[AMPS] Watt meters and PEP

[Jim Reid]

I thought the following input was especially good,
helped me!  More words on the topic,  but useful:

"Set up and operate the rig such that you measure 1500 
watts into a dummy load on a Bird or equivalent wattmeter 
under CW key-down conditions. Connect a scope to the 
output line and set the gain for a vertical pattern of a
selected height. That pattern height represents the output 
level of your rig when it is outputting 1500 watts.

Now, modulate the rig. If the voice modulation peaks exceed 
the previously identified 1500 watt level, then you are 
exceeding 1500 PEP on voice peaks. Reduce gain and try 
again. Operate always to keep your voice envelope peaks 
under the previously determined level that represents 1500
watts PEP output.

Mathematically, the first formula you cited is correct. Peak 
voltage of the r-f signal (consider no modulation for now) times 
0.707 to convert to RMS voltage. Square that equivalent RMS 
voltage and divide by the load resistance, say 50 ohms. That is
the power of that unmodulated r-f signal. It is also the PEP 
power level since the signal is not changing amplitude with 
modulation. 

If the modulation causes a higher peak voltage to be 
developed, then that higher voltage is what is multiplied by 
0.707 for the conversion. You can't follow those voice-
modulation voltages with a meter, so use a scope
instead. Calibrate the scope as described above. Modulate 
to keep all voice peaks below the 1500-watt level on the screen 
and you are legal.

Since a CW or continuous signal is not changing its peak 
amplitude with time, its PEP value is a constant which is 
readily and accurately measured on a meter.  The CW signal
is at its maximum possible peak value with every dit and dah.

On the other hand, a SSB signal is constantly changing its 
peak value with time according to the modulation waveform 
and the method of modulation. Thus, a meter with its ballistic 
delay, etc. cannot follow the modulation peaks and reads in 
error. Only a scope can keep up with the ever changing voltage.

While you have that scope out, you can also make sure that your
setup is not doing any peak-clipping or flat-topping which causes
unnecessary QRM to all concerned and actually decreases the
communications effectiveness of an SSB signal.

Hope this is of some help.
 
72/73, George, W5YR"

Unfortunately, I do not have access to a scope out here,
but believe I now know,  from what Bruce, W8GN,  (and also
the designer of the VFD Wattmeter) wrote the other day,
that I am opeating my rig correctly.  That is,  set up to read
1500 watts out on the VFD using CW key down ( and I can
verify this is essentially correct,  as my Bird 43 with the
2500 watt slug also reads 1500 watts,  key down, to 
within +/- 5% of 2500,  or within 125 watts of the VFD reading,
in fact each agrees with  readings with 40 watts of each),
then increase the drive to the linear during SSB,  until
voice peaks also produce the 1500 watt output on the
VFD.  This is true,  since the VFD is looking at the
voltage peaks of both the CW and the SSB modulation
envelope  signal.

My initial question/curiosity is satisfied,  hi!

73,  Jim,  KH7M





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