[Boatanchors] 813 grid to filament short

[Glen Zook]

My experience, at least with the Dallas, Texas, FCC office, has been that a tolerance of +/- 10% is a reasonable value.  The Bird 43 wattmeter is pretty much the "standard" for the commercial two-way radio market.  That wattmeter has a maximum tolerance of +/- 5% of the total scale value.  However, at lower power levels, the actual tolerance, of the individual reading, can exceed the 5% value.  As such, the FCC engineers that I have dealt with, home in a +/- 10% for the total accuracy of the measurement. Glen, K9STH 
 Website: http://k9sth.net
      From: D C _Mac_ Macdonald <k2gkk at hotmail.com>
 To: "boatanchors at mailman.qth.net" <boatanchors at mailman.qth.net> 
 Sent: Thursday, October 1, 2015 3:04 PM
 Subject: Re: [Boatanchors] 813 grid to filament short
   
C'mon guys, you are forgetting a very important factor here. 
 
Yes, the DC "carrier" input AND output power can easily be measured fairly accurately, probably within 5%. 
 
If you are talking about full carrier, AM modulated transmitters, remember that the modulating power (generated in a separate audio amplifier with its own power supply) is in addition to the "carrier" power.  
 
Assuming everything is "kosher" 100% modulation requires modulating power equal to one half of the RF carrier power; i.e., 500 Watts of audio for a 1 kW output carrier.  250 Watts (peak) is added to each sideband. 
 
Again, assuming 100% efficiency, instantaneous voltage applied to the plate will swing between zero and double the DC plate supply voltage.  Ohms law tells us that if voltage is doubled, current will also be doubled.  This means that the peak power at the plate will be quadruple the static power.  Ergo, a 100% plate modulated RF output of 1 kW will have peaks of 4 kW. 
 
And that, gentlemen, is why with a maximum PEP output allowed of 1500 Watts, 375 Watts of carrier with 100% modulation is what is allowable. 
 
This CAN be accurately measured with calibrated lab equipment, but the FCC can get close enough with fairly simple equipment to gig the operator who exceeds that limit.  Could be done with just a calibrated dummy load and calibrated o'scope? 
 
If we assume 75% efficiency in a class C RF amplifier, you could safely run 500 Watts carrier input with a 250 Watt modulator and stay within the authorized power output limits. 
 
What is the BFD about this? 
 
Who can guess what the FCC would allow as tolerance?  5%?  10%?  20%?  50%?