[TenTec] OT 75 Ohm twin feeder

[Jim Brown K9YC]

On Wed, 11 Jun 2008 11:41:48 -0600, Dr. Gerald N. Johnson wrote:

>In short lengths, so long as the coax wasn't tied to a metal structure,
>coax connected between balanced loads might not cause much unbalance
>despite the lack of symmetry in the coax. 

Ham antennas are unbalanced by their surroundings. Coax simply ADDS 
(algebraically) to that imbalance. 

>Certainly the common mode
>choke would improve the balance seen at the coax terminals. But when
>only a couple feet is needed, will the antenna still work when the coax
>is twice that long, all coiled up around the toroid(s)?

I don't understand the question. The differential mode circuit through the coax 
is simply that length of coax, whatever it is. In other words, it's a short 
(mis-)matching section. The common mode choke places a high resistive impedance 
between the antenna and feedline, which effectively disconnects the antenna from 
the feedline for common mode current. In this condition, only differential 
current can flow (that is, inside the coax). 

Sorry to be late with replies -- we've had a major wild fire here, and have been 
off line for several days.

73,

Jim Brown K9YC