Tuesday, September 13, 2011

FCC authorized "Controlled Carrier" AM

FCC DA-11-1535A1:

Some hams may remember something called "controlled carrier". ISTR this feature was installed in my Heathkit DX-60 ham transmitter.

The idea was to adjust the level of the AM carrier signal dynamically, as the level of the modulating audio changed. If your 500-watt AM transmitter is only being modulated 50%, you only need 250 watts of carrier to avoid overmodulation; the remaining 250 watts are essentially wasted.

Apparently this feature has been available in broadcast transmitters for some time, and used by high-powered stations overseas. It's also been tested domestically on KOTZ-720, Kotzebue, Alaska.

The FCC has now authorized its use in general by U.S. AM stations. Stations wishing to use it must request a "MDCL Waiver" of 73.1560(a).

("MDCL" = "Modulation Dependent Carrier Level")

(I gave up on AM phone with the DX-60 & went back to Morse Code when I got a 5 by 4 report from a station six miles away...)

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