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An antenna is a device or a system, which is responsible for the region of transition between guided and free space (normally air) propagation. For GPR, it is desirable to have most of the radiation from the antenna coupled into the dielectric medium; therefore, boresight (directive) antennas are efficient in doing so. There are two regions for the waves to interact with the medium, near-field (close to the antenna) and far-field. If the GPR is to operate in the near-field, the radiation and coupling performance (input impedance) affect the antenna design to a considerable extent. In far-field operation, antenna efficiency is more of concern (Lacko and Clark, 2003).

Other antenna parameters that can affect the system include the phase centre location (although this can be calibrated for in signal processing), the gain and the radiation efficiency. Moreover, the polarisation, the corresponding polarisation diversity and different effects of self-clutter, which are known as ring-down or reverberation, have to be taken into account. Such ring-down events, which occur due to an impedance mismatch at the aperture of the antenna, could be misinterpreted as multiple reflections (Radzevicius, Guy and Daniels, 2000) and, therefore, have to be avoided in the antenna design process. The following subsections describe the basic parameters of the antenna and the considerations that have been taken into account in regards to GPR.

5.1.1.1 Energy Transfers from the Antennas

It is important to understand the detection capability of GPR antennas operating at near-field or far-field regions. If we consider a small electric dipole radiator with a radiation pattern similar to a doughnut, the total field, radiated field, induction field and quasi-stationary field are all equal in field strength at distance equal to the ratio of wavelength to 2 𝜋, which is the boundary between the near-field (less than 1) and far-field (greater than 1). The approximate field boundaries are shown in Fig. 5.1. (Kraus and Marhefka, 2002):

 Electromagnetically short antennas – for antennas shorter than half of the radiating wavelength, the near-field is 𝑟 ≪ λ and the far-field is 𝑟 ≫ 2λ

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 Electromagnetically long antennas – for antennas physically larger than half of the radiating wavelength, the near and far fields are defined in Eq 5.1 and 5.2.

Figure 5-1: Antenna field regions. Source: Ihf (2006)

Any consideration of the signal detected in GPR receiver should therefore fully account for the physical proximity of the antenna and the target. In the case of electromagnetically short antennas and a radiating frequency centred at 0.5 GHz, it can be seen that targets closer than 1.2 m will have increased field contributions, whereas radar radiating shorter wavelengths will lose out on the increased field contributions because it moves into the far-field zone and becomes dependent on the largest dimension of the antenna aperture (𝐷).

5.1.1.2 Directivity and Gain

An antenna gain is a key performance factor, which combines the antenna’s directivity and electrical efficiency. In GPR to achieve an acceptable plan resolution (along track resolution), a high gain antenna is required. The directivity of an antenna refers to the degree of sharpness of its beam. This requires the antenna dimensions greater than the wavelength of the lowest operating frequency. To avoid a large antenna dimension, a high carrier frequency should be used. The gain of an antenna is defined as follows:

𝐺 = 4𝜋 ⁄ λ

𝐴

74 where 𝐴_𝑒𝑓𝑓 is the antenna effective aperture and λ is the wavelength of the operating frequency.

Directivity is similar to gain but, resistive losses are not included. In Chapter 3, the signal level was investigated for different dimensions of the buried target, therefore, depending on the penetration depth and target size, a rough level of gain for the antenna can be estimated.

5.1.1.3 Bandwidth

An antenna is only effective over a certain frequency range where the radiated energy is of low return loss or, in other words, the Input Voltage Standing Wave Ratio (VSWR) is less than 2. To achieve a good depth resolution, large bandwidth should be used. The GPR bandwidth, sometimes called antenna fractional bandwidth is defined by the following expression in percentage:

𝐵𝑊 = [^{2 (𝑓𝑚𝑎𝑥− 𝑓𝑚𝑖𝑛)}

The polarisation of an antenna is the orientation of the transmitted or received electric fields. The most common forms of polarisation are linear, elliptical and circular. A further consideration in the design of an antenna for GPR is the shape of the target. For instance, when the target is a planar surface then linear polarisation is adequate. However, if the target has a mixture of edges and curves, the backscattered fields experience a characteristic polarisation independent of the state of the polarisation of the incident waves. Therefore, dual or circular polarised (e.g. rotating the polarisation vector in space) antennas can provide better information.

5.1.1.5 Phase Centre

The phase centre of an antenna is considered to be the point from which the electromagnetic radiation spreads spherically outward, with the phase of the signal being equal at any point on the sphere. The importance of the phase centre is related to the characteristics of the radiation field.

For instance, some antennas such as log periodic or spiral, the physical position of the source of radiation at a particular frequency will vary along either the length of the antenna or the position

75 of the antenna. This may introduce signal dispersion particularly in spiral antennas (Drabowitch et al., 2005). In the case of waveguide antennas such as horns, the phase centre depends on the aperture distribution and the taper of the horn, and the resultant far-field pattern is affected by the variation of phase centre. The GPR antennas should have the same phase centre for all used frequencies in order not to change the pulse shape. For dispersive antennas, frequency domain radar systems (e.g. SFCW) are usually used, where the dispersion can be corrected by suitable post-processing of measured data. The phase centre can be estimated by far-field or near-field techniques as well as 3D simulation techniques. To estimate the phase centre of the antenna described in this chapter, a calibration technique was performed by measuring the reflections coming from a metal plate for heights varying between 5 < ℎ < 18 cm above the ground.

5.1.1.6 Radiation Pattern

The radiation pattern of an antenna is related to the gain versus angle in 𝜃 (azimuth) and 𝜑 (elevation) planes. From radiation pattern, mainlobe gain, beamwidth, sidelobes and backlobes can be visualised. For downward-looking GPR systems, boresight radiation pattern is required with minimal side lobes and back lobes.

5.1.1.7 Antenna Time Sidelobes and Ringing

Ground penetrating radar systems often operate at short ranges from the ground. Therefore, the rate of decay of energy stored within the antenna is a key factor in the contribution of self-clutter to the complete radar system. The time sidelobes or the impulse response of the antenna can obscure the targets close in range. Control of the range sidelobes in any GPR system is an essential issue for the GPR system performance.

5.1.1.8 Antenna Footprint

The antenna radiation pattern and signal processing govern the azimuth and elevation resolutions of the GPR system. In low attenuation media, the resolution obtained by the horizontal scanning technique is degraded, but employing Synthetic Aperture Radar (SAR) techniques increases the plan resolution. However, in high attenuation media, SAR techniques may not be as useful as it is restricted by the ‘window’ placed across the SAR aperture. The plan resolution of a GPR system is important when there is a need to distinguish multiple targets close to each other at the same depth. The effect of radiation footprint on the ground also needs to be considered, as the distance

76 of the antenna from the ground is related to the 3 dB footprint and this affects the image resolution as well as increasing the level of ground clutter. Figure 5.2 (b) and equation 5.5 show the conical spreading of radar energy propagating through the medium (with dielectric constant 𝜀_𝑟) and the footprint that it imposes on and below the ground (Conyers, 2004):

Figure 5-2: Antenna footprint at different angle of incident (a) and antenna footprint estimation (b)

Moreover, the reflection at the air-soil interface usually appears much stronger than the reflection of the target object itself. In (Magg and Nitsch, 1998) the successful implementation of a Brewster angle configuration has been reported which eliminates the air-surface reflection by applying a certain inclination angle to transmitting and receiving antenna (Fig. 5.2, a). Unfortunately, the size of the GPR system increases significantly, because the inclination angle of the antennas demands a certain distance between them which increases with higher permittivities of the soil. Moreover, it is very difficult to apply such a setup if the dielectric properties of the soil medium are not constant or completely unknown as it is the case for most field measurements.

5.1.1.9 Antenna Mainlobes, Sidelobes and Backlobes

Mainlobe of the antenna is the lobe that contains the maximum signal strength. The radiation pattern of most antennas shows a pattern of lobes at various angles in the polar coordinate system.

In directional antennas where the requirement is to emit the waves in one direction, the lobe in that direction is designed to have the highest field strength than the others (e.g. mainlobe). The other lobes at other angles are called sidelobes. These are known as unwanted radiation in undesired

77 directions. The lobe that occurs in the opposite direction from the main lobe is called the backlobe.

The maximum level of the sidelobe/backlobe and their radiation patterns are often dictated by the requirements of the application or regulatory bodies. In GPR depending on the mode of operation, the side lobes and back lobes are regulated. For instance, in the case of bistatic operation, reduced sidelobes are important to avoid saturation of the receiver and Direct Signal Interference (DSI) from the air and soil interface. Backlobes are often minimised to avoid the reflections that could build up from the structures behind the antenna such as mounts and moving platforms.

5.1.1.10 Antenna Efficiency

Antenna efficiency of an antenna relates to the fact that all antennas suffer from ohmic losses. This is regarded as the power delivered to the antenna and the power radiated or dissipated within the antenna. A lossless antenna would be an antenna with an antenna efficiency of 0 dB (e.g. 100%).

Figure 5.3 shows an input signal (red) injected into a lumped dipole (a) and a pin fed circular patch antenna (b). The reflection to the point of excitation from the antenna aperture is shown in green.

A good efficiency antenna (a) has most of the power present at the antenna’s input radiated away (less reflection). A low-efficiency antenna (b) has most of the power absorbed as losses within antenna or reflected back to its point of excitation due to impedance mismatch. For GPR, the antenna needs to have a high efficiency so it can introduce maximum power into the ground.

Figure 5-3: Antenna efficiency of two antenna examples, lumped dipole (a) and circular patch antenna (b)