The VNA uncertainty is less at lower attenuation levels. Supposedly better
than 0.1dB at 50 dB attenuation. And obviously it is zero at 0 dB (straight
thru coax) as that is where it is calibrated, The Array Solutions test
set-up uses a 30 dB pad so that 100 watts looks like 100 milliwatts on their
HP436A. Anyway, .The PowerMaster is supposedly within 5% accuracy across
the full range (not just at full power for a given element like the Bird) -
and that's what the ARRL lab tests showed as well.
I don't know why, but I've been obscessed with measuring power accurately
for awhile. I actually feel pretty good about measurements made with a
carefully built detector. I use a 1N5711 and SMD capacitors in the
detector, and use this across a 50 ohm load that has over 40dB return loss.
And then I built a precision voltage reference (0.5%) just to check my
digital voltmeter.
I need to spend more time on the air.
Phil - AD5X
----- Original Message -----
From: "Gary Schafer" <garyschafer@comcast.net>
To: "'Phil & Debbie Salas'" <dpsalas@tx.rr.com>; <amps@contesting.com>
Sent: Monday, December 07, 2009 7:01 PM
Subject: RE: [Amps] FW: Decent HF directional coupler for RF power meas.
> While that accuracy may be good for general tasks around the ham shack it
> comes no where near what is needed to do any kind of decent calibration.
> When changing from 60 to 70 db attenuators and getting .2 db difference I
> would wonder what the error would be at different levels.
>
> If you don't know that your attenuator is any better than .5 db and then
> you
> add that error to the calibration source you are not any closer than a
> bird
> wattmeter again.
>
> 73
> Gary K4FMX
>
>> -----Original Message-----
>> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
>> On Behalf Of Phil & Debbie Salas
>> Sent: Monday, December 07, 2009 6:10 PM
>> To: amps@contesting.com
>> Subject: [Amps] FW: Decent HF directional coupler for RF power meas.
>>
>> Good point. I just emailed W5BIG (the VNA designer) with this question.
>> He
>> said that the VNA2180 has been lab-tested with precision calibrated
>> attenuators. The VNA2180 was off a maximum of 0.3 dB with a 60 dB
>> attenuator, and a maximumn of 0.5 dB with a 70 dB attenuator through 140
>> mhz.
>>
>> Phil - AD5X
>>
>> > -----Original Message-----
>> > From: amps-bounces at contesting.com [mailto:amps-bounces at
>> > contesting.com]
>> > On Behalf Of Phil & Debbie Salas
>> > Sent: Monday, December 07, 2009 12:32 PM
>> > To: amps at contesting.com
>> > Subject: [Amps] FW: Decent HF directional coupler for RF power meas.
>> >
>> > "The cable and attenuator errors can be known without error, all he
>> has
>> > to
>> > do is pay a
>> > cal laboratory to characterize each at the test frequencies."
>> >
>> > I've watched the Array Solutions guys calibrate their PowerMaster
>> > couplers.
>> > They use their VNA2180 to accurately measure their cables and
>> > attenuators.
>> > So the only inaccuracy is the tolerance of the HP power meter.
>> >
>> > Phil - AD5X
>> >
>>
>> There is also the inaccuracy of the vna that has to be added in also.
>> They
>> don't say what that is.
>>
>> 73
>> Gary K4FMX
>>
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
>
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