[TenTec] OT 75 Ohm twin feeder

[Dr. Gerald N. Johnson]

On Thu, 2008-06-12 at 23:05 +0100, Steve Hunt wrote:
> Jerry,
> > Two pieces of 75 ohm coax in parallel (corresponding conductors
> > connected together at each end) gives 37.5 ohms. Two sets of those in
> > push-pull gives 75 ohms shielded balanced. Or the same with 50 ohm coax
> > gives 50 ohms balanced.
> >   
> That would work, but using four lengths of coax to replace one seems a 
> bit OTT
> > The jacket is the least consistent part of the coax and I believe it to
> > be loaded with carbon to make it black and UV resistant. That would make
> > it lossy (and slightly improve the shielding properties) to some extent.
> >   
> I've just ordered a short length of RG-400 to try in place of the 
> RG-58.  It seemed to me the outer jacket  (FPE) would have better RF 
> characteristics than the PVC.

It should. See my post in response to K5KVH created in the last few
minutes. I remembered an instance of known RF loss in the jacket of
RG-58 coax.
> 
> Tried the RG-58 option on my Hexbeam today and it took the max I could 
> throw at it - 400W.

Jackets do vary. They have always varied in how they protected the
center insulation from the sun and how much plasticizer they let migrate
to the center insulation drastically increasing its RF loss. I have
little trust in them as RF insulation.
> 
> Steve G3TXQ
> 
73, Jerry, K0CQ