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The 2-, 4- and 6-arm planar spiral antennas were excited by various feeding arrangements, and the pattern, phase variation and polarization of their fields were measured. It was found that the modes predicted by the solution of the infinitely-many-arms spiral antenna could be excited by spiral antennas that had only a small number of arms. In particular, we succeeded in exciting the "unpreferred" mode of the spiral antenna. Measurements also showed that in addition to the main mode of the spiral antenna, the field also contains higher order modes, which actually dominate the main mode in certain regions.

The effects of ferrite loading on the characteristics of the rectangular cavity slot antenna, the equiangular spiral antenna, and the log conical spiral antenna are studied. The object is size and weight reduction and this is accomplished at the expense of bandwidth and/or efficiency. The characteristics of the rectangular cavity slot antenna are analyzed and compared with experimental data. The operating frequency may be shifted by magnetic bias. Loading the spiral and log conical antennas lowers the low frequency limit by a factor of two or three. Data are presented on the temperature sensitivity of ferrite materials for aerospace antennas.

A wide cone-angle conical spiral antenna which has been developed for use as a circularly polarized vertical-incidence ionosonde antenna is described. It exhibits an 80-degree beamwidth, 5-dB gain, axial ratio of 1.2:1 and SWR less than 2:1 typically over the band 2-20 MHz. The entire structure is only 38 meters high and 52 meters in diameter. Extensive measurements on models and the full-size antenna are presented which show that the polarization and radiation patterns are quite uniform over the entire operating band. In particular, the radiation pattern shows no evidence of sidelobes at low elevation angles which could cause unwanted oblique returns. The discovery of a beam-reversing phenomenon in spiral antennas is described along with its elimination by geometry control. An example of ionosphere mode selection by means of polarization control with the full-size antenna is presented.

A problem associated with a two-dimensional spiral doublet array is concerned with the fact that use of the spiral doublet as an element to obtain linear polarization leads to spacing difficulties, since the distance between elements is always twice as much in one dimension as in the other. Thus, the allowable scan is seriously limited in one principal plane. A method is proposed for obtaining linear polarization from a single spiral antenna by using it in combination with a polarization grating and a ground plane. Experimental results were obtained by combining eight of these elements in a one-dimensional array. For the broadside position of the main beam, the field was found to be linear, with the cross-polarized component of the main beam being 40 db down from the principal polarization. As the beam was scanned out to \pm 45\deg , the linearity decreased quite rapidly (to approximately 10 db at 45\deg ) and the plane of the linear polarization was rotated; sidelobes were higher than predicted. Use of this array rather than an array of doublets would involve a compromise concerning specifications, dependent upon the particular application of the array.

An N -arm log-spiral antenna may be excited in N - 1 orthogonal modes of N -tuple input. A rigorous solution to the spiral antenna of more than two arms is obtained by numerical solution of the integral equations. Results for a four-arm spiral, including current distribution, half-power beamwidth, beam orientation, and gain and phase centers, are presented. The propagation constant of the current waves along each spiral arm is found to be approximately the speed of light. An approximate k-\beta diagram is given which may be used to estimate the size of the antenna for each mode of operation.

A pair of equally excited but oppositely sensed Archimedean two-wire spirals situated close to one another in the same plane-a doublet-is used to generate a linearly polarized field in which the direction of polarization and phase are controlled or varied independently of each other by rotation of the spiral radiators. An array of these doublets can be made to scan by rotation of the several spiral elements; an eight-doublet array which was made to scan over an 83\deg sector with small amplitude variation is discussed. A doublet fed from a ring network can be employed as a polarization diversity circuit. A virtual doublet is achieved by placing a single spiral in a fight angle trough. A preliminary scanning array comprising four spirals in a trough was made to scan \pm36\deg . The possibility of using a parasitic spiral in conjunction with a driven spiral for obtaining linear polarization of variable direction and phase is indicated. Also, a brief simplified analysis of the two-wire Archimedean spiral is presented, which leads to the concept of higher-order modes of radiation.

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 far 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.

A study was made of a method previously suggested for constructing a passive direction finder capable of determining the azimuth and elevation of an electromagnetic wavefront. This direction finder consists of a two-wire, Archimedian, spiral antenna and a phase comparator. Angular information of accuracy too low for the intended application is obtained when the theory and experimental data previously presented are used. A general theory for the direction finder is established and methods for accuracy improvement are suggested.

From an experimental investigation of the foil-type kind of a conical, logarithmic spiral antenna, the relative phase velocity along the antenna arm as a function of the cone angle has been found. It is probable that a reduction of the antenna length by one half can be achieved by using a special dielectric loading.

Expressions for obtaining the radiation patterns for a directive continuous spiral antenna array, with a desired main direction of radiation, are presented. A suitable approximation has been made in the general formula for numerical computations. The vertical and the horizontal radiation patterns are graphically shown and discussed.