To achieve the goal of transmitting on 3 frequencies at the same time, it would be highly desirable to erect one outdoor tower that could handle all 3, to avoid building more than one tower. Although it is true that FM stations and TV stations use "multiplexed" antennas, where several stations combine or mix their signals via a reverse splitter where all the transmission lines get piped together by giant plumbing. We have never heard of AM stations doing the same thing, but maybe they could.
With micro-wattage making a blender, combiner, mixer would be cheap, but would it work?
In the VHF bands and above the filters and diplexers (cavities) can be made physically small, on the order of a few feet to a few inches. These devices exhibit a very high attenuation (60 to 90 dB) at the notch frequency and low insertion loss (about 1.5 dB or so). This is due to the wavelengths being practically short.
I have seen the diplexer plumbing at a local plant where several FM stations share the same stick. It is a plumbing nightmare (or dream according to your point of view). Another method used in two way services is vertical separation of the antennas though the isolation is not as good as using the cavities.
Transmission lines can also be used for isolation but the Q is nowhere near that of cavities due to line losses. I know of no AM applications for combiners but it should be possible with the properly phased transmission lines but the losses can be high due to the lengths of cable involved. I have used tx lines for isolation at 146 MHz and also stubs for signal suckout with varied results. It depends on the relative strength of the signals, the separation of frequencies, and the loss which can be tolerated. My best "home brew" results were obtained with a home made strip line cavity but again, this was at VHF.
So I haven't directly answered your question but I hope some background information is helpful.
Neil
Yes, it is possible to diplex multiple AM (medium frequency) transmitters to one antenna. It is done all the time in areas where real estate is expensive. I think there is a site in Hawaii with 7 stations on one antenna array. They don't use cavities but tuning networks since the frequency is so low.
For a part 15 application it might not be practical, since there would be signal losses to an already low powered signal. Also, the part 15 regs count any feedline as part of the antenna length, so the combining network would have to be at the base of the antenna anyway. Theoretically possible though, and at the low power levels the components might be of manageable size.
Maybe someone would know if there would be a conflict with other aspects of Part 15 regulations?
Welcome to this board! Your comments about AM diplexing are interesting. I, a long time ago, visited the Bethany Relay Station (VOA) in Mason, Ohio where they ran many simultaneous short wave transmitters but as I recall they had separate antenna arrays for each tx. Since they changed frequency and patterns often it would seem to be a real nightmare to share antennas. But, as you mentioned, for fixed frequencies it could be done.
Neil
I have a perfect solution which will last right up until someone tells me why it won't work.
Here's what you do. Make your 3- AM transmitters up tp and including the buffer stage but MINUS the final! Then mix and match those buffer outputs into a common point, ratchet the available steam right up to 100 mW and THEN inject it into one final stage!
Aha! One 3-meter stick emitting 3-signals!
Bring it on.
You would need to use a circulator to keep each xmtr from getting in to each other. With the normal losses when using a circulator, you would need a few watts from each xmtr to get the 100 mw..
But it would be legal!
Would it be legal?