The simplest way is mechanical beam tilt, where the antenna is physically mounted in such a manner as to lower the angle of the signal on one side. However, this also raises it on the other side, making it useful in only very limited situations.
More common is electrical beam tilt, where the phasing between antenna elemnts is tweaked to make the signal go down (usually) in all directions. This is extremely useful when the antenna is at a very high point, and the edge of the signal is likely to miss the target (broadcast audience, cellphone users, etc.) entirely.
Occasionally, the two will be used together for odd situations, in order to create greater beam tilt in one direction than the other, mainly to accommodate unusual terrain. Along with null fill, beam tilt is the "focus knob" of radiocommunications, and together can create almost infinite combinations of 3-D radiation patterns for any situation.
The velocity of the imaging member is controlled relative to the beam, and the tilt of the beam is adjusted relative to the imaging member.
The output from summer 102 is a beamtilt error signal which adjusts the orientation of the acousto-optic deflector.
The beamtilt signal is also used to adjust the start of scan signal to account for the conicalness in the drum causing the image to drift across the page.
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