Reflective array antenna

Reflective array antennas usually have a number of identical driven elements, fed in phase, in front of a flat, electrically large reflecting surface to produce a unidirectional beam of radio waves, increasing antenna gain and reducing radiation in unwanted directions.

The larger the number of elements used, the higher the gain; the narrower the beam is and the smaller the sidelobes are.

A basic goal of antenna design is to make the currents add up to a maximum at the point where the energy is tapped off.

To do this, the antenna elements are sized in relation to the wavelength of the radio signal, with the aim of setting up standing waves of current that are maximized at the feed point.

Thus, in order to improve reception, antenna designers often use multiple elements, combining them together so their signals add up.

This makes the array more sensitive horizontally, while stacking the dipoles in parallel narrows the pattern vertically.

[3] As more elements are added to an array, the beamwidth of the antenna's main lobe decreases, leading to an increase in gain.

Active array antennas, in which groups of elements are driven by separate RF amplifiers, can have much higher gain, but are prohibitively expensive.

Since the 1980s, versions for use at microwave frequencies have been made with patch antenna elements mounted in front of a metal surface.

[8] When driven in phase, the radiation pattern of the reflective array is a single main lobe perpendicular to the plane of the antenna, plus several sidelobes at equal angles to either side.

The main lobe of the antenna can be steered electronically within a limited angle by phase shifting the drive signals applied to the individual elements.

Each antenna element is fed through a phase shifter which can be controlled digitally, delaying each signal by a successive amount.

Another option for steering the beam is mounting the entire antenna structure on a pivoting platform and rotating it mechanically.