[TenTec] OT 75 Ohm twin feeder

[Dr. Gerald N. Johnson]

On Tue, 2008-06-17 at 12:24 +0100, Steve Hunt wrote:
> 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).

If you can actually buy it by the yard. Many of the exotic coaxes at US
distributors are sold only by the 100' or 500' roll. Or you buy it by
the roll and sell by the yard, hoping the roll runs out just as the last
order comes in.
> 
> 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
> 
> 
> 
73, Jerry, K0CQ