Antenna Design

The design of the antenna will have a large bearing on the effectiveness of the system. The antenna must be capable of radiating the required bandwidth with the desired gain and have a characteristic such that the integrity of the transmitted pulse is maintained. If this is not possible, it must be possible to mitigate the shortfalls in the characteristics of the antenna during signal processing. This is a wide remit and considering the wide range of targets and ground properties it is not surprising that there are many designs that have been used in this context.

Most useful are broadband antennas having non-dispersive characteristics through a linear phase response. An example of this type of antenna is the TEM horn, a class of travelling wave antenna.

Of greater popularity are element antennas such as monopoles, dipoles, bi-conics and bow-ties. While not having such ideal characteristics they are simple to design and are well understood. Most effort has gone into increasing the bandwidth and improving the impulse response through use of distributed loading.

Another class of antennas that has found favour is spirals, either planar or conical. These antennas are broadband in nature, though dispersive, but their major attribute is the ability to radiate circularly polarised radiation. In certain instances this is crucial, for example in the detection or randomly orientated linear structures such as pipes and cables. These targets have significantly anisotropic scattering response and it is important to excite them with a suitably polarised wave. For random orientations the only way to ensure this is with a circular polarisation.

In order to design the excitation function and signal processing requirements it is crucial to understand the interaction between the antenna and the ground where the antenna is in close proximity to the ground. Analytic solutions can be constructed most easily for simple antenna types (loaded monopoles and dipoles), but more complex antenna configurations are not amenable to this analysis technique.

Furthermore most GPR systems use separate transmit and receive antennas to protect the receiver from the transmitted power. In this situation the cross coupling between the two becomes a critical factor in the design. The added complexity of multiple coupled antennas may render the analytic solution intractable. This is exacerbated by the additional complexities of ground inhomogeneities and dispersion characteristics. For example consider the cross coupling of a crossed dipole transducer in response to a spatially varying permittivity or conductivity in the local soil medium. Classically, the only method available to derive such information has been experimentation.

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