Ultrawideband transverse electromagnetic mode horn transmitter a

Details Ultrawideband transverse electromagnetic mode horn transmitter a Ultrawideband transverse electromagnetic mode horn transmitter a

1996-11-20

1999-03-30

Le, Hoanganh

Communications: radio wave antennas

Antennas

Wave guide type

343772, 343785, H01Q 1300

Patent

active

058894977

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

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.


SUMMARY OF THE INVENTION

The present invention aims to provide an antenna which offers good impedance matching at the interface of the two dielectric media so as to preserve a rapid risetime at high voltage operation.
Thus according to the present invention there is provided an antenna comprising an electromagnetic pulse generator, a first transverse electromagnetic mode transmission line containing a first dielectric medium, and a second transverse electromagnetic mode transmission line containing a second dielectric medium, serially connected so as to enable transmission of a signal from the generator to the second transmission line, wherein the first transmission line incorporates a transition element providing an interface between the first and second dielectric media which is so configured that a signal from the generator is incident on the interface at an angle substantially equal to the Brewster Angle.
It will be appreciated that for a given pair of homogeneous dielectric materials the Brewster Angle represents the angle at which a plane wave incident on a planar dielectric interface with the magnetic field in a direction parallel to the plane of the interface will undergo no reflection. For a wave passing from medium 2 of higher refractive index and permittivity .epsilon.2 to medium 1 of lower refractive index and permittivity .epsilon.1 the angles .PSI.1 and .PSI.2 are given by tan.PSI.1=.sqroot.(.epsilon.1/.epsilon.2) are .PSI.2=58.7.degree. and .PSI.1=31.3.degree.. In order to use this principle to match the impedances of two TEM horns which are located on either side of the media interface and meet along it consider firstly two striplines in media 1 and 2 of heights H1 and H2 of equal width W. Then ##EQU1## This implies that the impedance of the striplines (given approximately by Z.sub.O (H1/W) and Z.sub.O .sqroot.(.epsilon.1/.epsilon.2) (H2/W) when H1,H2<<W) are matched across the interface (

REFERENCES:
patent: 2202380 (1940-05-01), Hollmann
patent: 2829366 (1958-04-01), Armstrong et al.
patent: 3659203 (1972-04-01), Ross et al.
patent: 4641528 (1987-02-01), Clayland, Jr. et al.
patent: 5729470 (1998-03-01), Baier et al.
Measurement Techniques, vol. 37, No. 2, Feb. 1994 New York U.S., pp. 199-204, XP 000468274 AL Betkov et al., "Experiments on Measurement Transducers Using a TEM Horn".

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