Chapter 2
Small Circularly Polarized Antenna
2.1 Introduction
Small-size CP antennas are of interest for a variety of applications, such as wireless sensors, hand-held GPS terminals, RFID readers, small WLAN access points and satellite communications mobile terminals. This chapter will start with an introduction to the basic theory of small antennas and the relationships amongst antenna size, quality factor and bandwidth are discussed. Then, different techniques of designing small CP antennas are presented and examples of different types of antennas (microstrip antennas, helix, QHA, PQHA, slot antennas, DRA) are illustrated. Finally, a table summarizing the performance of some typical small CP antennas is provided.
2.2 Basic Theory of Antenna Size Reduction
A small antenna means that the antenna is electrically small as a function of its overall size or occupied volume relative to the wavelength at its lowest resonant frequency. Usually, a small antenna is one for which [1–4]:
2.1 
where 
is the free space wavenumber, and 
is the radius of an imaginary sphere enclosing the maximum dimensions of the antenna. The lower bound of the antenna quality factor is given by [3, 4]:
where ...
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