[TowerTalk] Re: coax as balanced line

[Jim Reid]

At 06:32 PM 8/1/97 -0400, you wrote:
>Hi Jim,
>	I vote for the loss being identical to that of a single run of the
>same coax.

Hi Pete,

Well if the two transmission lines are identical
and everything is matched, then one half of the 
RF current is going to flow in each line. Note that
to be matched the source and load impedances must
be double the characteristic (surge) impedance of the
transmission line used. 

In fact,  would be very weird if this were
not the case!!  

Power lost is current squared times the loss resistance.  
So if half the current is in each identical line,
then one half squared is one fourth.  Add the two fourths
in each line together and you have one half of the power
lost now as compared to a single line with all of the
current flowing through it to the load.

So QED,  there is less loss by half when using two parallel 
lines to feed a load.

Note that to feed a 50 ohm antenna system,  you must use four
parallel coax lines:  a pair of lines for one side of the transmission
line,  and a pair for the other side, as in the 
ARRL Antenna Book.  Then you will be back to a 50 ohm line,
and have one fourth the RF signal loss experienced with
just one of the coax lines used by itself.

73,  Jim,  KH7M


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