Small Antennas

The function of an antenna is to transmit and receive energy in the form of electromagnetic waves. The size of an antenna is measured in terms of the wavelength associated with the transmission frequency. For an antenna to deliver maximum efficiency and energy transfer, its length must be arithmetically related to the frequency of the wave. However, even so, the shorter the antenna, the less efficient it is. This is one of the bottlenecks in miniature antenna design, which comes down to finding a compromise between achieving good efficiency and a wide bandwidth while remaining miniature.

In this article, we review this topical subject, without being exhaustive. The reader will first discover the definition of the key parameters characterizing the performance of miniature antennas, as well as their limits in terms of size, bandwidth, radiation quality coefficient and gain. The contribution of several authors to the fundamental limits of small antennas is also summarized. Secondly, we look at the many miniaturization techniques devised to reduce the size of existing antennas, with varying levels of complexity and ingenuity. In particular, we look at some of the techniques proposed over the years to miniaturize dipole and planar patch antennas. Finally, we present some examples of antennas for cell phones and the role that the PCB (printed circuit board) plays in overall antenna performance.