A poster presented at:
The ESA Workshop on Millimetre-Wave Technology and Applications
Antennas, Circuits, and Systems.
May 27-29 1998, MilliLab, Espoo, Finland.

Antennas which cover 360° in azimuth and a restricted range of angles in elevation are useful in many applications, for example identification systems, mobile communications, wireless LANs, beacon systems and electronic warfare. In the case of communication systems operating between mobile platforms, circular polarisation is also desirable to avoid signal loss when the platforms are tilted with respect to one another.

A dipole is the simplest antenna which has a radiation pattern of this form, but is inherently linearly polarised and has a very wide elevation beamwidth1. It is possible to use a conical reflector to divert a vertically launched, axially symmetric beam into 360° of azimuth and a range of elevation angles. However, the illuminating beam often has a power maximum on axis and this causes severe scattering from the tip of the cone resulting in a ragged elevation pattern.

We present a millimetre wave antenna which overcomes this difficulty by using an illuminating beam which has an axial null in intensity thus virtually eliminating the scattering problem and gives a very clean elevation radiation pattern with low sidelobes. It supports circular polarisation and is simple to manufacture.

The design of this antenna is
patent applied for:-

Omnidirectional Antenna

UK Patent Application
No. UK 9602395.7
Filed 6/2/96

Duncan A. Robertson & Peter B. May

Registered to:
The Secretary of State for Defence

Hermite-Gaussian to Laguerre-Gaussian mode conversion is achieved using a spiral phaseplate. A spiral phaseplate is a disc of dielectric material whose radial thickness varies with azimuthal angle. This introduces a screw dislocation into the phasefront of the HG00 beam, and converts it into an LG mode beam. The step height is chosen to correspond to an integer number of cycles of phase. Strictly speaking, an infinite set of LG modes is produced but the lowest order ones are dominant.

The phasefronts of LG modes are helical which means the phase response of the antenna varies with azimuthal angle. Whilst this is not generally likely to be a problem, it is a feature which could be exploited in certain applications, such as direction finding.

Spiral phaseplates can be made to have very low insertion losses by minimising the overall thickness, and by blazing the planar surface. Potentially, the quarter-wave plate structure could be machined into the flat side to produce a single integrated device.

Provided the input HG00 mode is pure and rotationally symmetric, spiral phaseplate mode converters can produce very high quality LG mode beams. Some example 2D scans and axial cuts through HG00 and LG10 mode beams are shown on the following pages.

Opticspage 2

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