[TowerTalk] Re: coax as balanced line

[John Brosnahan]

>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.
>
>Convinced parallel line has lower loss?
>Want to know how much lower?
>Do the math.
>Comparing a sheet to a solid cylinder around the wires 
>should be give a fair approx of the difference.
>
>-bob
>wb4mnf


Bob, the original discussion was about using the two CENTER
conductors of coaxial cable as a balanced and SHIELDED 
transmission line.   In that case you gain nothing as far as reducing 
the losses over what you would have with a single line.  You end up 
with a 100 ohm balanced line that can be useful for certain 
applications where a shielded, balanced line is needed--or where a 
balanced 50 to balanced 200 1/4 wave transformer is needed.  (Or 
use RG-11 and end up with a 50 ohm balanced to 450 ohm balanced
transformer--useful to get from coax (with a 1-1 balun) to regular 
open wire line.

It was then proposed to parallel the two (or more) coaxial lines by 
connecting shields together as well as connecting the center 
conductors together in order to reduce the cable losses by reducing 
the ohmic losses by making 25 ohm cable.  Although this reduces 
the ohmic losses by a factor of 2 it also reduces the characteristic 
impedance by a factor of 2 and nothing is gained.  In addition IF you 
need to make a match to the 25 ohm line you will incur additional 
losses associated with whatever matching scheme is used.

What you are propossing, using the two shields as a balanced
line, will certainly work--and in fact it is the answer to the TRICK
question that I have posed to others previously about how to 
make a low loss transmission line out of a pair of coaxial cables.  
The parallel connection or the serial connection are the obvious 
answers that really gain you nothing (but the need for more coax).  
The actual answer to my trick question is, as you have proposed,
to use the shields as an open wire line.  

Of course you still have to concern yourself with keeping the 
line away from conductors as you would with any open wire line.
The PVC jacket is a lossy dielectric material, but it is a relatively
small percentage of the total dielectric, with air being the larger
part of the dielectric.  It would be relatively easy to calculate the 
loss of this type of open wire feedline.  The PVC jacket MAY
be the limiting factor for how much power this type of feedline
could handle, rather than the heating of the conductors, due to 
heating in the PVC from the relatively strong fields.

73  John  W0UN


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