[AMPS] RMS Power

[Radio WC6W]

On Thu, 28 Jun 2001 04:16:26 -0000 "Billy Ward"
<billydeanward@hotmail.com> writes:

Hi Billy,

>
>I am having quite a debate with a group of hams and cb'ers about the
subject 
>of RMS power. I have been a Ham for almost 44 years and have never had 
>anyone deny that there is such a thing as RMS Power.  I am aware that
there 
>are other amplifier manufacturers on this reflector and I would like
some 
>input on how to calculate RMS Power of a linear amplifier. I have heard
or 
>seen in a book somewhere that Average power is equal to the RMS voltage 
>times the RMS current.  It seems to me that if one is measuring RMS
voltage 
>and RMS current, the product would be RMS power but it also seems that
the 
>reference that I read that said otherwise was one of the main
respectable 
>ARRL OR Orr handbooks.

   (True) RMS power is probably best measured with a caloric instrument. 
If you are dealing with anything besides a single tone signal, the peak
to average ratios will deceive most simple meters.

>One of the gang that I have had the discussion with says that when you 
>measure a modulation envelope peak to peak symetrically modulated signal

>across a dummy load with a calibrated scope and calculate the power
using 
>Esquared/R = P, that the Bird 43 will show 1/3 of that value when
looking at 
>the dead key instead of 1/4. He therefore thinks that the Bird is a POS.
I 
>happen to like the Bird 43 and have a lot of respect for it. Has anyone
here 
>got a comment on that?

  The Bird 43 (without a peak reading module) is an average reading
meter.  

  Sounds like AM... then, 1/4 is the correct figure above.  If the peaks
aren't 4X the carrier value then, either you don't have 100% modulation
or something is operating non-linearly.

>It has been years since I had everything set up on the bench to make
these 
>measurements accurately but I would swear that when I used to frequently

>make these type of tests that I came up with carriers that were 1/4 the 
>modulated peaks.  I know that the modulation envelope is double the size

>during 100% modulation and that calls for 4 times the power of the dead
key 
>value.  I also am aware that the carrier does not actually change in
size 
>and that the modulation is a mixing process.

   Likewise correct.

   Note that during modulation the average power, contained in the
carrier plus the two sidebands of a 100% modulated AM signal, is 150% of
the unmodulated carrier power which is what a Bird 43 would read.

73 & Good evening,
  Marv WC6W





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