[AMRadio] FCC's AM power


John Coleman jc at pctechref.com
Wed Sep 28 13:38:33 EDT 2016


Power definitions can be very confusing.
And as many have said " It don't really matter anyway for a number of reasons".

A lot of the stuff below is for new comers so pardon the length.  It was all very confusing to me durring my high school years and several years after.

Power, when determind at the peak of any wave by the peak voltage and peak current, is relitively useless, in my humble opinion.  It is posisble to have a peak power calculated at 1000 watts, but only be able to light a #47 lamp.  Power is a measurment of the amount of work done or capable of doing.  When you look at the light of a 100 watt, 115 volt lamp running on 115 volts DC and measure the current it is found to be just under 1 amp (100/115)  If you measure the peak voltage when the lamp is running on AC you will find the peak voltage to be about 160 volts. and the peak current to be just under 1.6 amps.  This is almost 250 watts but not really because it is not there all the time only on peaks and constantly changing. Yet the lamp gives off the same heat and light as it did when running on DC.  So to find the the average power from the AC source, we must use the RMS values of voltage and current.  RMS voltage would be about 115 volts and the RMS current would be the same as the DC current (100w/115v).  (BTW - Most AC volt meters are fixed to measure RMS values of a sine wave only, not peak, and not any other wave shape.)

The same is true when meassuring RF. If you had a metering device that measured the RF peak voltage across a 50 ohm dummy load with a carrier wave only, then to find the power you would multiple .707 time the measured peak voltage to get the RMS value, square that value and divide by 50 (Erms * Erms) / 50.

The same measurement and calulation would be true when measuring an AM wave.  But the measuring device would need to have the capability to measure the peak voltage and and hold  the measurement while you read it.  A scope will give you a close idea if using a audio signal generator for input regardless of wave form as long as it is repeditive.  This is why many hams speak the word "four" into the microphone so as to get some simbalence of a repatative wave form.  But there are voltmeters that will measure the peak voltage with modulation and not change for a while so that you can read it.

Another way to look at it is to imagine a XMTR whose power is varying by a knob and some one is turning the knob back and forth.  The output power is varing along with the knob. on a slow sweep scope it would look like an AM signal with very low freq modulation depending on the speed the knob is turned up and down.  The peak envelope power is the power (Erms * Erms/50ohms) at the time the knob is at its max.

I made a peak reading voltmeter adapter once using a 6AL5  tube and connected one plate to the dummy load terminal and the coresponding cathode to a .05uf capacitor of which the oposite end was grounded to the dummy load ground.  Measured the voltage across the capacitor with VTVM and found the the discharge time for the .05 cap was very slow and was able to make very acurate measurments. The Capacitor charges to the peak of the RF voltage but discharges very slow having only the VTVM as a bleader.   I built it into the top box on the heath cantenna.  and lit up the 6AL5 with a 6 volt latern battery so I could move it around with out having to plug in a filiment XFMR.

I think the regulation reads something like this.  "The Power of a single sine wave (360 deg) RF  at the peak of the modulation envelope"  The mesurment and calculations above should satisfy this statement.

Hope this clears up some stuff.

73 , John, WA5BXO



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