New member here, and my apologies if this subject has already been raised. I did a quick search on the site and couldn't find anything right-off-the-bat.
New member here, and my apologies if this subject has already been raised. I did a quick search on the site and couldn't find anything right-off-the-bat.
I'm wondering about the legality of maintaining a multiplicity of Part 15 transmitters, all on the same frequency with perfect phase-lock, to maximize radiation and coverage. Properly positioning and phasing the individual radiators could even provide a 'directional array.'
This would not amount to multiple antennas from one transmitter, which would no doubt violate Part 15 rules, but rather a number of individual transmitter installations, all locked to a common reference oscillator and fed the same program material. Placed on a 1-mile grid, you could probably expect good coverage over an area limited only by the boundary of the grid; that is, the number of transmitters. (Of course the phasing would introduce some definite nulls in the coverage area.)
Common ownership of installations might constitute violation of the 'spirit' of Part 15 operation, but for a "radio ministry," for example, each installation could be owned and maintained by individual church members. Same could go for a school.
There would be two technical issues to overcome. First, phase-locking scattered transmitters. This could be accomplished simply by using a commercial AM station as a frequency standard, deriving a 10kHz reference from that station at each installation, and then multiplying it back up to the Part 15 operating frequency. Secondly, piping the same audio to each transmitter site might prove a bigger problem, but could be solved with license-free Wi-Fi links or even dial-up telephone connections.
This is just an idea that, doubtless, someone has already presented, but my innocence of, and interest in, this technique is genuine.
.
Welcome to the board.
Your idea regarding phase locking transmitters is not without merit and apparently you understand that the patterns of the transmitters will be dependent on the phasing, especially where the signals overlap.
The idea of locking on to a commercial AM as a reference is good with regards to keeping the transmitters on frequency but this will be very difficult to maintain in terms of phase. Many things affect the phase of a received signal including multipath and skywave signals from other stations. Even a vehicle moving nearby will affect the phase.
Another problem will be with delay of the audio signals to each transmitter, especially if fed via the internet.
A different approach you might consider is to operate the transmitters on different frequencies. This eliminates the need for phase control, frequency control, and audio synchronization and will produce reliably defined signal patterns. Of course, the down side is that the listeners will need to try different frequencies and select the one which works best for them. On the plus side, it will eliminate a lot of engineering and maintenance headaches.
Neil
This is very important thinking. Basically, it is a sincere inquiry into the ideal of legally covering a useful patch of geography to present programming that the licenses ignore. It's really what spirit of Part 15 is all about.
Rangemasters are designed for synchronization in phase, and there are several stations using a number of synchronized Rangemasters. Check:
http://www.am1000rangemaster.com
.
Ermi wrote "Rangemasters are designed for synchronization in phase, and there are several stations using a number of synchronized Rangemasters.".
This is true but I had the impression the OP was proposing to do this wirelessly with an AM station signal, thus my mention of the difficulties. I do note that mention is made on the Rangemaster site about using GPS for synchronizing so it would be wise to contact them about this.
Communication systems have done so by other means for years. For example, the 8 cycles on the back porch of a NTSC network broadcast video waveform (soon to go away) was used as a phase reference. I am aware of some systems which use GPS or other satellite signals for phase reference. It can be done but this would seem to be beyond the reach of most experimenters.
No harm in thinking beyond the box however since a new method may emerge.
The only legal limit I know of refers to user built transmitters which are limited to 5. I doubt this will apply to assembled certified units but I am not a qualified legal authority.
Neil
Yes, homebrew units are limited to five, but you can use any number of certified units.