On 12/18/16 9:48 AM, rfman45 wrote:
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.
How does he do impedance matching?
But, in general, no, this doesn't help - Consider the loss of a length
of coax as being, say, X dB. Now split your power into two paths, and
send each half down a separate piece of coax. You'll get X dB loss in
each path, and when you combine them at the other end, you'll have X dB
total loss.
There are some cases: where the lines are mismatched, or very short -
the "X dB/100 meters" is for a "long transmission line" where theres
multiple wavelengths. If you've got a 1/8th wavelength and the system
isn't perfectly matched, the actual loss will depend on whether you're
at a high current point or a high voltage point. For HF, the dominant
loss is IR losses not dielectric dissipation.
SO let's think of another case:
Say the resistance of the coax is 1 ohm. If I transform my signal to
100 ohms, the current will be halved, the voltage doubled, and run two
pieces of coax in series (so it's 100 ohms). But now I have 2 ohms in
the transmission line, and the loss is exactly the same.
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