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There are issues when synchronizing the operation of two nearby AM radiators operating on the same carrier frequency that may be unrecognized by some Part 15 users. This is not as simple as just phasing the program audio fed to the two transmitters.
For the most useful configuration, the carrier frequencies themselves need to be accurately synchronized. But even when they are synchronized, the two radiated fields will combine at each receiving location, reinforcing each other in some locations and canceling in other locations (see graphics below).
These patterns are valid for receiving locations about the same distance from each transmit antenna. Reception quality would be different for receivers much closer to one transmit antenna than the other. But the patterns show how the useful coverage areas can vary in this situation.
Total posts : 45366Rich has posted some valuable information which applies to those considering the synchronizing of two or more AM transmitters, which can be done with Rangemaster 1000’s as we have been informed, because some members plan to explore the possibility and others are curious.
To add to the discussion I present two related things…
First an analogy.
The placement of more than one loudspeaker in a public address array results in a similar problem. At certain locations the waves arriving at the ear at slightly different times will result in “filter combing” in which cancelations and other interferences obscure the clarity of the sound.
Second, a question for Rich to which I only guess at the correct answer but Rich will know precisely:
Is there perhaps an ideal distance between antennas in a synchronized transmitter setup, probably based on wavelengths of the frequency in use?
If so, the real world might not make ideally located antenna sites available, in which case the “best possible” synchronization would not be achieved.
Thank you.
Total posts : 45366Is there perhaps an ideal distance between antennas in a synchronized transmitter setup, probably based on wavelengths of the frequency in use?
“Ideal” depends on the needs of the broadcaster and location(s) of potential listeners. Various radiation patterns are possible from these configurations that may provide good service to most of the desired coverage area. Determining that would require an engineering study.
As a heads up: there could be an FCC issue, in that they may consider these setups to be “coordinated,” and intended to provide a useful service area greater than can be produced by single transmit systems operating legally under either §15.209 or §15.219.
Total posts : 45366Rich, Thanks very much for the very useful info. So is there a section of Part 15 banning such “coordinated” setups?
Jim
Total posts : 45366… So is there a section of Part 15 banning such “coordinated” setups?
The only official FCC publication I’m aware of on this subject is quoted below:
Publication Number: 708832 Rule Parts: Publication Date: 04/05/2007 Keyword: Multiple, Low Powered Transmitters First Category: Radio Service Rules Second Category: Part 15 General Question: Are multiple transmitters addressed in the Commission’s Part 15 Rules? Answer: There are no specific regulations that address the use of multiple Part 15 transmitters. In 1987-1989, the Commission revised its rules for unlicensed operation (GEN Docket No. 87-389). In the original Notice of Proposed Rule Making, the Commission proposed to prohibit the use of multiple transmitters to extend coverage area. However, the final Report and Order in this proceeding did not adopt that prohibition. In paragraph 137, the Commission stated that it concurred with the comments that multiple devices should be permitted provided, the individual transmitters comply with the rules and any emission resulting from the simultaneous operation of the individual, non-coordinated transmitters complies with the rules. 
Total posts : 45366Thanks again.
Total posts : 45366that considering the complex and mostly unknown reception at varying locations that the simple approach to multiple transmitters would be to use different frequencies for each. RF and audio sync. would not be needed and the patterns illustrated by Rich’s post would not occur.
Neil
Total posts : 45366i think the worry about coordinated transmitters is class b compliance with transmitters in very close proximity to each other such as multiple synced transmitters on a single tower.
i think the fcc busted a couple setups like this and they stated as long as the tx is a suitable distance apart a single frequency network would be ok.
it is buried in the archives of this site somewhere. was way back in the early to mid 2000’s when this site was first created. some of the conversation might have started on the old community radio usa board it goes that far back.
Total posts : 45366What is “class b compliance” ?
Total posts : 45366https://www.cableorganizer.com/images/kensington/microsaver-alarm-lock/FCC-rules.pdf
class b explanation is in this document.
FCC Part 15 Sub B (Class B)
Total posts : 45366Of. note, There are numerous 1970’s highway and park documentations confirming their were several installations of multiple miles stretch of roads with multiple Part15 free radiating transmitters in use. Each xmtr had repeating taped messages which were all identical in content. They called them “zones”.
In fact the very first utiization of outdoor free radiate part 15 transmitters to have ever occurred (or at least documented) was first in Yosetimite in 1968, and then in 1969 a string of transmitters along a 5 mile stretch of one-way road in Yellowstone was their very first utiozationa. A few years later the Highway Dept. began doing the same thing in certain areas in a few states.
Total posts : 45366utiozationa
That is one helluva word that I intend to explore.
Total posts : 45366In official FCC terminology that means: utilization.
Total posts : 45366This is a section of the rules for digital devices (unintentional radiators) operating in a residential environment. It has nothing to do with intentional radiators like an AM transmitter.
Total posts : 45366Radio 8Z advizes: “The simple approach to multiple transmitters would be to use different frequencies for each.”
I have wondered what the result would be to operate on side-by-side frequencies, say 1670 and 1680 kHz and perhaps more.
If my best expectation would hold true it would give the station the advantage of having a double-width swath of the dial or better.
By locating multiple transmitters in a step-pattern along a geographic path two or more frequencies might appear to dominate the dial as listeners could select the strongest and clearest signal closest to them.
Only part 15 would allow such multi-frequency bunching.
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