Some considerations on the correct measurement of the gain and bandwidth of electrically small antennas

Abstract

The maximum realizable gain and bandwidth of an antenna of given electrical dimensions are governed by physical laws of nature, and do not depend on the particular technology used to realize the antenna. Thus, data sheets and papers presenting electrically small antennas (typically with their largest dimension smaller than a quarter of a wavelength) having a high gain (>2 dBi) for a tolerable bandwidth (a few percent) should be regarded with some caution. What usually happens is that the measurements done by the manufacturer to establish those data sheets neglect a typical characteristic of electrically small antennas: they are never unbalanced and seldom completely balanced, which means that the current distribution at their feeding point is neither symmetric or asymmetric. The usual measurement setup feeds the antenna through a coaxial cable, which is an unbalanced feed. Thus, if no precaution is taken, the shielding of the coaxial cable will radiate as much or even more than the antenna under test, and the measured gain or bandwidth will correspond to the radiator formed by the antennas under test and the feeding cable. This paper shows what happens when elementary measurement precautions are not taken, and illustrates this phenomenon with examples of data sheets of commercially available antennas. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 17: 156–160, 1998.