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

[Larry Benko]

Phil,

Since the TDS-2XXX scopes are not 50 ohm input, don't ignore the 
capacitance in the probe.  See the appendix in the manual on page 153.  
The capacitance will be less at the 10X probe setting but not 
insignificant at 30MHz.

73,
Larry, W0QE

Phil & Debbie Salas wrote:
> Hi Paul - I haven't looked at the PowerMaster below 5-watts.  I don't 
> believee it measures SWR much below 5-watts for that matter.  However, I'm 
> curious.  I'll look at it tomorrow.  I have a Tektronix TDS-2022 that is 
> NIST-calibrated.  I'll see what that indicates compared to my PowerMaster 
> from 1-5 watts.
>
> Phil - AD5X
>
> "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" 
>
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