[TowerTalk] common mode chokes, baluns and multiband doublets

Steve Hunt steve at karinya.net
Tue Dec 12 07:12:26 EST 2017


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:

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

On 11/12/2017 21:33, k7wxw wrote:
> I am putting up a centerfed doublet to cover 80 through 10 meters, focusing on 80, 40, 30, and 20 meters. I plan on feeding it with a random length of 450 ohm window line. I have a Johnson Matchbox and Dentron Supertuner and can use either to match the antenna to my rigs.

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