Date: Tue, 12 Dec 2017 12:12:26 +0000
From: Steve Hunt <steve@karinya.net>
To: towertalk@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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|