[TenTec] OT 75 Ohm twin feeder

[Dr. Gerald N. Johnson]

On Sat, 2008-06-14 at 15:16 -0700, Jim Brown K9YC wrote:
> 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)?

Some antenna designs depend on the connections being as short as
possible (like the crossed feed line in the middle of the 8JK) and some
common mode chokes involve winding several turns of coax through the
toroid so the transmission line length is longer and that can upset the
phasing of the antenna elements.
> 
> 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
> 
73, Jerry, K0CQ