A plane micro-strip type adaptive ESPAR (electronically steerable passive array radiator) antenna array has advantages of low profile, compact structure, low weight, low production cost and the like.
The operation principle of an ESPAR antenna has two types of radiant elements in an array. One central element is called active element and the other is called passive element (or parasitic element). These passive elements are disposed symmetrically to the central active element in such a way to behave like a passive radiator by picking up electro-magnetic coupling.
The parasitic element is connected to a variable reactance passive circuit. The amount of coupling basically depends on a variable reactance value changing an input excitation phase. Thus, beam steering is generated in the array and the directivity of the antenna can be controlled.
In the design of a plane ESPAR antenna, conventionally a multi-layer type in which a variable reactance RF circuit is installed in a layer different from the layer of a printed-circuit board (PCB) mounting an antenna element is popular.
The followings are prior technical documents related to a plane ESPAR antenna technique and presume multi-layer structures.
Patent document 1: Japanese Laid-open Patent Publication No. 2005-159401
Patent document 2: Japanese Laid-open Patent Publication No. 2007-267041