[TowerTalk] Re: coax as balanced line

[Pete Soper]

Bob Duckworth writes:

> The frequency determines skin depth but it doesn't
> change. So Ohmic loss for parallel wires is half
> of that of one wire. Loss will be less than the coax.

The inner surface of the shield is on the job for the single 
unbalanced coax run, replaced by the second center 
conductor for the balanced case. So there is some difference
in ohmic loss but wouldn't the trend be the opposite from
what you're asserting? And wouldn't it be dominated by the 
resistance of the center conductors for a very small net 
difference? 

> 2)Dielectric loss.
> 
> In a piece of coax E field (dielectric loss part) lives 
> in the dielectric pretty much entirely.
> 
> In two pieces of Coax (a balanced line with a shield
> for each conductor)  the field lives between the two 
> center conductors. In other words, there is a portion 
> of the field living in the air which has extremely low 
> dielectric loss. Loss will be less than the coax.

If this was true the "immunity to nearby metal objects"
of the balanced line case would be out the window. I think 
the fields are confined to the dielectric and inside the 
shields for this kind of balanced line. Note that the ARRL 
docs instruct us to tie the shields together and ground 
them on one end. It seems like this would make it tough 
for the fields to get out into the air!

Regards,
Pete
KS4XG

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