[TowerTalk] re: coax as balanced line

[Jim Reid]

At 01:32 PM 8/1/97 -0600, John Brosnahan wrote:
>
>I have used a balanced RG-213 feedline on my 8/8/8/8 10M array
>that got such good EME echoes when it was up.  It was my contention 
>that the loss would stay the same--simple thought problem.  

Depends upon how you connent the parallel runs of coax:
to quote from the ARRL Antenna Book, 17th ed, pg. 24-18:

"  Shieled balanced lines have several advantages over open-
wire lines. Since there is no noise pickup on long runs, they
can be buried and they can be routed through metal buildings
or in side metal piping.  Shieled balanced lines having impdances
of 140 or 100 ohms can be constructed from two equal llengths of
70 or 50 ohm caslbe.  Paralled RG-63 (125 ohm line) cable would make
a balanced line mdore in acord with traditional 300 ohm twinline,
actually yielding about 250 ohm Z.

The shields are connected together and the two inner conductors
constitue the balanced line.  At the input, the coaxial shields 
should be connected to chassis ground; at the output (the antenna side)
they are joined,  but LEFT floating.

A high power,  low loss, low-impedance balanced line can  be
made using four coaxial cables, see Fig 23B for connection
set up.  Again the shields are all connected together, and
the center conductors of the two sets of coaxial cables
that are connected in parallel provide the balanced feed."

Eveidedntly the cables do act as parallel resistances, so
the R that causes loss is cut in half,  so down goes the
loss.

73,  Jim, KH7M


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