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

[Phil & Debbie Salas]

Yes - I'll be using the 10X probe setting and doing the measurement at 
1.8mhz.
Phil
----- Original Message ----- 
From: "Larry Benko" <xxw0qe at comcast.net>
To: "Phil & Debbie Salas" <dpsalas at tx.rr.com>
Cc: <amps at contesting.com>
Sent: Monday, December 07, 2009 10:05 PM
Subject: Re: [Amps] FW: Decent HF directional coupler for RF power meas.


> 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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>>
>