Hunt <steve at karinya.net>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] common mode chokes, baluns and multiband
doublets


<Bill,

<I use a very similar antenna system. I measure best feedline current 
<balance when I use an unbalanced tuner (Ten Tec 238) connected through a 
very short length of RG213 to a 1:1 CM choke mounted just outside the 
shack window where the window line terminates. The choke comprises 11 
bifilar turns on 2 stacked FT240-52 cores, similar to Balun Designs 
model 1171:
http://www.balundesigns.com/model-1171-1-1-atu-current-balun-1-54-mhz-5kw/

To illustrate Jim's point about antenna systems rarely being balanced - 
visually mine appears symmetrical, however the star-network equivalent 
circuit at 3.5MHz measures:

Leg1/Centre: 15.1 - j79 Ohms
Leg2/Centre: 1.6 - j109 Ohms
Centre/Ground: 30.7 + j110 Ohms

Looking at the top two impedances - Leg1/Centre & Leg2/Centre - there is 
clearly some significant electrical imbalance despite the apparent 
physical symmetry.

Steve G3TXQ

##  The balun designs  Tuner balun..and also the similar tuner balun made by Danny
Horvat  http://myantennas.com/wp/product/atu130-5k-tuner-balun/  are ideal
for the task in hand.   The horvat version uses  3 x torroids in the stack and is good from 1-30 mhz. 
For 40-10m the  balun designs http://www.balundesigns.com/model-1161-1-1-atu-current-balun-3-54-mhz-5kw/
is ideal.   For optimized lower bands the balun designs http://www.balundesigns.com/model-1171-1-1-atu-current-balun-1-54-mhz-5kw/
works good.
  Polyimide wire in the bigger gauges is rated for 15 KV. Slide
it into terflon spagetti tubing and now the peak V rating is sky high.    Then if you want,
follow it up with a CM line isolator, like the http://myantennas.com/wp/product/cmc-230-5k/
Use a double male coax connector or a real short jumper to connect the two boxes together. 
Then a short piece of coax to the shack, into the coax to coax tuner,  typ a T type.   That
eliminates bringing open  wire line into the shack. 

## IF u do bring open wire line into the shack, use a balanced tuner, not a  T type, that
is re configured for open wire line, none of those are truly balanced.  Balanced tuner
would consist of  2 x roller inductors and one cap, like a 500 pf vac cap.  Both rollers
driven simultaneously.  Or use a Johnson match box, or modified match box.  

##  The old timer, VE7BM, who was my elmer back in 1972, used a HB  arrl  link
coupled tuner, and hb open wire line  to a windom.    Ran flawless  from day 1  till he
was sk. 

Jim  VE7RF