SMALL ANTENNAS Basic Principles of Electrically Small Antennas

E lectrically small antennas have been an important part of communications engineering since the beginning. Whether they are small compared to the extremely long wavelengths used at the lowest radio frequencies, or intended to save space in GHz-range wireless devices, the basic principles are the same. This tutorial will review those principles, with primary attention to describing the performance tradeoffs of small size. Definition of " Electrically Small " There are various rules of thumb for considering an antenna to be electrically small. The most common definition is that the largest dimension of the antenna is no more than one-tenth of a wavelength. Thus, a dipole with a length of λ/10, a loop with a diameter of λ/10, or a patch with a diagonal dimension of λ/10 would be considered electrically small [1]. This definition makes no distinction among the various methods used to construct electrically small antennas. In fact, most work on these antennas involves selecting topolo-gies suitable for specific applications, and the development of integral or external matching networks. Most readers will be familiar with several common uses of small antennas. Loop antennas and short monopoles (whip) for medium-wave (AM broadcast) reception are common in home and vehicle entertainment systems. With wavelengths in the 200 to 600 meter range, these antennas far exceed the λ/10 criterion. Antennas for FM and television broadcast reception are sometimes reduced in size for convenience and portability. The ubiquitous 315 or 433 MHz wireless remote control and telemetry systems for key-less entry, garage door openers, wireless doorbells and remote-reading thermometers rarely have " full-size " resonant antennas, since a wavelength is around 1 meter. A λ/4 monopole would be 17 cm long, and requires a similarly-sized counterpoise. The developing RFID market demands low cost and small size. A 3 cm square RFID tag will have an antenna that is considered electrically small at any frequency below about 1 GHz. Handheld RFID readers will allow somewhat larger antennas, but will still fit the λ/10 criterion at many of the commonly used frequencies. Finally, of course, are wireless phones, which now have integrated GPS, Bluetooth™ and other radio systems. Only the largest form factors can support antennas that are large enough to be outside the electrically small definition. The most common structures used in electrically small antennas are the short dipole (or equivalent monopole and ground place), the small loop, and the dielectrically-loaded patch. Each …