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“Would you say that it’d be difficult to modify the other transmitters”
Should not be too difficult. If you have all of the schematics on hand, and have electronic skills, should be a piece of cake.
There is a way to do this without even spending 80 or 100 or 240 bucks on additional kits for the “slave” units.
If you take the main oscillator/modulator feeding a main feed coax to all of the slave points, each slave merely needs to be a linear amplifier module with coupling circuitry for the 3 meter radiating element. The “slave” module could consist of a simple two transistor amplifier powered via the feed coax as well as getting its main oscillator signal with modulation.
This approach keeps things simple and easy to service in the field, as well as keep the slave units as small as possible so they do not take up too much room, or look out of place with the rest of the surroundings.
I believe there is a diagram of this kind of setup somewhere in the library resources.