I'm assuming that he is putting the 2 cables in parallel. This will result in
a composite cable with a surge impedance of half of the single cable, so two
50Ω cables in parallel will look like a 25Ω cable. First, remember that you
will need to match the 25Ω cable to 50Ω at each end.
Now, let's assume we have 100 watts of RF. In the 50Ω system that means that
1.4 amperes of RF is flowing in the cable (I²R). In order to move 100 watts in
a 25Ω system, I² must be doubled so I=2 but since there are two identical
cables, each cable carries half of that current or 1 ampere. At HF
frequencies, most of the coax loss is attributed to the RF resistance of the
conductors so the loss will be proportional to I².
Let's look at the I²R loss in each scenario assuming Rx = conductor
resistance;1 cable: I=1.4 amperes so I²Rx=2Rx in the single cable.
2 cables: I=1.0 amperes so I²Rx=1Rx in each cable or 2Rx for the composite
cable.
This shows that the power lost from the conductor resistance is the same for
both scenarios.The same line of reasoning will apply to the dielectric losses
since the voltage in the line decreases by a factor of 0.707 and losses in the
dielectric are proportional to E²/R.
I think the losses in both systems will be identical except for losses incurred
in the matching systems.
-Mike-WA6ZTY
From: rfman45 <rfman45@hotmail.com>
To: "towertalk@contesting.com" <towertalk@contesting.com>
Sent: Sunday, December 18, 2016 9:48 AM
Subject: [TowerTalk] Double Coax Runs to Reduce Loss?
I was at a hamfest recently and a seller there was discussing how he uses two
equal length runs of the same coax (an LMR type) to halve his losses on a long
run to a distant tower some 300 or so feet away. His point was that he had a
lot of this type of coax around and therefore it was better to do this than to
buy something more efficient.
I never heard this before. What does the TT community say?
Mike W2LO
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