I would like to have a modern VNA. You should be able to do a pretty good
job with it. But most hams at best only have access to the older stuff.
While it is not too bad and can make some pretty accurate measurements it
must all be in cal to start with.
I even have some of the old Fluke resistance calibration equipment and while
it is pretty good it is a pain to use while checking everything several
times to get it right. Modern and even not so modern high end voltmeters
will run rings around the older cal equipment.
As far as I know Bird used the calorimeter method to do their wattmeter
calibrations.
73
Gary K4FMX
> -----Original Message-----
> From: Carl [mailto:km1h@jeremy.mv.com]
> Sent: Thursday, December 03, 2009 4:59 PM
> To: garyschafer@comcast.net; 'Dennis OConnor'; amps@contesting.com
> Subject: Re: [Amps] Dummy Loads & Wattmeters
>
> I used that spec knowing your probable response. Thats for the early
> 80's
> VNA's that are in many home shops and is a worse case spec at the higher
> frequencies using a 300 kHz to 3 gHz VNA. At HF I daresay it is a lot
> more
> accurate.
>
> If you step up to a more recent unit the accuracy is .05 dB , no fudging
> required.
>
> The current requirements for extreme accuracy in commercial and military
> equipment has obsoleted the 80's idea of performance. The Bird predates
> computers and was designed in the WW2 era.
>
> The still unanswered question is how Bird determined their calibration.
>
> Carl
> KM1H
>
>
> ----- Original Message -----
> From: "Gary Schafer" <garyschafer@comcast.net>
> To: "'Carl'" <km1h@jeremy.mv.com>; "'Dennis OConnor'"
> <ad4hk2004@yahoo.com>;
> <amps@contesting.com>
> Sent: Thursday, December 03, 2009 3:11 PM
> Subject: RE: [Amps] Dummy Loads & Wattmeters
>
>
> >
> >
> > Yes Carl, .5 db power accuracy plus .2 db frequency accuracy is .7 db
> > accuracy. That is approaching 12% error if everything else is right
> on.
> > Trying to calibrate a bird meter at full scale accuracy to 5% isn't
> going
> > to
> > happen with that setup.
> >
> > 73
> > Gary K4FMX
> >
> >> -----Original Message-----
> >> From: Carl [mailto:km1h@jeremy.mv.com]
> >> Sent: Thursday, December 03, 2009 2:03 PM
> >> To: garyschafer@comcast.net; 'Dennis OConnor'; amps@contesting.com
> >> Subject: Re: [Amps] Dummy Loads & Wattmeters
> >>
> >> Using a VNA it is possible to calibrate Birds, directional couplers,
> >> pads,
> >> cables. etc, at any frequency to within the instruments accuracy.
> Since
> >> the
> >> 80's, power accuracy is +/- .5 dB or better and frequency linearity
> is
> >> +/-
> >> .2 dB.
> >>
> >> That is close enough to keep most happy.
> >>
> >> The 436A power meter accuracy in dBm mode is +/- .02dB
> >>
> >> Using available instruments it is then possible to take any Bird line
> >> section and compatible element and use your own garden variety 50 ua
> >> meter
> >> as the indicating device. Using Jim Tonnes software any scale desired
> >> can be
> >> created including a red zone so element ratings are not exceeded.
> >>
> >> Carl
> >> KM1H
> >>
> >>
> >>
> >>
> >> ----- Original Message -----
> >> From: "Gary Schafer" <garyschafer@comcast.net>
> >> To: "'Dennis OConnor'" <ad4hk2004@yahoo.com>; <amps@contesting.com>
> >> Sent: Tuesday, January 01, 2002 12:52 AM
> >> Subject: Re: [Amps] Dummy Loads & Wattmeters
> >>
> >>
> >> It sounds good but,, you won't come as close as a bird meter doing
> that.
> >> Measuring the resistance of the load is the easy part.
> >> Measuring the RF voltage is the trick. Throwing together a diode
> setup
> >> will
> >> not get you very close. All diodes are non linear. Just like you
> can't
> >> replace the diode in a bird slug with any old diode. Bird has
> calibrated
> >> the
> >> meter scale to the response curve of the diodes that they use. If you
> >> look
> >> close at the meter scales you will note that they are not linear nor
> are
> >> they a true log response either. Also they are calibrated with the
> diode
> >> working into a specific load resistance.
> >>
> >> Some people try and make multiple scale wattmeters by using one slug
> and
> >> switch a pot in series with the meter for a second scale. This only
> >> works at
> >> one specific reading on the meter. The rest of the scale will not
> track
> >> because the diode in the slug is not working into its expected load
> >> resistance when this is done.
> >> There are a few people on the net selling kits and "calibration
> >> services" to
> >> do this. Don't be fooled.
> >>
> >> Trying to measure RF voltage with a scope "accurately" is not easily
> >> done
> >> either. First there is the "eye balling" of the reading on the scope
> >> that is
> >> not very good. Even if you have a more expensive scope that gives a
> >> digital
> >> readout of the voltage, you need to have it calibrated in a lab to
> know
> >> what
> >> you have. Then there is the accuracy spec on the scope itself.
> Getting
> >> one
> >> that has 1% accuracy at RF is not cheap.
> >>
> >> The next problem is how do you couple the scope to the load to read
> it.
> >> You
> >> can't take a BNC cable with one end connected to the scope and the
> other
> >> end
> >> connected to a T connector at the load input. It won't work.
> >> Placing a compensated scope probe at a T connector at the input of
> the
> >> load
> >> is better but you still have errors doing that. No way are you going
> to
> >> approach 1% accuracy. Remember when measuring voltage to find power
> that
> >> any
> >> voltage reading error is multiplied by the square of that error when
> >> calculating power.
> >>
> >> You would probably do better calibrating your scope using a bird 43
> as a
> >> reference.
> >>
> >> 73
> >> Gary K4FMX
> >>
> >> > -----Original Message-----
> >> > From: amps-bounces@contesting.com [mailto:amps-
> bounces@contesting.com]
> >> > On Behalf Of Dennis OConnor
> >> > Sent: Thursday, December 03, 2009 7:58 AM
> >> > To: amps@contesting.com
> >> > Subject: [Amps] Dummy Loads & Wattmeters
> >> >
> >> >
> >> > The simplest way for a ham to accurately measure to within 5% power
> is
> >> > to measure the RF voltage impressed across a known impedence...
> >> > There are a number of ways to refine your voltage measurement to
> >> > be within 5% deviation from NBS, but quick and dirty works just
> fine
> >> for
> >> > me...
> >> >
> >> > The very easiest is to get a dummy load... And get a 1% precision
> >> > resistor that is near to 50 ohms - Mouser, et. al. about a $1.70...
> >> > Use the resistor to measure your VOM and calculate a correction
> >> > factor then measure the dummy load, apply the correction factor
> from
> >> the
> >> > precision resistor and you know the DC resistance of your dummy
> with
> >> > near 1% precision... (ya, I know DC and RF ain't the same - I said
> >> > quick-n-dirty)
> >> > Once you know that, you apply the RF from the amp to the dummy load
> >> and
> >> > measure the RF voltage... That measurement can be direct, with an
> >> > oscilloscope, or by rectifying the RF to DC and measuring that...
> >> > If you are using a Fluke, or other quality VOM, you have a known
> >> > precision factor for the DC measurement and can directly calculate
> >> your
> >> > supposed precision value - just remember to add the 0.7 volt you
> lose
> >> > across the diodes, to the measurement...
> >> > If you are using an oscilloscope you can take the RF voltage number
> at
> >> > face value compared to the scope's built in calibrator - or there
> are
> >> > other ways, just like the VOM + Precision resistor, to derive a
> >> > correction factor... It would take a very long post to describe all
> >> the
> >> > ways to do that - but i trust hams to come up with simple
> solutions...
> >> >
> >> > So for most hams, a dummy load, precison resistor, a couple of
> diodes
> >> > and a cap, and a VOM, will get you into the ballpark of laboratory
> >> > precision for a few bucks... Then this can be used to tweak your
> >> > wattmeter to be right on (within your precison range) at the power
> >> > output you normally run - or to make a correction card for the
> >> readings
> >> > it currently gives (just like the compass on my boat)...
> >> >
> >> > denny / k8do
> >> >
> >> >
> >> >
> >> > _______________________________________________
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> >> > Amps@contesting.com
> >> > http://lists.contesting.com/mailman/listinfo/amps
> >>
> >> _______________________________________________
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> >> Amps@contesting.com
> >> http://lists.contesting.com/mailman/listinfo/amps
> >
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