That's what I thought.
There are two basic issues with using multiple transmitters, and perhaps people are getting them confused.
The first is to get the signal out to the transmitters. You can use wireless point to point, over fixed wires, over the Internet, over another Part 15 radio - lots of ways.
The second is to eliminate the interference/synching problems with multiple transmitters. One solution, and the simplest, although probably least desirable, is to use different frequencies on the transmitters. If you are going to use the same frequency, you must get the signals to the various transmitters almost simultaneously (so the Internet is a poor option) and then, hopefully adjust them to eliminate that interference (and some don't allow for that, the Hamilton Rangemaster advertises that it does).
"If you are going to use the same frequency, you must get the signals to the various transmitters almost simultaneously (so the Internet is a poor option) and then, hopefully adjust them to eliminate that interference (and some don't allow for that, the Hamilton Rangemaster advertises that it does)."
Phase corrections and time domain corrections is going to be crucial. Each TX, even the first TX in the chain would need to be able to adjust the phase and time domain of that signal waveform so that everything will precisely match, both phase and time as well as frequency error.
Can those wireless link options give elbow room to allow for phase and time domain correcting, as well as monitoring and real time correcting?
RFB
Have to be honest, don't know. Experimentation would once again be in order.
And once it starts getting this complicated, I start looking for a simpler solution. At least one that is simpler to someone with my skills.
That's why I'm looking at the hotspot approach, as opposed to multiple transmitters. Never mind the fact that we can't legally use multiple transmitters here in Canada.
Never mind the fact that we can't legally use multiple transmitters here in Canada.
What about feeding them with different programing? Shift the programing on one TX by a 24 hour period from the other for example.
Though I'm not sure that would be acceptable.
RFB
Artisan Radio's post above addresses the transport and synching of the same audio program to transmitters at different physical locations, which is an important part of multi-site installations using the same carrier frequency.
But it is also very important to synch the r-f carriers of transmitters using the same frequency (such as 1650 kHz), where such transmit sites provide r-f signals to the same geographic area.
The r-f phase of a radio wave changes as it travels away from the transmit antenna. In some locations and directions, radiated fields from two systems on the same carrier frequency will add, and in others they will subtract.
These pattern effects occur even with perfectly synched r-f carriers, and no modulation. This is the reason for the "daisy petal" appearance of the radiation pattern in the link I posted earlier in this thread.
A few AM broadcast stations use synchronous "boosters" to improve coverage in a particular area not receiving the main station well. But even though the boosters use much lower power than the main station, for most useful and stable operation the r-f carriers of both transmitters need to use exactly the same r-f carrier frequency. Typically this is done by using oscillators locked to GPS hardware.
These AM stations still have areas where the mutual interference of the two transmit systems makes listening impossible, but try to set up the system so that these areas have relatively few listeners.
"The r-f phase of a radio wave changes as it travels away from the transmit antenna. In some locations and directions, radiated fields from two systems on the same carrier frequency will add, and in others they will subtract."
This is comparable to the old analog video systems where vertical and horizontal sync pulses have to be compensated with delay modules in order to have everything in the system meet at the same phase and time domain. Even the chroma burst signal requires synchronization using the same delay modules. The common reference to all of it in the television engineering field is called "timing".
But even with perfect synchronized timing of all those pulses, each pulse from each device MUST also have the exact time domain...ie pulse duration/width. If they are not exact, even perfectly timed pulses with different time domains/durations, or pulse widths will cause errors such as picture shifting during a transition from one video source to the other through a video switcher console.
This is why I believe that a Part 15 AM system with extension transmitters linked together must also have the phase and time domain error correction in real time by taking samples and correcting with a master reference pulse like that in an analog video system, known as sync reference or color reference or "black", so that all units, including the master transmitter are adjusted "on the fly".
At that point, the Part 15 219 system with extension transmitters linked together becomes rather impractical, but not impossible.
But I would imagine such an elaborate compensating system with real time monitoring and correcting would become quite expensive to implement.
"These AM stations still have areas where the mutual interference of the two transmit systems makes listening impossible, but try to set up the system so that these areas have relatively few listeners."
Great advice! And should be at the forefront of anyone considering multi-transmitter systems that are linked. In most cases that approach would probably resolve many problems.
RFB
With respect to fully qualified RF engineers I call myself a "non-engineer" because my technical background is weak in the RF department. But I know enough to follow much of what's being said on this interesting subject, and perhaps my "outsider's thoughts" might help to clarify the discussion.
In both audio and video technical production the questions of "timing", "phasing" and "pulse duration" become very familiar, especially in years of analog work.
But today we are also talking about the added dimension of "space", where transmitters are physically apart by some distance. Comments have already mentioned that unpredictable phase-shifts take place because of random objects that populate the RF field between two transmitters, and this makes for a special category of difficulty that I think makes AM transmitter co-ordination even more complicated than timing a video system in a single location.
It would seem to me that results would vary depending on the exact spot where measurements are taken. Is there a "preferred spot" where measurements should best be taken?
"Comments have already mentioned that unpredictable phase-shifts take place because of random objects that populate the RF field between two transmitters, and this makes for a special category of difficulty that I think makes AM transmitter co-ordination even more complicated than timing a video system in a single location."
Your missing the point. The point is a comparison of the two with the same cause and effect, thus require what was pointed out, some method of compensating for the phase shifts, time domain shifts in order to properly synchronize the signals.
Even in a video system in one location, the many runs of cabling separating each piece of gear is the "space" like that in where the radio signal travels through the air.
As to a measuring point for adjustment to a reference, that might get even more complicated because unlike a video system with cabling carrying signals, the signals here are riding in the air, and a lot of things can vary that signal as it makes it's way through that air or space. In a video system, it's just more predictable and controllable, whereas in air or free space, it is nothing but a big fat variable that varies constantly to which is not predictable, thus some kind of sampling system has to come into play and talk to a compensating system that is constantly looking at everything, and adjusting according to a reference so that all the transmitters are singing the same song, at the same time, phase and duration.
RFB
But "comparison of the two" requires taking readings in the open air, does it not? If so, at what one or two locations in the open air are these measurements best taken?
If all measuring is done internally within the equipment, then my worldview needs to be dismantled and rebuilt.
In the experience of AM broadcast stations, it is not required to sample and correct for phase variations producing the net radiated field in the open air, because due to the relatively long wavelengths in use, those differences are determined essentially by the physical locations of the transmit antennas, the receive location, and the carrier frequency (except for very localized effects adjacent to an a-c power line, for example).
Synchronous AM broadcast stations use GPS control of their carrier frequencies, accurate/stable control of the program audio amplitude and audio phase fed to each transmitter, and matching modulation swings of each transmitter in the positive and negative polarities.
This is enough to ensure that the net radiation pattern is stable with respect to the coverage area, and that the modulation recovered by receivers at most locations will not be (highly) distorted.
"But "comparison of the two" requires taking readings in the open air, does it not?"
Yes. And in a video suite, measurements are taken with a video waveform monitor and vector scope, both referencing the "house sync" or "black" or "chroma reference". Usually it's "black" from a reference generator producing all of the necessary timing pulses so that everything else is "timed" to that reference.
For radio in the air, well that's the big question. Where would that measurement reference be, since the air is vast.
But the cause and effect are the same in both cases. IE...improper timing means out of sync in video, or out of phase in radio.
Indeed, finding that measurement point to lock to a reference so that compensating can take place will be quite a task considering the cause is the variable in the medium..ie air in which the signal travels. So do we put our measuring point half way down the line between transmitters and lock that to the reference so the compensating can take place? Do we put it somewhere else?
Your right about synchronizing radio signals is more difficult than a video system. But the comparison is in the cause and effects which are the same in both cases.
RFB
"Synchronous AM broadcast stations use GPS control of their carrier frequencies, accurate/stable control of the program audio amplitude and audio phase fed to each transmitter, and matching modulation swings of each transmitter in the positive and negative polarities."
That would be the best way to go to synchronize multiple transmitters. There is still the problem of expense of a GPS locking system, which come to think of it, there isn't any Part 15 AM transmitter currently sporting such a system.
Commercial AM stations yes..but Part 15 AM transmitters..not yet anyway.
RFB
Although it appears that no means presently exist to lock part 15 systems to a GPS reference from satellite, an unrelated story I encountered this morning comes to mind.
A news service at KPFA in Berkeley, California, sent notice that due to exceptionally moisture laden cloud cover, their KU-band satellite channel was completely blocked.
Thus is born the question, how reliable are GPS signals during the constant periods when they would be controlling AM transmitter sync?
Any and all satellite transmissions are vulnerable to sun outages as well as iced up satellite receiving dishes, bad LNB's, leaky seals and water ends up in the feedlines, etc etc.
RFB
Incoming knowledge about the workings of GPS are fragmented for me, because I as yet have no GPS devices, but I gather that automobiles and even hand held devices are able to link with GPS co-ordinates without use of a dish. That opens the next question.
Are the GPS systems employed to synchronize AM transmitters fed by dish, or by smaller reception antennas?
Bonus question: in the case of synching two AMs, do the receiving GPS devices need to speak to each other?