Active antennas - Design principles

The concept of the active antenna has its origins in that of the antenna array, which is characterized not only by a general functional definition, but also by a wide variety of implementations.

To illustrate the diversity of appearances and methods of realization of antenna arrays, we cite (cf. article Antennas - Techniques [E 3 284] , § 1, in this treatise):

• the "Yagi" type antenna, known as a "rake" because of its appearance, used on our homes for ultra high frequency TV reception;

• antennas made up of slotted guides, isolated or combined to form a flat or cylindrical surface. Very common, though often invisible, they are used on airliner weather radars, ship navigation radars, fighter aircraft radars, satellite radars and telecommunications stations;

• antennas made from printed circuit boards (printed antennas), radiating via a network of "blocks", interrupted lines, meanders, slots, etc. These are the very low-cost version of array antennas, and are used wherever there are few constraints on the transmitted power balance (radar gauges, intruder detection radar, short-distance radio links). At low frequencies (100 MHz to 2 GHz), they can also prove highly efficient in terms of performance for low mass and low cost, as in the Landsat satellite antenna;

• antennas made up of radiating elements and electronic modules, assembled and installed on a structure. This type of solution is often adopted for electronically scanned antennas (see the article Antennas - Different types [E 3 282] , § 8, in this treatise).

In the first three examples, radiation is organized once and for all: the antenna is internally inert, the pattern is fixed relative to the antenna, and the beam is steered by mechanically setting the antenna in motion.

• The electronically scanned antenna, on the other hand, features electronic devices with at least phase-shifting functionality, enabling internal state changes and changes in the direction and shape of the transmitted beam.

  These antennas come in a wide variety of designs, each with different degrees of complexity. A distinction is made between passive and active antennas.

  • The passive antenna performs beam shaping and beam splitting, but the transmit function (amplification of the signal to be transmitted) and the input for the receive function (low-noise amplification of the signal received), are performed...