RadioBanter - View Single Post - Length of Coax Affecting Incident Power to Meter?

August 14th 03, 04:13 PM

(Tom Bruhns) wrote in message om...
....
The resistance of the copper in the transmission line changes with
temperature. If ambient is 20C and you put in enough power to heat up
the line (center conductor) to 70C, that's a 50C change, and will
result in about an 18% increase in resistance. So if you had a line
which had 3dB loss at 20C, it would increase to about 3.5dB at 70C.
If the load end has a 2:1 SWR, then the sending end will have about
1.40:1 SWR at 20C and about 1.35:1 at 70C. It's not a _big_ change,
but it should be observable on an SWR meter that is accurate over a
wide range of powers.

Correction... First, thanks to Reg for his posting on the same
general topic. He pointed out that the RF resistances changes as the
square root of the DC resistance, and though I know that perfectly
well, had not put that into the above statements. With that
correction, I believe the result will be more like a change of 8% in
RF resistance, a loss change from 3.00dB to 3.26dB, and a change from
about 1.40 to 1.37 SWR. The point is the same; it would be observable
on a good SWR meter, though not distinguishable from other effects on
a typical cheap one.

Cheers,
Tom