[TenTec] OT 75 Ohm twin feeder

[Dr. Gerald N. Johnson]

On Thu, 2008-06-12 at 15:04 -0700, Stuart Rohre wrote:
> The rated breakdown voltage of the coax would be increased if only using 
> the shields as conductors, as you would have two jacket thicknesses 
> between shields.  You could even sleeve the coax outer jacket with 
> additional heat shrink tubing for thicker and thus higher voltage 
> insulation.

Yes, but the jacket is not made as a RF dielectric, but as a climate
protection for the braid and good center conductor insulation that is
quite sensitive to sun and ozone. It has no voltage breakdown rating. It
may have a voltage breakdown value. But that's not part of the coax
specification. Making the jacket thicker would raise the characteristic
impedance too.

Long ago (mid 60s), a friend, K0DOK made a VHF vertical out of 1/2 wave
(in coax) pieces of RG-58 (as published in QST and I think the ARRL
Handbook) connected with braid and center conductor alternating. That
cross connection puts the pieces in phase. Its intended to be supported
in a hollow fiberglass fishing pole. It worked poorly for him. He had
access to the antenna range where he worked and he tested it there. He
found the pattern (vertical anyway) as expected but the field strength
low (poor efficiency). He had left the jackets on each piece. He
stripped the jacket pieces off, then had the same pattern, but field
strength he expected. That jacket was providing lots of loss.
> 
> Ten Tec L net tuners are lower loss than most common tuners, since they 
> have one less loss producing component, and are typically built well. 
> (Rugged).

And the L network has only one match point, always at the optimum loaded
Q for the impedance ratio. The T or Pi match has many tuning points, all
with higher loaded Q that makes the circulating current in the match
circuit higher than the L and thus the loss higher for any given
impedance ratio for the same ruggedness of components.
> 
> Stuart Rohre
> K5KVH
> 
> 
73, Jerry, K0CQ