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The directional antenna idea comes up occasionally on various forums.
Commercial AM broadcast stations achieve a directional pattern by using multiple towers.The towers are usually built in a straight line. Then the transmitter signal is distributed to the towers over separate coax lines. The trick is that the coax lines are not all the same length. Coax delays a signal directly proportionally to its length. By precisely adjusting the length of each separate coax run, the phase of the signal can be controlled at each of the antennas in the array. the result is that the combined radiated signal from all antennas is cancelled in some directions and reinforced in other directions producing a directional pattern.
Part 15 antenna phasing could theoretically be done the same way, but not leagally by the 3 meter rule, and determination of the correct length of each coax run would be a very difficult problem to solve.
Another idea I have seen is to set up multiple transmitter/antennas in a similar array and spacing them precisely to achieve the required phase differences. The killer for this idea is that the phase of all tranmitters must be PRECICELY controlled. This is not achievable by relatively simple circuits. You can phase-lock all the transmitters to a single frequency source over equal length lines, but relatively simple circuits will not lock exactly due to circuit variations and low-level noise on the common frequency distribution line. The result is some amount of “jitter” of the frequency of each transmitter. Phased-array antennas won’t work with jitter. Coax distribution in commercial transmitters doesn’t have jitter.