Fred L. Cain

Statistical Prediction Model for EMC. Analysis of Out-of-Band Phased Array Antennas

A statistical model was developed for predicting and describing the out-of-band pattern characteristics of phased arrays containing ferrite phase shifters. Statistical analysis techniques are necessary because experimental investigations showed that the propagation of higher-order modes at out-of-band frequencies cause phased arrays containing ferrite phase shifters not only to respond differently at out-of-band frequencies but also to exhibit random behavior. Equations were derived which relate the out-of-band pattern scanning properties, relative gain levels, median gain, and standard deviation to the in-band scan angle and the ferrite phase shifter statistics. Computer algorithms were written to both compute and graphically display the out-of-band antenna patterns. Experimentally derived phase-shift data were used as inputs to the model. The results of these investigations show that statistical analysis techniques are potentially very useful in an EMC context for characterizing out-of-band responses of phased array antennas

Investigation of Choke Blinders for Feed Horns of Radar Antennas

Previous studies with some reflector type antennas have shown that wide-angle radiation and consequently radiofrequency interference (RFI) can be reduced by using choke blinders on the E-plane edges of feed horns. These choke blinders, which are arrays of choke slots or grooves, were designed such that the depth and the separation of the slots were a quarter wavelength. Subsequent efforts have been concerned with how various design parameters affect the performance of the choke blinders. Experimental studies were conducted over a wide-frequency band to observe the effects of varying the number, width, spacing, and depth of the choke slots or grooves and of varying the angle, position, and length of blinders on an X-band hoghorn. The data presented enable one to make a judicious choice of appropriate design parameters.

Statistical Gain Characteristics of Radar Antennas at Very Short Fresnel Zone Distances

This paper extends the existing knowledge of statistical gain characteristics of radar antennas to include very short Fresnel zone ranges. The data indicate that the median gain and the standard deviation remain essentially the same as those observed in the Fraunhofer zone until the range begins to approach the physical dimensions of the antenna; however, as the range is decreased closer toward physical contact, the median gain decreases and the standard deviation increases. The site-effects tests which were conducted in the Fresnel zone indicate that the geometrical configuration of an object, the position of an object, and the clear-site median gain of an antenna help to determine the magnitude of the effects of objects on the clear-site statistical gain characteristics.

Electromagnetic Models for Antenna Performance, EMC, and Biological Effects

This paper describes several models which have been developed at Georgia Tech for the analysis of antennas, EMC, and biological effects. These models include algorithms based upon the plane-wave-spectrum scattering-matrix analysis, the geometrical theory of diffraction, and the method of moments. In addition, statistical models for antenna coupling, for out-of-band antenna performance, and for scattering by complex structures are discussed. In each case, the model results are compared with measured data to assess the accuracy of the modeling technique.

Effects of Near-Field Obstacles on Far-Field Antenna Performance Characteristics

Considerable research has been conducted to investigate near-field coupling between antennas, both with and without obstacles located in the near-field of the antennas, but until now, very little research has been conducted to provide information for predicting the effects of various near-field obstacle blockages on far-field antenna performance characteristics. Recently completed research investigations at Georgia Tech for the U. S. Naval Ship Engineering Center have yielded considerable design information and have provided much insight into potential problem areas such as those dealing with nearfield obstacles and their effects on far-field gain loss, beamwidths, beamshifts, and maximum sidelobe levels. The effects of several variables on these far-field antenna performance characteristics were investigated, and various trends as well as typical design curves will be discussed. The usefulness of the information is illustrated by insertion of the far-field gain-loss data into a computer program for shipboard siting of antennas. Moreover, based on theoretical and experimental investigations with a dielectric-coated solid metal circular mast, applications of dielectric coatings to portions of ship superstructures to improve the far-field antenna performance is possible. A correlation of the effects of near-field obstacles on far-field antenna characteristics and on near-field antenna-to-antenna coupling is also possible.

Prediction of Near-Field Coupling between Misaligned Antennas

A technique for estimating the main-beam to mainbeam mutual coupling between misaligned-near-field antennas is presented, and predicted and. measured data are compared. The technique is based on calculated main-beam near-field patterns as a function of range for individual antennas. Curves that display the gain reduction along the boresight direction as well as the gain reduction for angles off boresight have been calculated for various types of illumination functions for circular and rectangular apertures. To account for beam distortions caused by asymmetrical illumination due to antennas being misaligned as well as being in very close proximity to each other, experimental data were utilized to improve the prediction technique.

Relations of Site Effects to Statistical Gain Characteristics of Radar Antennas

In this paper, the results of a study of site effects on the clear-site Fraunhofer zone statistical gain characteristics of several radar antennas are presented. The study was based on two types of site-effects tests. The first type of test was concerned with the illumination of objects in the immediate vicinity of the antenna under test to show that site effects may depend on the clear-site statistics of the receiving antennas under test, on the geometrical configuration of the objects, and on the position of the objects relative to the transmitting and receiving antennas. The second type of test was concerned with the illumination of objects in the distant surroundings of the antennas under test to determine how the frequency, beamwidth, and clear-site median gain of the antennas are related to site effects.