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

[Bob kb8tq]

Hi

Having spent a *lot* of time doing this sort of thing with
gear that is “in cal” according to the little stickers on the
front pannel …. 

1) Scopes tend to be a bit questionable for measuring power.
There are normally a number of error sources. That’s not to say 
all are poor in all cases, just that I’ve seen a lot of examples that 
did not make them the first thing I’d grab for an accurate reading. 

2) Good spectrum analyzers usually did pretty well at modest levels 
of input attenuation. Low cost spectrum analyzers …. maybe not 
so much. 

3) Dedicated power meters ( HP 436/437 style devices)  
typically all agreed quite well with each other. Yes, both the 
“head” and the “meter” needed to have valid stickers on them. 

4) Watt meters ( of any sort ) typically tended to disappoint. 
Full scale might be pretty close at some specific frequency.
Anything below about half scale …. yikes ….

Is this a knock on scopes? Most certainly not. They are a 
very useful device. It’s as much a knock on not having things
like passive probes and fancy adapters handy as anything 
else. 

Does this mean that a HP437 was perfection? Certainly not. 
They are fine for measuring a single carrier. Once you get into
something complex …. umm …. errrr …. Get something 
different.

One “interesting” way to deal with all this are calibrated / high 
power attenuators.Your Ham transmitter puts out way more than 
the typical / affordable surplus device will handle. I would focus 
more on getting one or more of them first. Next would be a 
fixed frequency / fixed level reference source at a reasonable 
frequency. Then you can calibrate whatever you use right as 
you go to use it. 

Bob

> On Aug 7, 2022, at 3:19 AM, David Stinson <arc5 at ix.netcom.com> 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
> 
> -- 
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