List of Symbols I Introduction II Definition of the Antenna 2,1 The Design Principle 2"2 The Equiangular Spiral 2.3 The Antenna 2.4 Physical Construction of the Antenna III The Radiation Pattern 3.1 The General Far Field Pattern 3,1a The Typical Antenna 3"lb The Pattern Bandwidth 3.2 Variation of Antenna Parameters 3.3 Pattern Beamwidth and Pattern Rotation 3,3a Pattern Rotation 3, 3b Pattern Beamwidth 3.4 Polarization of the Field 3.5 Variations in the Basic Antenna 3,5a Designing for a Specific Band of Frequencies 3,5b Effect of Construction with Thick Arms IV The Near Fields 4.1 The Tangential Field in the Slot Arms 4.2 The Normal Field on the Arms 4,2a General Considerations and Theory of Measurement 4,2b Adjustment of Equipment 4,2c Measured Fields 4.3 Estimation of Error V The Input Impedance VI Efficiency of the Basic Antenna 6.1 Method of Measurement 6.2 The Experimental Investigation 6,2a Measurement of Surface Resistivity 6,2b Ratio of Power Inputs 6, 2c A Check on the Current Distribution 6, 2d Measured Efficiency VII The Cavity Backed Antenna 7.1 Efforts toward a Unidirectional Antenna 7.2 Antenna with Absorbing Termination 7.3 The Antenna over .a Conventional Cavity IX

The balanced conical logarithmic-spiral antenna is considered as a slow-wave locally periodic structure with a slowly varying period. A study of the near fields and their relationship to the f a r fields has led to the identification of the active region or effective radiating aperture on the antenna and to a clearer understanding of its operating characteristics. Information of the near-and far-field characteristics and on the input impedance for a wide range of parameters is presented in a form suitable for use in the design of practical antennas.

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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

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