[AMRadio] League pulls RM-11306

Geoff/W5OMR ars.w5omr at gmail.com
Sun Apr 29 20:53:25 EDT 2007

Bill Smith wrote:
> Hi Geoff,
> Been there, tried that.. have run increased carrier, and with an
> asymmetrical limiter have limited positive peaks to comply with the 1500 W
> PEP limit.  

I've gone to the trouble for you (and anyone else that cares, for that 
matter) of the 'heart' of the discussion that John/BXO, Bacon, Tim and 
Don/KYV came up with...

*"The maximum audio modulator power requirement for 100% modulation of a 
1000 watt high level modulated class C final is 500 watts, /but only if 
the modulating signal is a sine wave/. The reason for this will become 
apparent from the following example. *

* The following example will show that the required audio power 
necessary to achieve 100% sine wave modulation is 50% of the value of 
the DC Plate input power supplied to the final class C RF amplifier. * * *

* Assume that the final amplifier has 2000 volts applied to the plate 
with a current of 500ma. This makes the power equal to 1000 watts. To 
achieve 100% modulation, the peak modulation voltage of the sine wave 
must be 2000 volts and because of the symmetry of the sine wave will 
yield a 4000volt peak-to-peak swing (0 – 4000V when the quiescent DC is 
at 2000V) thus providing the desired 100% modulation. To calculate 
average power rather than instantaneous peak power we must convert the 
peak voltage output from the modulator to a “Root Mean Square” voltage 
(Peak voltage X square root of 0.5 = RMS Voltage). RMS voltage is equal 
to 0.707 of the peak voltage. *

*Again if the plate voltage on final is 2000 and the current is 500ma 
then in the above asymmetrical example the peak-to-peak swing of the 
plate voltage would be (0 – 6000 volts with the quiescent plate voltage 
at 2000 volts DC). (6000-2000 = 4000 volts peak) This is twice as much 
peak audio voltage as was needed before. (4000 * 0.707 = 2828 V RMS) and 
P = E*E/R or ((2828 * 2828) / 4000 ohms) = 2000 watts. Since we have 
doubled the needed peak voltage, then we have quadrupled the power 
required to produce it. This does not mean that you will be putting that 
amount of power into the final continuously, but you will need that 
capability in order to get the needed voltage swing on the peaks. *

* An RF ammeter using a thermocouple actually responds to average power 
rather than RMS current, because it is heat that causes the indication. 
This type of ammeter does not indicate true RMS current. Because it is 
calibrated with an un-modulated sine wave signal, it will read 
un-modulated signal currents correctly, but its reading will be elevated 
when normal amplitude modulation is present. It will display a current 
reading equivalent to the RMS current that would produce the same 
average power as the modulated wave. In fact, the true RMS current of an 
AM waveform does not change until modulation exceeds 100% in the 
negative direction, unless there is nonlinearity or some sort of carrier 
level shift action taking place. *

* With a 50 ohm load, a 750 watt carrier will cause the thermocouple RF 
ammeter to read 3.87 amps. If this carrier is modulated 100% by a sine 
wave, the average power will increase by 1.5 times (the 50% added power 
comes from the sideband energy created by modulation). The total average 
power will be 1125 watts with modulation at 100%. The RF ammeter will 
show about 4.74 amps, which is the current that would be necessary to 
produce an un-modulated signal of this power level. If a person’s voice 
is used to modulate the rig, and the modulation envelope looks something 
like the scope picture in figure 4, you will see approximately the same 
increase in RF current, even though PEP with voice modulation is 6750 
watts, and PEP with sine wave modulation is 3000 watts. This is because 
the speech waveform is spiky, so its peaks need to be relatively high in 
order for it to have the same RMS power as a sine wave. With all this in 
mind, it boils down to the fact that in order to faithfully reproduce my 
voice with the legal carrier level of the time, it was necessary to have 
a modulator capable of 2000 watts. Let me say again that I was not 
putting 2000 RMS watts of audio into a 1000 watt (input power) rig. But 
if I had not had the 2000 watt modulator, then the peaks of my voice 
would have been chopped off or clipped, resulting in distortion and 
splatter. *

* Now with the 1500 PEP ceiling, I can only legally run 220 watts input 
and 165 watts output if I want to properly reproduce my voice."

*Asymmetrical Audio.

The whole article can be seen at



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