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
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>
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