The effect of geometry on the feed impedance for a PCB-dipole antenna and the time domain radiation emission from the feed point

The measured reflected radar waveform from an object in free space depends on many factors including the antenna’s geometry, the impedance of the balun and feed-cable, the object position in relation to the antennas, and the object’s angular scattering function. Analytical methods can be simplified when the object is a long way away from the antenna. However, for near-surface GPR applications, such as landmine detection, the objects are generally in the near-field region of the antennas. The ultra-wideband scattering function of objects can be complex even in the far-field.

To optimise GPR antenna design, it is necessary to be able to quickly estimate the Spatio-Temporal Point Spread Function (ST-PSF) for a bi-static antenna pair. Conventionally, the PSF is considered only in the far-field and in the frequencies domain at spot-frequencies. This paper outlines the steps needed to create an analytical approximation of the ST-PSF, and it describes the first step in this process - the parametric modelling of the antenna geometry on feed impedance. The described analysis uses the case of a PCB dipole for both the excitation and the receive antennas as an example of the approach.

The results show the importance of understanding the antenna feed impedance for both the compactness of the radiated pulse and the transfer function of the receive antenna. The paper discusses the optimum cable impedance, assuming a balanced source, and consequentially, the optimum case for matching to heavily damped antenna designs. This paper covers the first step to an analytical approximate of more commonly used (non-damped) dipole antennas.