Analysis and design of conical spiral antennas in free space and over ground.

摘要

In this research, the two-arm, conical spiral antenna (CSA) is investigated for its use as a transmitting and receiving antenna for all applications in free-space, e.g., telecommunications and electromagnetic compatibility and for its use when placed over the ground, e.g., in a ground-penetrating radar (GPR). This antenna belongs to the class of frequency-independent antennas, and has uniform performance characteristics, such as input impedance, gain, and circular polarization over a broad frequency range. These are desirable features for the above applications. The CSA is analyzed using the finite-difference time-domain (FDTD) numerical method and validated by comparison with measurements of the input impedance and the realized gain. A parametric study is performed using the FDTD analysis, and the results from the study are used to produce new design graphs for this antenna. The graphs supplement and extend the existing, mainly empirical, design base for this antenna. Two resistive terminations, intended to improve the low-frequency performance, are examined. One is a termination formed from two lumped resistors, and the other is a new termination formed from a thin disc of resistive material. When the CSA is placed directly over the ground with its axis normal to the surface of the ground, it may be useful for applications in which signals must be transmitted into the ground and/or received from within the ground. Qualitative arguments based on the geometrical and electrical properties of the CSA isolated in free space show that the beneficial properties of the CSA in free space are preserved when placed over the ground. Results from a complete analysis of the CSA over the ground are used to quantitatively verify these arguments. Illustrative examples are presented in which the CSA is used in a monostatic GPR.