Could multiple transmitters on the same frequency work if phase-locked to GPS? (gedanken)

Question

I've been noodling this concept for a while; it seems to me that if it were actually workable somebody would be doing it already, but I haven't heard of it happening. So where do I go off the rails here?

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If two transmitters operate on the same frequency, a receiver will generally get audio that is annoying-to-unusable. This is due to heterodyning between the RF oscillators and between the differing program material (modulation).

But what if the transmitters were phase-locked to a common standard? Now the RF oscillators would be identical in frequency and phase. If the modulation came from a digital file with a timecode, it could also be perfectly synced to the same standard, and then the transmitters would not interfere. The synchronization standard could be GPS receivers.

Is GPS accurate enough to pull this off in, say, the AM broadcast band?

I'm thinking this might be useful for e.g. a "pirate" radio broadcast that doesn't run afoul of FCC regulations (albeit not addressing copyright issues :- ). It would also be difficult to locate the transmitters, if they were camouflaged or hidden. The transmitters could be done pretty cheaply and you can fit a lot of audio on a cheap SD card.

Answer 1

Yes, absolutely. The physics work how you think they do - the radio signals just add up. In fact you don't necessarily need GPS as (I think) you could use the radio waves themselves for synchronization.

However, the receiver will still see a different propagation delay from each transmitter, causing them to receive multiple copies of the signal slightly out of phase. How slightly? Depends on the frequency and position. If the receiver is halfway in between two transmitters transmitting identical signals, they receive both signals in-phase with constructive interference. If they move a quarter wavelength towards either transmitter, now they have destructive interference and the signal is very weak.

If the receiver is next to one transmitter, they'll receive the signal from the nearby transmitter, plus a delayed signal from the distant one. If you transmit AM audio it will sound like an echo. If you transmit analog video it will look like "ghosting".

You need a modulation scheme which can tolerate these "echoes". Some can, such as OFDM. In fact, this is how digital TV and radio is modulated in Europe, and they can use the same frequency for multiple transmitters because of it. I suspect they generate the analog signal once and send it to each transmitter (e.g. via microwave), but with enough engineering work and a way to synchronize there's no reason you couldn't generate the same signal independently at each transmitter.

Answer 2

WWVB uses two antennas (called "North" and "South") at \$f_C=60 kHz.\$
Wavelength 5000 m. Antenna separation is 857 m.
For sure, they are synchronized from a primary standard, not GPS.
Transmitted pattern at various time-of-day is calculated, one of which (0600 UTC) is shown here:

Perhaps the herringbone pattern is a result of phasing of those two antennas? Receivers in a trough might be getting destructive interference from each antenna, while a receiving site at a peak might be getting constructive interference from each antenna.
Or possibly ionospheric reflections/refractions are the source of peaks/troughs, since these differ at various hours of the day.

Answer 3

You mention "pirate transmitters" that won't "run afoul of the FCC." That sounds to me like you want to transmit your own audio on the same frequency as an existing transmitter without causing interference.

That will not work. If your transmitter is on exactly the same frequency as an existing radio station, then you will cause interference - you will "run afoul of the FCC." Your transmitter will block reception of the licensed transmitter - that's interference, and will get you in trouble.

If there were no interference at the radio frequency level, then it still wouldn't help your pirate radio station. If you receive two signals at the same time with the same transmitter frequency on an AM radio, the audio will be mixed. Listeners would hear your pirate music and the licensed broadcast at the same time - they are normally only separable by the different frequencies of the carrier.

Your undetectable pirate radio transmitter can't work, no matter how well you synchronize it to an existing station.

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The other answers seem to be concentrating on the legitmate uses of multiple transmitters on a single frequency rather than your concept of an "uncatchable pirate transmitter."

Legitimate uses include licensed operators using licensed, synchronized transmitters to cover a larger (licensed) area using lower power transmitters. In all such cases, all the transmitters are transmitting the same content on a single frequency.

I know of one system that a police organisation used to extend coverage over a major highway network. There were many transmitters spaced long the major roads to make sure they had complete coverage.

All of the transmitters sent the same content. The transmitters all used the same radio channel, and hence (almost) exactly the same frequency. That "almost" is important.

If the transmitters had been on exactly the same frequency, there would have been dead spots where the signal from one transmitter exactly canceled out the signal from a neighboring transmitter. That kind of cancellation is no surprise - it works just like this demonstration of cancellation using speakers and sound.

The solution was to have all the transmitters tuned to within a few hertz of one another while making sure that no two transmitters used exactly the same frequency. The signals could cancel each other out but there were no standing dead zones. The received signal strength would wobble a bit due to the signals from the transmitters having constructive and destructive interference, but that didn't bother things.

Answer 4

Using the low power AM transmitters spaced say 200 feet apart, by the time you get to the destructive interference zone of TX1, you are already so close to TX2 that TX1's signal will not cancel TX2's. Just make sure the half-way point between the TXs is not equal to or near the 1/4 the wavelength of the transmitting frequency. I think the US government takes so long to update their rules and being that they are not the smartest tools in the shed, they probably haven't thought of people using a SFN of AM TXs. The US Government headlight regulations for cars for example, are nearly always 10 to 30 years behind the latest car headlight technologies. (e.g. sealed beam, halogen, HID, LED, Adaptive, Laser, etc.)