Investigation of the Radio Source 3C 273 by the Method of Lunar Occultations

A theoretical and numerical analysis of square Van Atta reflectors has been carried out with or without a conducting plate, used for mounting of the antenna elements. The Van Atta reflector investigated has antenna elements which are parallel half-wave dipoles interconnected in pairs by transmission lines of equal electrical length. The dipoles are placed in a plane which is parallel to the conducting plate when this is present. In the theory, each pair of antenna elements with the interconnecting transmission line is represented by an equivalent circuit. The mutual impedance between the antenna elements and the reradiation from the elements as well as from the conducting plate have been taken into account. The influence of the conducting plate on the induced dipole currents has been treated using the theory of images. The scattering cross section of Van Atta reflectors with or without a conducting plate has been examined numerically using an electronic computer, and some of the results have been compared with experimental results obtained by others.

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Square Van Atta reflector with conducting mounting flame

This communication reports the results of an experimental investigation of a new log-periodic (LP) antenna. Tbe antenna consists of an LP array of radial waveguides excited by a two-wire transmission line. The antenna exhibits a vertically polarized omnidirWtiOM1 radiation pattern which is essentially constant over the designed bandwidth. Much progress has been made in the development of frequency-independent antennas. The main features of this class of antennas are directional radiation patterns and impedance which are constant over a very m-ide bandwidth. These antennas are generally not capable of omnidirectional operation. Dyson [ 1 ] has shown that a logspiral antenna of the proper configuration would have an approximately Omnidirectional radiation pattern which is circularly polarized. The discone antenna, which does not belong to the log-periodic (LP) class, has an omnidirectional vertically polarized radiation pattern over an approximate 6 to 1 bandwidth [2], [3]. The purpose of this investigation was to determine whether the log-periodic principle could be applied to a structure to give omnidirectional and vertically polarized characteristics over a wider range of fiequencies than is now possible. The basic element chosen for such a structure was a pair of parallel circular disks which are closely spaced compared to the wavelength. Such a basic element, when excited at the center of the plates, operates as a radial waveguide in the lowest E-type mode [4], thus producing the desired far-field radiation patterns. The LP structure is composed of an array of such disks with the diameter of the plates and the spacing between them varying in a log-periodic manner. The elements are alternately connected to a narrow two-wire transmission line running through the center of the disks. The LP antenna is shown in Fig. 1. Tests were made on three models of the structure having values of the LP parameter 7 of 0.90, 0.96 and 0.98. The ratio of the plate spacing to the diameter u was held constant at 0.115. The antennas were designed to operate over a 3: 1 bandwidth (500 MHz to 1500 MHz). The diameter of the largest plate was approximately 0.4 wavelength and the height was approximately 0.93 wavelength at 500 MHz (T =0.96). Measurements of impedance, far-field radiation patterns, and polarization were made. For all models, the impedance was relatively constant over the designed frequency quency range, being within an SWR of 2: 1 relative to I5+,20 ohms. The far-field pattern measurements showed that the structure was omnidirectional within 3 dB over most of the designed bandwidth. Better omnidirectionality was obtained with r closer to 1 than when T was a lower value. Fig. 2(a) shows the typical azimuth variation. The eleManuscript received March 13, 1968; revised April 1, 1968 and July 8. 1968. This work was supported in part by the Small Industries Research Program through The Pennsylvania State University, and in part by National Science Foundation Traineeship Grant GE7919.1. Fig. 1. The log-periodic parallel plate antenna. vation patterns showed that the angle of maximum radiation occurred between 45 to 75 degrees with the axis of the antenna and scans with frequency. A typical elevation pattern is shown in Fig. 2(b). The polarization measurement, shown in Fig. 2(c), was made by changing the orientation of a linearly polarized transmitting antenna with respect to the axis of the test antenna. Zero degrees on Fig. 2(c) represents the point where the polarization of the transmitted wave was parallel to the axis of the test antenna, i.e., vertically polarized. Ninety degrees represents the point where the polarization of the transmitted wave was perpendicular to the axis of the test antenna, i.e., horizontal polarization. Thus the structure is, indeed, vertically polarized. More details of the experimental investigation may be found in [5]. The investigation indicated that the structure is capable of vertically polarized omnidirectional radiation and an impedance characteristic which are both relatively constant over a very wide bandwidth. However, the deviations from the desired omnidirectional characteristics indicate that the lowest E-type mode may not be the dominant one on the antenna at some frequencies. Near-field measurements indicate that higher-order modes are present. The nonpropagating modes will not affect the far-field radiation characteristics, but can account for the large reactive component of the input impedance. A comparison of the LP disk antenna with a discone antenna of the same bandwidth (3 to 1) indicates that the LP antenna is considerably larger in size. For a lower cutoff frequency of 500 MHz, the largest diameter of the LP antenna is 9 inches whereas the largest diameter of the discone antenna is 5.5 inches. The height of the LP antenna is 20 inches compared to 5 inches for the discone antenna. Theoretically, the LP disk antenna should be I capable of operation over a wider bandwidth than is obtainable with a discone antenna. L Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on October 2, 2009 at 07:12 from IEEE Xplore. Restrictions apply.

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T. Larsen

Papers

The influence of the scattered field on the reradiation pattern of reflector arrays