[TenTec] OT 75 Ohm twin feeder

[Steve Hunt]

I just got a length of RG400 to experiment with in the "twin feed" 
configuration - I'm quite pleased with the results which should be fine 
for my HexBeam application.

With the measured braid diameter (3.9mm) and FEP jacket diameter 
(4.9mm), you'd predict a Zo of 58 Ohms and Vf of 0.69. However, in this 
configuration air still forms a significant part of the dielectric and 
the effective Dielectric Constant reduces from 2.1 to 1.56, increasing 
the Zo to 67 Ohms and the Vf to 0.8

Assuming the FEP is not modified in some way, its low loss-tangent 
(0.0002) results in a predicted loss of 0.024dB / meter at 30MHz. 
Together with a Dielectric strength of 50kV per mm I reckon the power 
handling should be fine.

The cost of RG400 probably makes it prohibitive for normal feeder 
applications, but it'll be fine for the short lengths needed to 
interconnect HexBeam elements (18" max).

There's a photo on my website of the configuration before I swapped the 
RG58 for RG400:
http://www.karinya.net/g3txq/hexbeam/gallery/index.html?cat=3&n=20

73,
Steve G3TXQ