[AMPS] Power Meter Readings Question

[Peter Chadwick]

	>SWR changes, in connection to line lenght changes, when the antenna
	>impedance is different than that of the line and the generator.

The only thing that changes SWR with change of line length is the
attenuation of the extra cable. Input impedance changes, SWR is constant
with line length for a zero loss line.


	>It's obvious that a 50 Ohm wattmeter, unless derived by a
thermocouple,
	>doesn't indicate a real power if the load ( as seen through the
line or an
	>antenna tuner ) is largely reactive, or even resistive, but away of
the
	>nominal value.

Even a thermo couple will give the wrong result if the SWR is high, since
the impedance at the point is unknown. Incidentally, the accuracy of
thermocouples needs checking with frequency. It's often said that the
advantage of the thermocouple is that you can calibrate it with DC.
Unfortunately, the resistance of the heating element varies with frequency
because of skin effect - more so if it's a magnetic material. This can lead
to major errors.

73

Peter G3RZP




--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm