Synchronous Operation of Nearby AM Transmitters

[rfry]

It has been suggested elsewhere that up to five unlicensed AM transmitters with 100 mW input power each can operate physically close to each other without mutual interference as long as their r-f frequencies are the same, and their program audio is the same and is timed (delayed) properly.

The following link leads to a NEC analysis of this for just three such transmitter systems each using 1700 kHz, spaced at 200 foot intervals in a straight line.

The net radiation pattern from such an array can be highly directional, which may be different than expected and/or desired.

Changing the spacing and orientation of the radiators and/or the power and r-f phase in each of them can change the net radiation pattern shape, but the pattern will still be directional in some form.

http://i62.photobucket.com/albums/h85/rfry-100/3x3mMonopolesat200ftIntervals.gif

RF

 

[pianoplayer88key]

It has been suggested elsewhere that up to five unlicensed AM transmitters with 100 mW input power each can operate physically close to each other without mutual interference as long as their r-f frequencies are the same, and their program audio is the same and is timed (delayed) properly.

The following link leads to a NEC analysis of this for just three such transmitter systems each using 1700 kHz, spaced at 200 foot intervals in a straight line.

The net radiation pattern from such an array can be highly directional, which may be different than expected and/or desired.

Changing the spacing and orientation of the radiators and/or the power and r-f phase in each of them can change the net radiation pattern shape, but the pattern will still be directional in some form.

http://i62.photobucket.com/albums/h85/rfry-100/3x3mMonopolesat200ftIntervals.gif

RF

Interesting. Question, though - why the 0dB max scale? Is that because the 5 transmitters will only have the same field in the lobe as the single non-directional transmitter?
Also, what happens when you space them closer together, like less than 30-40 feet?
Or, what about using a single antenna, and somehow feeding all 5 transmitters to the same antenna? (Obviously this would involve a bit of a transmission line and/or some redesign of the antenna terminals, I think.)
Another one I was thinking about... what about using 5 transmitters and 5 antennas, feeding the transmitters into the antennas as normal, but stacking the antennas one on top of each other?
Also, what does someone do who has very limited space in which to put a ground radial system? (I maybe could do 10-foot radials, but not much more, and proably much less. What about putting in something like a 2" deep, 5-foot wide water hole, put heavily salted water in it, and put the transmittter and antenna over that? Or is it better to just use the very short ground radials?

 

[rfry]

Question, though - why the 0dB max scale?

The outer ring of the plot is not zero dB max, it is 0 dB with respect to the -13.85 dBi gain shown for the Outer Ring at the bottom of the plot. The other dB values on the plot are referenced to the value of the outer ring, so the real gain at the -10 dB ring is -23.85 dBi, etc.

Also, what happens when you space them closer together, like less than 30-40 feet?

The pattern would become less directional, and their proximities will have a greater affect on the feedpoint impedance of the other radiator(s).

Or, what about using a single antenna, and somehow feeding all 5 transmitters to the same antenna?

"Somehow" is the key issue there. The transmitters would affect each other unless they are kept electrically separate using an expensive combiner. It would be easier to use one transmitter running five times the power. Of course, the FCC likely would not consider either of these configurations as meeting 15.219.

Another one I was thinking about... what about using 5 transmitters and 5 antennas, feeding the transmitters into the antennas as normal, but stacking the antennas one on top of each other?

More likely to work with less mutual coupling than the above scenario, but probably could bother an FCC inspector.

Or is it better to just use the very short ground radials?

Yes.

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