The conical spiral antenna over the ground

The two-arm, conical spiral antenna (CSA) is a well-known antenna that has several desirable features when isolated in free space, such as uniform input impedance, gain, and circular polarization over a broad frequency range. In this paper, this antenna is investigated when placed directly over the ground with its axis normal to the surface of the ground. In this configuration, the CSA may be useful for applications in which signals must be transmitted into the ground and/or received from within the ground, such as communication links to underground tunnels and ground-penetrating radars. First, the performance of the CSA in free space is briefly reviewed, and then qualitative arguments based on the geometrical and electrical properties of the isolated antenna are used to predict the performance when this antenna is placed over the ground. Next, results from a complete analysis of the CSA over the ground, performed with the finite-difference time-domain method, are used to quantitatively verify these predictions. The paper ends with an illustrative example in which the CSA is used in a monostatic ground-penetrating radar. Index Terms--Broad-band antenna, conical spiral antenna (CSA), finite-difference time-domain (FDTD), frequency-independent antenna, ground-penetrating radars (GPR).