When m = 0 (ZOR mode), the resonance frequency is independent of physical length. The resonance frequency ω0 is either one of√ωse or ωsh. When the transmission line is a balanced structure, ω0 = ωse = ωsh = 1/ L R CL . The property of ZOR structure, i.e., size-independence of the resonance frequency, can be applied to reduce the antenna size; i.e., design of ESAs, as the physical size does not depend on the frequency, but on the circuit parameters C and L [56, 57]. Examples are introduced in Chapters 7 and 8.

6.3Techniques and methods to produce FSA

Since an FSA does not necessarily have its size much smaller than the wavelength as in an ESA, techniques and methods to produce FSA differ from that of ESA. Generally, an FSA is constructed by integration techniques, by which either antenna performance can be improved or enhanced, or some function is integrated into an antenna structure so that the antenna can perform additional functions as a consequence of integration.

6.3.1FSA composed by integration of components

Integration of components such as device, circuitry, or materials, including metamaterials, with antenna structure has been commented upon in the previous sections; for the purpose of generating uniform current distributions, composing SW structure, or self-resonance structure, and modifying antenna geometry so that various types of small antennas, not only FSA, but also ESA, can be created. The major objective of integration is generally to change current distributions on an antenna structure so that modification of antenna performance, change in antenna characteristics, and improvement or enhancement of antenna performances, are achieved. These techniques enable us to produce various interesting FSAs such as those having enhanced gain or bandwidth or capability of varying or controlling radiation patterns.

6.3.2FSA composed by integration of functions

By integrating some function into an antenna, either enhancement of the antenna performance or improvement of the antenna characteristics can be expected. The functions to be integrated include amplification, oscillation, frequency conversion, power combining, and pattern control. By integration of amplification, gain increase can be expected, although extreme care about internal noise in the amplifier should be taken into account. The concept of smallness in the FSA should be understood in such a way that when, for instance, an enhanced gain or wider bandwidth that can be attained only by increasing the antenna size is realized, then the antenna of unchanged size can be said to be a small antenna. Integration of oscillator circuit, frequency converter, etc. will contribute to produce varieties of interesting FSAs such as a power oscillating antenna, an antenna having frequency mixing function, power combining in space, and so forth.

The integration technique may bring evolutional antenna systems that cannot be realized by conventional methods. Either passive or active integration, or a combination