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[quote:c2b32990f6=”PhilB”]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. (etc)[/quote:c2b32990f6]
To clarify, the electrical length of the coax run feeding each tower of a directional AM broadcast array can be whatever length is necessary for site layout purposes. Those coax lengths are important to developing the directional pattern, but the phasing of the towers, and the currents in each tower are controlled by adjustments to variable Ls and Cs in a large “phasor” cabinet in the transmitter room, which is driven by the transmitter. Each tower also has a network to match the base Z of the tower to the Z of the coax feedline.