1. Field of the Invention
The invention relates to an ultrawideband antenna incorporating a pulse generator for use in transverse electromagnetic mode (TEM), particularly for use at high voltages.
2. Discussion of Prior Art
There is a desire to develop antenna capable of operating with pulses of increasingly high voltage, short duration and rapid pulse risetime and repetition rate, for example for application in wide bandwidth high definition radar systems. The transmission line geometry of such an antenna required to ensure good transmission of pulse energy is a principal area of concern, having associated problems which are exacerbated under higher voltage and rapid pulse risetime conditions.
In these antennae, the dielectric medium which contains the generator and the dielectric medium from which the signal is radiated away from the antenna are likely to be different, the former for example a dielectric polymer or transformer oil and the latter for example air or other suitable gas. Problems are encountered where the pulse signal output from the generator makes the transition between the dielectric media. To minimize degradation of the signal by reflection at an interface it is desirable to minimize impedance variation across it. The two media will possess different dielectric properties however, so that a transmission line having a continuous geometry at the interface will necessarily produce impedance discontinuities, so that reflection will occur. Impedance matching for a normally incident signal can be achieved by incorporating geometric discontinuities at the interface but this can also be expected to produce degradation of pulse quality. A further important consideration in determining geometry arises from the need to maintain an insulating condition in the transmission line to avoid breakdown. Voltage holdoff may reach 30 kV/cm in air at atmospheric pressure which will determine the minimum plate separation for an air filled transmission line at a given voltage. These factors place limits on the voltage or necessitate use of a pulse having a longer risetime.