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
> >
> >
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|