[Elecraft] OT - RG6 dual coax as feedline?

[Alan Bloom]

On Sun, 2009-04-26 at 10:33 -0700, Jim Brown wrote:
> On Sat, 25 Apr 2009 17:29:15 -0400, Don Wilhelm wrote:
> 
> >My reason:  The paralleled line certainly will work, but the loss is the 
> >same as a single coax of the same material, 
> 
> I don't agree, Don. Loss in coax at HF is all I squared R. If you match to 
> twice the impedance, you'll have half the current. 

Actually you'll have 1/sqrt(2) or 0.707 the current.  So you'll have 1/2
the power loss in each coax, but there are two coaxes so the total power
loss is exactly the same as with a single coax.

As long as everything is matched it doesn't matter how many coaxes you
use.  The loss is always the same.

> This means that the 
> fraction of the loss in the center conductor will be 6 dB lower at the 
> higher impedance. On the other hand, the loss in most coax shields is lower 
> than the loss in the center conductor, so I would expect losses to be about 
> 3 dB lower, not 6 dB. 
> 
> >and in a multiband 
> >situation, it certainly will be run at a high SWR. 
> 
> Agreed. This is not a good solution for antennas of random impedance. But it 
> IS a good solution for antennas that match to about 2.5:1 or better. 

The same could be said about using a single coax as well.  One advantage
to using two coaxes is that each carries only half the power (0.707 the
voltage), so you get twice the power-handling capability.

One issue with making parallel-conductor line with two coaxes is that
they must have the same electrical length.  If they are cut from the
same roll of high-quality coax, then identical physical length should
equate to identical electrical length, but it still might be worth
checking to be sure.

> 73,
> 
> Jim K9YC

Al N1AL