[Amps] AL 1500 P.O. Meter Function

[Larry Benko]

Joe,

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). 
>
> 73, 
>
>    ... Joe, W4TV 
>  
>   
>