5 Subjects related with small antennas

5.1Major subjects and topics

The major subjects that have long been discussed on small antennas are investigation into the fundamental characteristics and performances of small antennas, and exploitation of methods or ideas to realize small antennas, particularly ESA (Electrically Small Antennas). Also, there have been practical matters discussed frequently as critical problems involved with designing small antennas. They are rather urgent problems that are concerned with design of antennas in unusual or uncommon environmental conditions often encountered in practical applications.

5.1.1Investigation of fundamentals of small antennas

The essentials of small antennas have long been studied by many investigators; however, there are still many problems that must be discussed or solved. Among them, the limitations of small antennas is the most significant and interesting subject. It is important because knowledge of the limitations may give a clue to realizing novel small antennas. The physical-size limitation was first discussed by Wheeler in 1947 and then by Chu in 1948, as was mentioned in Chapter 4. Since then small antenna problems have been continually discussed down to the present day. Chief problems are those concerned with major antenna parameters regarding antenna quality factor Q, gain, bandwidth, and efficiency, in conjunction with the limitations of small antennas. How to obtain antenna Q precisely still remains a controversial problem, since Q directly relates with the radiation power, and exact calculation of the reactive part of the radiation power still remains a controversial subject. The antenna Q is significant in relation to the bandwidth, since when the antenna Q is appreciably high it is very nearly the reciprocal of the bandwidth. Thus achieving required bandwidth with an antenna of given size is related to realizing the Q of a corresponding value. When the Q is small, the bandwidth is wide; however, with small-sized antennas, it is usually a hard problem to obtain a small Q.

5.1.2Realization of small antennas

How to achieve a small antenna with reasonably high gain, efficiency, bandwidth, etc. is one of the most serious propositions in the field of antennas and communications.