Our debate over annual NRSC mask testing continues and expands a little. John Schmidt, chief engineer of Peconic Public Broadcasting’s WPPB in Southampton, N.Y., has this response.
I’d actually be more concerned about harmonics and true spurs than compliance with the “close in” mask. I’m of two minds about testing. I’d like to think that today’s gear is reliable enough and designed so as to make routine tests unnecessary. But there is a lot of not so “today’s” stuff out there still on the air. And a couple of examples from perhaps 20 to 30 years ago that may or may not be pertinent.
Some years ago I was listening to the shortwave bands on my Sony 7600 at a relative’s about 200 miles from home. By chance, I tuned to 2480 kHz. There was a very strong signal from a station licensed for 1240 kHz — 200 miles away!
The 1240 fundamental was nowhere to be heard, buried in co-channel junk. But the second harmonic provided a great signal far beyond their usual coverage area! I called up the chief, a friend. “Oh, impossible, it must be propagation.” Well, it wasn’t and he soon repaired it. But how long was that second harmonic there? Who knows? He never got cited by the FCC.
A few years earlier, I was tuning my (analog, that dates it) car radio around the FM band and picked up a local class A all up and down the dial. My first thought was intermod in the radio. But when I got home and heard the same thing on my receiver there, again, I called the chief. “Oh, your receiver is overloaded.” But it turned out that their brand new synthesized exciter from one of the big name manufacturers had a little problem. Momentary power hits would put it in “all kinds of spurs” mode, until it was power cycled. It took the manufacturer several attempts at mods to put this one to bed. So there is something to be said for occasional measurements.
How often? I don’t know.