[Amps] FW: Decent HF directional coupler for RF power meas.

[Paul Christensen]

>Anyway, .The PowerMaster is supposedly within 5% accuracy across
> the full range (not just at full power for a given element like the 
> Bird) -

Phil,

Do you have a way to check the accuracy of your PowerMaster below 2-watts? 
Below 1-watt, my PowerMaster deviated substantially from the Alpha 4510, 
LP-100, and a 200 MHz scope measured across a precision 50-ohm load, all of 
which measured very closely against one another.  The PowerMaster showed the 
typical signs of inaccuracy at low power due to diode detector 
non-linearities as discussed here:

http://www.arrl.org/members-only/tis/info/pdf/9002019.pdf

I am not stating with certainty that's the cause for the deviation with my 
PowerMaster, only the behavior is identical to inaccuracies seen with other 
common wattmeters using simple diode detectors.   And, as I recall, the 
LP-100 and Alpha 4510 use more complex, temperature-compensated detector 
designs.

Since this is the AMPS group, not too many folks here will care about 
low-power results.  But I think it's important to note some limitations of 
the PowerMaster that are not limitations with some of the other commercial 
wattmeters.

Paul, W9AC