Measurement of near fields of antennas and scatterers

Abstract

A tutorial review of techniques used for the measurement of near-fields of antennas and scatterers, and an extensive bibliography of the readily available literature in this area, are presented. M I. INTRODUCTION OST of the cxperirnental measurements made on antennas are concerned with the determination of the fundamental characteristics of the antenna t.hat. have to do wit.h their immedia.te application: the input, impedance, the far-field radiat.ion patt,ern, and the gain or the efficiency. There are however, occasions when it is desirable to havc in addition t,o, or in lieu of, t.he preceding properties, information about. the current. or charge distribution on the ant,enna, and/or t.he distribution of the near fields, i.e., the fields in the immediate vicinit.y of the antenna. The near fields of the antenna can be conveniently divided int.0 the "reactive near field" and the "radiating near field" [l]. The former may be considered to be that, region immediately surrounding t,he ant.enna, with a usual out,er linit approximately a wavelength or less. The latter is that region, beyond the reactive near field, in which t,he radiation pattern is dependent upon t.he distance from the antenna. This latter region is usually considered to extend to a distance 2D2/X from a uniformly illuminat.ed radiating aperture of diamet,er Dl and much greater distances for apcrtures excited with noncophasal distribut>ions. These two regions may present different measurement. problems. Rleasuremcnt, of the current or surface charge distributions on t.he surface of an antenna involves measurement of the normal electric field and tangential magnetic field very near this surface. Similarly, the measurement. of aperture field distributions usually are made within or very near this aperture. These measurements are thus usually made in the reactive nea.r field and the requirement that they be made in very close proximity to the a.ntenna may dictate the use of very small probe ant,ennas to sample these fields. There is a large class of measurements which can be made at, somewhat greater distance from the antenna but still well within the radiating near field. This would

IEEE Standard Test Procedures for Antennas

“…For measurements made at short distances the antenna under test may be used as the transmitting antenna and a simple receiving antenna or probe may be used to sample the radiated Þeld [65], as shown in Fig 37(a). A reference signal may be coupled out from the transmission line leading to the test antenna and compared with the received signal in a suitable circuit.…”

Analysis of electromagnetic wave propagation by using Poynting vector in automobile

“…Measurements for this paper were carried out using the near magnetic field probe [6] shown in Fig. 1.…”

Determination of the magnetic complex antenna factor of a double‐gapped shielded loop antenna

“…Figure 2 shows the structure of a double-gapped shielded loop antenna with a square element. This antenna has two feed points and has the characteristic that the magnetic field can be measured by itself without being affected by the electric field [5,6]. …”

“…To resolve this problem, a double-gapped shielded loop antenna with two feed points has been proposed. This structure is less likely to be affected by the electric field [5,6].…”