SOME CONSIDERATIONS ON THE CORRECT MEASUREMENT OF THE GAIN AND BANDWIDTH OF ELECTRICALLY SMALL ANTENNAS

The maximum realizable gain and bandwidth of an antenna of given elect rical dimensions are governed by physical laws of nature, and do not d epend on the particular technology used to realize the antenna. Thus, data sheets and papers presenting electrically small antennas (typical ly with their largest dimension smaller than a quarter of a wavelength ) having a high gain (> 2 dBi) for a tolerable bandwidth (a few percen t) should be regarded with some caution. What usually happens is that the measurements done by the manufacturer to establish those data shee ts neglect a typical characteristic of electrically small antennas: th ey are never unbalanced and seldom completely balanced, which means th at the current distribution at their feeding point is neither symmetri c 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 e ven more than the antenna under rest, and the measured Rain or bandwid th will correspond to the radiator formed by the antennas under test a nd the feeding cable. This paper shows what happens when elementary me asurement precautions are not taken, and illustrates this phenomenon w ith examples of data sheets of commercially available antennas. (C) 19 98 John Wiley & Sons, Inc.