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The self-complementary antenna is a basic antenna for extremely broadband practical antennas.This antenna is an arbitrarily shaped antenna which is constituted with a half of an infinitely extended planar-sheet conductor such that the shape of its complementary structure is exactly identical, or "self-complementary" with that of the original structure with two terminals for the simplest case. The self-complementary antenna has constant input impedance independent of the source frequency and the shape of the structure.
The type of the self-complementary antenna is not limited only to the case of a planar antenna with two terminals, but there are more general types with various grades of complexity, such as, the number of terminals, the number of reference planes, and others. Moreover, there is infinite freedom in their shapes of the structures. They also have constant-impedance property independent of the source frequency and the shape of the structure for respective classes of structures with various grades of complexity. This general principle of self-complementarity is also called the "Mushiake Principle" by various sources.
Examples for two shapes of the structures are shown in the attached figures. The structures 1 to 3 are for square shape, and the structures 4 to 6 are spiral shape. Actually, the structures extend infinitely, but the figures show only finite portions near the feed point of each structure.
Explanation of SCA structure 1