[Amps] Time for New Power Meter

Mon May 4 13:20:43 EDT 2015

  Measuring power, especially over a substantial range of frequencies, has always been a difficult task.
If it is done by measuring voltage or current, a 1% error in voltage represents 2% error in power.
Taking that into account and the calibration of the element plugged into it which has a frequency response 
which limits its accuracy. 5% as an guaranteed maximum error is not that bad. Just look at the analog AC volt meters.
If you get one that is specified to have 2% of full scale accuracy you are doing real good and that translates to
4% in power. We have not even thrown in temperature concerns. If you were trying to measure the power into
a purely resistive load at 60Hz with a pair of good old reliable analog Simpson 260 meters, one to measure voltage and
the other to measure current via a resistor in series with the load, you would be lucky to get a result within 
5% or the actual value as measured by a calorimeter. Errors creep in from all directions and when they all add up
you find your measurement way off.  It was a long time before hams started actually measuring output power.
Accurate power meters were just too costly for most. I would have loved to have a good power meter in the 
60's so I could determine the efficiency and power gain of my amplifiers. 
  73
Bill wa4lav

___________________________________
From: Amps [amps-bounces at contesting.com] on behalf of Jim Thomson [jim.thom at telus.net]
Sent: Monday, May 04, 2015 7:47 AM
To: amps at contesting.com
Subject: [Amps] Time for New Power Meter

Date: Sun, 3 May 2015 21:38:01 -0700
From: Bill Turner <dezrat at outlook.com>
To: Amps group <amps at contesting.com>
Subject: Re: [Amps] Time for New Power Meter

------------ ORIGINAL MESSAGE ------------(may be snipped)

On Mon, 04 May 2015 08:09:47 +0800, you wrote:

>Might I be allowed to suggest that Accuracy +/- 5% of Full Scale Power"
>means that the best accuracy they guarantee is that 5% at full scale.
>
>At lower readings, it may still be 5% or it might be a little more.
>Whatever the figure might be, to cover their ass, the manufacturer is
>not going to make any guarantees other than at full scale.
>
>To suggest +/- 125 Watts anywhere on the scale, for a 2500 Watt range,
>simply does not make sense and it would make a total mockery of the
>using the instrument as a power measuring device.
>
>73, Alek.
>VK6APK

REPLY:

Perfectly stated. Thank you, Alek.

73, Bill W6WRT

##  Nice try Bill...it doesn’t work that way.   Its +/- 5%  of full scale.....anywhere on the scale.   Even W8JI harps
on upon that fact.   If you try and measure  say 100 watts..using a 2.5 kw slug, you will get BS readings.   You need to use
the smallest element that will do the job.   That’s why I have elements for 100-250-500-1000-2500-5000-10,000 and 25 kw.

##  from birds site...from the Bird Q+A  url the fellow posted..which u obviously didn’t read.
      Question / Issue

      Bird 43 wattmeter with 250H slug element. Does the 5% +/- accuracy at full scale mean it also has the same 5% accuracy at any point along that scale, or is it more accurate for levels below 250w?  I believe you understand my question. Thanks!

      Answer / Solution

      With the Bird 43 the +/- 5% accuracy full scale means at 250 watts the accuracy is based on the 250 W element in use. So using a 250W element the accuracy is 250 watts + 5% and 250 watts -5%, or 250 +12.5 watts or 250 -12.5 watts. The accuracy window is therefore 237.5 to 262.5 watts.

      If you try to measure 200 watts using the 250 watt element then the SAME +12.5 or -12.5 accuracy window is still in effect, as it is BASED ON FULL SCALE OF THE ELEMENT.

      If you need the accuracy to track the lower power then the accuracy spec you need to look for is +/- 5% of display. We also manufacture that type of metering.

      ##  Note, in my experience, if you use an element that is too big for the job, the results  will be on the high side..but not by a helluva lot.  Bird sez to use the smallest slug to do the job.   My L4B in low power mode indicates 700 watts..using a 5 kw slug. It indicates 650w, using a 2.5 kw slug.    It indicates 625 watts..when using a 1 kw slug.   The 625 result is close to dead on.   Some of it will depend on what freq you are on.  The elements cover 2-30 mhz... and I still don’t know what freq they are calibrated at...when they leave the factory.   They read BS on 160m too...way too high.   They make a 500 khz to 2.2 mhz slug that may be a bit more accurate on 160m.   Also, the readings will drift to the high side when the room temp is higher.   IE:  60 deg F in the morning..vs  85 deg F  in the late afternoon.     The array solutions power master meters are not affected by room temp.   Array solutions sez they hand calibrate each coupler to NIST standards.  If you want to calibrate it urself, go for it.  The frwd and rvs calibration points can be tweaked in 1% increments...from  -15%  to  +15%.   They mark the calibration settings to use on each coupler....once for 160-10m...and again for 6m band.   Worse case scenario is if a 1% calibration point straddled  the actual real power.   In which case the most it can be off is .5%.

      ##  1.5 kw is  5.477  amps into a 50 ohm load.  = 273.861 volts rms.   = 387.2398 V peak.   =  774.4797 peak to peak.    Its not rocket science to calibrate a scope to read 387.2398 volts.   IE: make sure a real 400V Peak indicates 400 V peak on the scope.   As noted b4, if the scope is off by just 1%..the calculated power results will be off by 2%...or double.   Since power = V squared / 50 ohms.

      ##  Everybody that I know that uses an AL-1500  / Om etc, etc,  brags how they run em at 2.5 kw pep out on 80m.  That has been going on for years.  Ok, so they are 2.2 db more than 1.5 kw...who really cares.  Measuring po at the back of the amp is silly anyway.   What really counts is ERP...which will be a function of feed line loss  plus gain of the ant plus po of the amp.  1.5 kw into a stacked array is...ok, meanwhile 1.6 kw into a dipole is a no-no...gimme a break.

      Jim   VE7RF

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