AM radio ‘flutter’ – EDN


Sometimes I like to listen to a couple of AM radio stations that transmit from southern New Jersey, which is rather far from here on Long Island. Their signals are pretty strong during the daytime but now and then there is a rapid in-and-out fading effect, which sounds very much like a flutter.

I’ve sometimes heard that same effect when listening to shortwave radio and I initially assumed it was an ionospheric phenomenon but now I don’t think so. There seems to be an alternative explanation. 

Consider yours truly listening to one radio station coming in from so many miles away while at the same time, there is another station transmitting on the same assigned frequency, also lots of miles away, but in a different direction from here versus the first station (Figure 1).

Figure 1 Two radio stations and me.

Although assigned to the same station frequency, the actual carrier frequencies of each station might be ever so slightly off, perhaps only by a few hertz—still within legal limits, but not exactly on the same frequency. Let those two station frequencies be called F1 and F2.

If F1 and F2 were exactly the same and their waves happened to arrive exactly in phase with each other, you could probably hear the audio of both stations at the same time (Figure 2).

Figure 2 Simultaneous reception of two radio signals with identical carrier frequencies F1 and F2.

The sum of the two signals would carry an amplitude envelope bearing both audio signals. You might hear Little Richard singing on top of Charles Collingwood’s report.

However, if the two incoming signals were slightly out of phase, there would be some net signal attenuation and even more strikingly, if the two carrier frequencies were ever so slightly different, the two signals would alternatively, cycle by cycle, reinforce and cancel at a rate set by the difference between the two frequencies (Figure 3).

Figure 3 Simultaneous reception with flutter.

If F1 and F2 are not exactly equal, then cycle by cycle, the incoming signals reinforce then oppose then reinforce then oppose then reinforce and so forth causing the acoustic effect that I call “flutter.”

In ham radio lingo, we used to call this phenomenon “rapid QSB.”

If F1 and F2 get further apart, the flutter speed increases until it becomes an audible tone and then an audible squeal, which is often called a heterodyne squeal, a sound very familiar to shortwave listeners and ham radio operators using AM communications instead of single sideband.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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