This technology is older than I am and has a rather checkered history. It was deployed quite widely in the '30s before AM DAs became popular as a means of allowing pairs of stations hundreds--or even 1000+--miles apart to share a channel "without interfering with each other." With the technology of the day, interference did occur, however, because it wasn't possible to lock the carrier frequencies (and phases) together closely enough or to maintain a constant phase relationship over long periods. This caused the technology to fall out of favor. Nevertheless, it never completely died, and today dozens of US AMs operate "experimental" synchronous transmitters--mostly to fill holes in their nighttime coverage. The best known may be KKOB, which operates a nighttime-only synchronous transmitter in Santa Fe to provide coverage east of Albuquerque in an area not served at night by the main 50-kW signal, which becomes directional to the west at night to protect WABC. There is also one licensed station (the FCC may think of it as two stations) not too far from me--WLLH, a Class C, which has synchronized transmitters in Lowell and Lawrence.
The widepread deployment of GPS technology has made it possible to lock together multiple AM carriers closely enough that the frequency and phase differences can be imperceptable. A contract engineer who has been (and may again be) CE of WLLH, has told me that has has carefully driven the WLLH signal(s) in the area between Lowell and Lawrence and beyond and monitored the signal with an FIM. He says that he simply can't detect any artifacts; the two signals meld seemlessly. I can understand his inability to detect any audible beat notes--using GPS for synchronization, the period of such notes could clearly be made days or weeks long. But he says there are no detectable standing waves and that seems impossible to me. I would think that there would have to be places where standing waves would create near-perfect nulls where the two carriers are equal in amplitude and exactly 180 degrees out of phase. The location of such areas could be remarkably stable, but how can they not exist? Any thoughts, anyone?
Is the secret perhaps that the amplitudes are equal where the phases are not exactly 180 degrees apart and the phases are 180 degrees apart where the amplitudes are unequal?
The widepread deployment of GPS technology has made it possible to lock together multiple AM carriers closely enough that the frequency and phase differences can be imperceptable. A contract engineer who has been (and may again be) CE of WLLH, has told me that has has carefully driven the WLLH signal(s) in the area between Lowell and Lawrence and beyond and monitored the signal with an FIM. He says that he simply can't detect any artifacts; the two signals meld seemlessly. I can understand his inability to detect any audible beat notes--using GPS for synchronization, the period of such notes could clearly be made days or weeks long. But he says there are no detectable standing waves and that seems impossible to me. I would think that there would have to be places where standing waves would create near-perfect nulls where the two carriers are equal in amplitude and exactly 180 degrees out of phase. The location of such areas could be remarkably stable, but how can they not exist? Any thoughts, anyone?
Is the secret perhaps that the amplitudes are equal where the phases are not exactly 180 degrees apart and the phases are 180 degrees apart where the amplitudes are unequal?
