Hi Gary - I have two PowerMasters. I live fairly close to Array Solutions,
so I went there and watched them cal the couplers on their HP set-up. It
gave me a lot of confidence. Since they put in trim factors to compensate
the coupler, there are no pots that are adjusted (or that could drift over
time and temperature). Anyway, just my particular preferance at this time.
But I still have my homebrew methods - though they are not as convenient
(gotta do some cypherin').
Phil - AD5X
----- Original Message -----
From: "Gary Schafer" <garyschafer@comcast.net>
To: "'Phil & Debbie Salas'" <dpsalas@tx.rr.com>; <amps@contesting.com>
Sent: Tuesday, December 08, 2009 11:46 AM
Subject: RE: [Amps] FW: Decent HF directional coupler for RF power meas.
> Hi Phil,
>
> I too have been obsessing about accurate power measurements over the last
> year or so.
> I made a coupler somewhat like that used in the typical wattcmeters, a
> torpid with the center conductor of the power cable passing thru it and
> this
> is placed right at the input to a 1kw bird load. But instead of going to a
> diode detector it feeds a small resistor load and then an attenuator then
> that goes to the power meter. I have a Boonton that is similar to the HP
> 436. I seem to get fair results with it but have not frequency compensated
> it yet.
>
> I thought that making a resistor divider right at the input to the dummy
> load might work rather well to, making the whole affair into a high power
> attenuator. With the rather high value of resistor required to tap into
> the
> 50 ohm line it should not upset the visor too much.
> I didn't have the proper value resistors on hand so it is on the back
> burner.
>
> However I found that measuring attenuators and getting different power
> meters and sources to agree is not simple. I thought that if I could get
> two
> or three things to agree with different test setups that I would be in the
> ballpark. Not easy.
>
> I have been thinking about the LP100 or one of the other wattcmeters but
> at
> this point I don't have high confidence in any. What have you found?
>
> 73
> Gary K4FMX
>
>> -----Original Message-----
>> From: Phil & Debbie Salas [mailto:dpsalas@tx.rr.com]
>> Sent: Monday, December 07, 2009 9:11 PM
>> To: garyschafer@comcast.net; amps@contesting.com
>> Subject: Re: [Amps] FW: Decent HF directional coupler for RF power meas.
>>
>> 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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