The Gopher Antenna: A New Type of Ground Penetrating Radar Antenna

Currently, ground penetrating radar (GPR) antennas for ground-coupled setups are inefficient. They use resistors and/or dissipative material to improve the impedance bandwidth and thus achieve optimal pulse shape. These antennas are mostly bowtie dipoles and have an omnidirectional pattern, although the casing and the electrical properties of the ground modify the pattern. The center frequency of the antennas in free space is often 500 MHz, and the required bandwidth is wide: the spectrum ratio is from 1:2 to 1:10 at a −10 dB limit. The phase center must remain stable over the whole bandwidth. The main objective of this dissertation is to develop an antenna that is more efficient and has good directivity. The concept of a patch antenna with a dual resonant structure was used as a basis. To achieve this goal, a feed antenna with three resonant frequencies was used. The novel point in the design is the second resonant frequency (not the first, as is typical) of the feed that is matched to the line impedance by a coupled dual resonant parasitic patch structure. Around the center frequency, the matching is good, and there are no dissipative materials. Therefore, the antenna is efficient. The radiated spectrum expands well beyond the matched impedance area and thus is not efficient in that part of the radiated spectrum, but it provides a Gaussian spectrum which is preferred by GPR users. The patch antenna design provides an inherent directivity, often 9 dBi, and combining the feed improves the directivity further. For practical reasons, this antenna type was given the name the "Gopher antenna". A reasonably priced GPR without an antenna was provided by one manufacturer. This was partly modified by the author to fit better the Gopher antenna so that the tests could be performed. Measurements were done in settings where there were known objects underground, and on a lake where the lake bottom was visible. The received data needed extensive processing. The sophisticated free software for the processing did not read the files from the system, thus a simple processing software was designed by the author. This had the advantage of enabling the testing of various pre- and postprocessing methods, and the trace integration method with optional deconvolution and cross-correlation methods were found to be useful with this data. The antenna was implemented, and the measured results validate the concept. The lake profiles are quite clear, and the ground profiles show reflections from known objects in the expected size range. As there was no standard way of describing the antenna radiation in the ground, I propose that the highest electric field magnitude is stored in each FDTD pixel during the simulation. This provides a useful graphic with which to compare antenna patterns. The pattern was also tested in a case study by tilting the antenna. With this efficient and directive antenna, the GPR is expected to see deeper.