High frequency effect of the junction between cone tip, coaxial feeder and ground plane on the input impedance of a long monocone antenna on ground plane

In the Time-Domain Scattering Range of the Royal Military Academy Brussels a long monocone on a ground plane is used as transmitting antenna to illuminate small-scale targets with short electromagnetic pulses (duration of about 300 ps). When properly fed, this antenna is indeed able to radiate efficiently and without any waveform distortion the transient voltage applied to its terminals in the form of a spherical TEM wave with which the target set on the ground plane can be illuminated. The monocone is base-driven from a coaxial transmission line and the cone angle is kept small enough to prevent the non-TEM modes from being excited at the junction of the cylindrical transmission line with the cone tip. At high frequency however the size of the aperture becomes a significant fraction of the wavelength, so the actual geometry of the driving point has to be taken into account for an accurate calculation of the impedance. This paper presents a theoretical and experimental study of the high frequency (short rise time) effect of the driving point geometry on the input impedance. It deals with the modelling of the junction and the TDR-measurements conducted on a small-scale monocone with rise times as short as 50 ps to validate the theoretical model of the junction at high frequency