-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Gary Schafer
Sent: Tuesday, January 01, 2002 12:52 AM
To: 'Dennis OConnor'; amps@contesting.com
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
>
>
>
> _______________________________________________
> 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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|