You are correct about trying to measure power based on voltage but in
your example an SWR of 2:1 is the same as a reflection coefficient (Rho)
of 0.333 meaning that the voltage on the line will be between 66.6% and
133.3% of the voltage on the line for a 1:1 SWR. This would mean than
any power calculated from the voltage could be between 44.4% and 177.8%
of the correct value. Not as bad as your numbers but still pretty far off.
73, Larry W0QE
Joe Subich, W4TV wrote:
> Based on the schematic of the amplifier, the PO function is
> not a wattmeter but a simple capacitive voltage divider and
> rectifier on the RF output. A circuit like that has no
> directivity and is easily confused by reflected power and
> common mode currents on the feedline.
> If you are concerned about accurate power measurements,
> get a good wattmeter and use it in a matched circuit -
> SWR = 1:1 - and use common mode chokes between the meter
> and antenna. A capacitive voltage divider/rectifier will
> only provide accurate readings with no reflected power to
> "boost" or "buck" the voltage in the line. For example,
> if the SWR is 2:1, "power" calculated by using a voltage
> measurement can range from 25% to 400% of the actual
> power depending on the distance to/from the antenna (load).
> ... Joe, W4TV
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