Wideband symmetric near‐field radiation pattern of sleeve dipole antenna by connecting additional ferrite‐loaded wire

A scaled‐down laboratory experiment has been performed in a well‐controlled situation to find an enhanced rule for detecting a man‐made underground tunnel by employing a cross‐borehole frequency‐swept radar system. A sleeve dipole antenna, fed by a coaxial cable passing inside its upper arm, is employed to easily put and pull it along an air‐filled thin and long glass tube in scaled‐down experiments. The tube‐guided wave along the coaxial cable can be suppressed by ferrite‐loading along the coaxial cable. However, the dipole antenna fed by the ferrite‐loaded coaxial cable provides an asymmetrical radiation pattern in the near‐field region. To improve the symmetry of its near‐field radiation pattern, an additional ferrite‐loaded wire is connected to the lower arm of the sleeve dipole antenna. According to the scattering measurement by an air‐filled circular cylinder, the additional ferrite‐loaded wire improves the symmetry of the electric field by more 30 dB in the frequency range of 1.5–3 GHz.