[ARC5] Silly Inaccurate Wattmeters and a Tool to Fix Them.

[Tom Lee]

Hi Dave,

Your maths is correct, but there are more steps than you need.

Square the peak, move the decimal to the left two places, and you're 
done. That's one of the benefits of a 50 ohm system.

No need to consult tables (unless it helps you with the squaring operation).

-- Cheers
Tom

-- 
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 8/7/2022 04:19, David Stinson wrote:
> I recently was graced and blessed by a dear soul
> with a modern, calibrated scope which has made it
> possible to begin calibrating my test gear, most
> of which has been shockingly "out to lunch."
> There is a web site with calculators which are
> invaluable for this.
>
> For instance:
> If you set your calibrated scope for 50V per
> vertical division (5 V if using a calibrated
> x10 probe), connect a *non-reactive*
> 50-ohm load to your transmitter, key a carrier
> and scope the output, you can read the peak-to-
> peak waveform.  Say it's 4 divisions, which is
> 200V Peak-to-Peak.  Easy way to tell your true
> power into that load: Divide the reading by 2,
> which gives you "Peak Voltage," then multiply
> by 0.707, which will give you RMS Voltage
> (70.7 V in this example).
>
> Take that figure to this page:
> https://www.pasternack.com/t-calculator-power-conv.aspx
> Select Input Type as "Volts," output type as "Watts,"
> enter the RMS voltage and hit "Calculate."
> The result: You are delivering a true 100W to the load.
> If you modulate your carrier and read the vertical scale,
> you can calculate peak power with modulation.
>
> These folks have a bunch of useful calculators at
> https://www.pasternack.com/t-rf-microwave-calculators-and-conversions.aspx 
>
>
> The point of all this is to talk about our typical ham Wattmeters.
> I have Drake W-4, the Drake 7-line meter, other makes and
> a couple in antenna tuners.  NONE of these are even close
> to accurate, nor can you validly calibrate them.  They
> are ridiculously non-linear.  If you pump a scope-confirmed
> 20 watts through them and set the adjustment for 20 Watts,
> then drop the power to 10 or raise it to 40 the meter
> will read way off.  All of them.  I have a Bird at
> work but no HF slugs- maybe that one would do better.
>
> I conclude that "ham grade" Watt meters are good for
> relative power indications and tuning-up, but any
> Power reading they give you is likely nonsense.
>
> What has been your experience?
>
> GL OM ES 73 DE Dave AB5S
>