[CW] Charts of Radio Morse and American Wire Morse - wattmeter

[Donald Chester via CW]

The Bird 43 and its ilk are true average-reading meters only for modes of emission with a steady carrier. They  work fine for CW, FM  and RTTY. With AM they read the average carrier power, but will not the total average power including modulation sideband power. Readings will be erroneous for SSB and many digital type modes. 

The reason is that they are actually average-reading RF voltmeters, not true RMS-reading RF voltmeters, with a scale calibrated to watts into a 50-ohm nonreactive load. AVERAGE power is the product of RMS volts X RMS amperes. Average a.c. volts X average a.c. amperes is a meaningless quantity, a substantially erroneous variation from average power.

OTOH, a thermocouple RF ammeter is a true RMS-reading instrument. With an AM signal 100% modulated with a sine-wave tone, the thermocouple meter  will show a 22.5% increase in rf current above the unmodulated carrier reading. A diode rectifier type of RF voltmeter as used in most "wattmeters" will theoretically show no increase with modulation. This makes them useful for checking carrier shift with an AM transmitter.

The APM-16 contains active circuitry to make the meter read true RMS voltage. Like the Bird 42, the scale is then  calibrated to indicate watts into a 50-ohm non-reactive load. I  seem to  recall that Bird wattmeters can be special-ordered for work at other load impedances, like 72 ohms for example. No doubt ones built for odd-ball load impedances would be very expensive.

The APM-16 was specifically built for work with digital type emissions, but they also work with SSB, AM and other analogue modes.


Don k4kyv


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