TRASFORMADORES .1 INTERCONEXIONES

GPR antennas are designed according to the type of radar system used. If the radar system is a time domain that applies an impulse to the antenna, there is a requirement for linear phase response across the operating band. Unless using a matched filter to reduce the effect of the frequency- dependent radiation characteristics of the antenna, only a limited number of antennas can be used in time domain radars (Daniels, 2007). When the radar system is frequency modulated (e.g.

78 FMCW) or synthesised (e.g. SFCW), the requirements for linear phase response from the antenna can be relaxed. The configuration and number of antennas used also add another level of complexity of the antenna design. In the case of bistatic operation where separate transmitter and receiver antennas are used, the cross coupling level between them is a critical parameter. Figure 5.4 shows three types of antenna arrangements and their main advantages.

Figure 5-4: Antenna arrangements

Most GPR antennas operate within short ranges of the ground interface; therefore, it is important to appreciate the effect of the ground material near the antennas. As discussed in earlier chapters, the ground material, in general, is regarded as a lossy dielectric medium and can play a significant role in the low-frequency performance of the antenna. Two approaches are taken when considering the dielectric medium in the antenna design, first is to take into account the ground material in the antenna model and optimise its performance accordingly. The aim is to reduce the interaction of the antenna with the interface and maximise the power coupled into the ground. In this case, the two key factors are the current distribution and the radiation pattern. Although this method provides a better overall performance, it suffers from two main practical problems for many applications. The first problem arises with the mechanical damage of the antenna due to an insufficient spacing between the interface and the antenna. The second problem is the specific performance of the antenna in regards to a particular material. For example, if the antenna is designed to couple well into sandy soil, it may not perform as well in other materials such as loamy soil or ice. The second approach in the design is to consider several materials of interest and

79 optimise other parameters such as the distance from the antenna to the ground. As the antenna spacing is increased, the antenna field patterns are modified by a reduction in the effect of the reactive field. The other parameter is the use of multiple frequencies of illumination. Since the dielectric medium imposes frequency dependent attenuation, frequency diversity can be used to overcome the fluctuations in the dielectric constant of the medium.

In contrast to conventional microwave antenna systems, which are designed for the transmission or reception of high-frequency electromagnetic waves in nearly homogenous free-space conditions, GPR antennas have to cope with various inhomogeneous media. Figure 5.5 shows the two-way propagation path of the signal to and from the antenna. The main boundaries where the signal enters different media is the antenna aperture to air and air to ground. These interfaces have different impedances, and if there is any mismatch between them, the signal is reflected back and can be regarded as a source of high return signals. Reduction of these high return signals can increase the amplitude of the weak target return signal. In contrast to conventional microwave antenna systems, which are designed for the transmission or reception of high-frequency electromagnetic waves in nearly homogenous free-space conditions, GPR antennas have to cope with various inhomogeneous media. Figure 5.5 shows the two-way propagation path of the signal to and from the antenna. The main boundaries where the signal enters different media are the antenna aperture to air and air to ground. These interfaces have different impedances, and if there is any mismatch between them, the signal is reflected back and can be regarded as a source of high return signals. Reduction of these high return signals can increase the amplitude of the weak target return signal.

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Figure 5-5: Antenna signal path and its main boundaries

Ideally, a well-designed GPR antenna should, a) have a stable antenna characteristic for a wide variety of acquisition conditions, b) radiate a consistent broadband pulse, and c) be characterised by relatively high transmitter efficiency. For efficiency, the antenna should be minimally resistive.

Un-damped antennas, however, tend to be distinguished by multiple reflections from the antenna ends (aperture). This results in ringing in the emitted and received signals that interfere with the weak return signal coming from the target buried in shallow subsurfaces. Table 5.1 summarises classification of antennas that are employed for GPR applications. The ticks correspond to their advantages and the squares correspond to their main drawbacks.

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Wire type antennas Aperture type antennas

 Single or Cross Dipoles

 Logarithmic or Archimedean Spiral

 Linearly polarised

 Linear, Dual and Circularly polarised

 Antenna array

 Ultra-Wide-Band

 Highly directive

 High gain

 Complex structures

 Relatively high fabrication cost

Table 5-1: Classification of antennas for GPR Applications