There are many radio transmitters in the megawatt range in full operation as of today. I don't know what the cosmic background noise and probability of detection is in the medium - short wave bands, but I imagine a crystal controlled oscillator has an extraordinary characteristic signature over white noise if you listen to it for years on end, especially if you expect a slight Doppler wobble somewhere in the habitable zone.
NTSC seems a bad example because it spreads the energy in wide spectrum using an unfamiliar encoding scheme and the carrier is mostly suppressed. Depending on the content, AM radio dumps over 50% of that megawatt into a single carrier frequency with a stability of +/- 10 ppm.
Can noise to signal ratio be reduced infinitely with larger radio telescopes? What if an advanced species built a light-month wide radio telescope network?
NTSC seems a bad example because it spreads the energy in wide spectrum using an unfamiliar encoding scheme and the carrier is mostly suppressed. Depending on the content, AM radio dumps over 50% of that megawatt into a single carrier frequency with a stability of +/- 10 ppm.