Multi-Polarized Spiral Antennas for RF Sensing

Spiral antennas are capable of dual polarized operation if the currents can propagate along their arms by going either out or in through the radiation region of the spiral. The wrapping sense of the spiral determines the polarization of the radiated field resulting from this current flow. There are two ways to accomplish this phenomenon over very wide bandwidths. Specifically, either have a feed point for the spiral both outside and inside the radiation region, or use of the modulated arm width (MAW) spiral antenna. The MAW spiral antenna has not been widely accepted and is seldom reported in open literature. Its geometry is however sufficiently unique from the other planar frequency independent (FI) antennas to require a complete explanation of where it fits for different applications, specifically RF sensing. The design of the MAW spiral antenna is detailed herein including geometry described by modulation period, modulation magnitude, expansion rate, total number of arms, feed point structure, termination, cavity, feeding and dielectric effects. The emphasis is on detailed understanding of its performance characteristics such as impedance, pattern control and quality. The relevance of these characteristics to the antenna being used as a sensor is explained. The specific concerns being quantified are location by angle of arrival techniques and polarization detection. The use of a four-arm MAW spiral for angle of arrival as well as polarization sensing is demonstrated theoretically and experimentally. This combined capability has not been mentioned in any literature previously and was investigated thoroughly under this thesis to determine the limitations since as found herein no other four-arm FI planar antenna has this capability. In addition, the application of several geometries of the MAW spiral are examined as possible improvements over the original equiangular geometry including Archimedean, bi-layer, and structures that are not self-complementary due to either the modulation ratio or the period. In particular, pattern performance improvement is demonstrated for modulation periods that do not produce self-complementary geometries while having minimal impact on impedance. Finally, an investigation into the asymmetric modes for an arbitrary number of arms was conducted to evaluate the performance limits of the highest available mode (mode with the largest phase change between arms) of a MAW spiral. Typically, this highest mode has significantly poorer performance than the other modes due to the inability of the MAW spiral to separate it from the modes that are not controlled by the beamformer

Experimental evaluation of shape memory alloy actuation technique in adaptive antenna design concepts

Creation of an antenna system that could autonomously adapt contours of reflecting surfaces to compensate for structural loads induced by a variable environment would maximize performance of space-based communication systems. Design of such a system requires the comprehensive development and integration of advanced actuator, sensor, and control technologies. As an initial step in this process, a test has been performed to assess the use of a shape memory alloy as a potential actuation technique. For this test, an existing, offset, cassegrain antenna system was retrofit with a subreflector equipped with shape memory alloy actuators for surface contour control. The impacts that the actuators had on both the subreflector contour and the antenna system patterns were measured. The results of this study indicate the potential for using shape memory alloy actuation techniques to adaptively control antenna performance; both variations in gain and beam steering capabilities were demonstrated. Future development effort is required to evolve this potential into a useful technology for satellite applications

Near-field testing of the 5-meter model of the tetrahedral truss antenna

This report documents the technical results from near-field testing of the General Dynamics 5-meter model of the tetrahedral truss antenna at the Martin Marietta Denver Aerospace facility. A 5-meter square side of the tetrahedral served as the perimeter of the antenna, and a mesh surface and extensive surface contouring cord network was used to create a parabolic aperture shape to within an rms accuracy of 30 mils or better. Pattern measurements were made with offset feed systems radiating at frequencies of 7.73, 11.60, 2.27, and 4.26 (all in GHz). This report discusses the method of collecting the data, system measurement accuracy, the test data compiled, and diagostics and isolation of causes of pattern results. The technique of using near-field phase for measuring surface mechanical tolerances is included. Detailed far field antenna patterns and their implications are provided for all tests conducted

Multi-polarized spiral antennas for RF sensing

Spiral antennas are capable of dual polarized operation if the currents can propagate along their arms by going either out or in through the radiation region of the spiral. The wrapping sense of the spiral determines the polarization of the radiated field resulting from this current flow. There are two ways to accomplish this phenomenon over very wide bandwidths. Specifically, either have a feed point for the spiral both outside and inside the radiation region, or use of the modulated arm width (MAW) spiral antenna. The MAW spiral antenna has not been widely accepted and is seldom reported in open literature. Its geometry is however sufficiently unique from the other planar frequency independent (FI) antennas to require a complete explanation of where it fits for different applications, specifically RF sensing. The design of the MAW spiral antenna is detailed herein including geometry described by modulation period, modulation magnitude, expansion rate, total number of arms, feed point structure, termination, cavity, feeding and dielectric effects. The emphasis is on detailed understanding of its performance characteristics such as impedance, pattern control and quality. The relevance of these characteristics to the antenna being used as a sensor is explained. The specific concerns being quantified are location by angle of arrival techniques and polarization detection. The use of a four-arm MAW spiral for angle of arrival as well as polarization sensing is demonstrated theoretically and experimentally. This combined capability has not been mentioned in any literature previously and was investigated thoroughly under this thesis to determine the limitations since as found herein no other four-arm FI planar antenna has this capability. In addition, the application of several geometries of the MAW spiral are examined as possible improvements over the original equiangular geometry including Archimedean, bi-layer, and structures that are not self-complementary due to either the modulation ratio or the period. In particular, pattern performance improvement is demonstrated for modulation periods that do not produce self-complementary geometries while having minimal impact on impedance. Finally, an investigation into the asymmetric modes for an arbitrary number of arms was conducted to evaluate the performance limits of the highest available mode (mode with the largest phase change between arms) of a MAW spiral. Typically, this highest mode has significantly poorer performance than the other modes due to the inability of the MAW spiral to separate it from the modes that are not controlled by the beamformer