Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2000-12-14
2003-04-01
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S825000
Reexamination Certificate
active
06539608
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to antennas employing dielectric such as flat plate antennas.
BACKGROUND TO THE INVENTION
One form of antenna that is in widespread use is the triplate antenna (also known as a layered antenna) which, in one form, comprises a radiating element including a pair of closely spaced correspondingly apertured ground planes with an interposed printed film circuit, electrically isolated from the ground planes, the film circuit providing excitation elements or probes within the areas of the apertures, to form dipoles, and a feed network for the dipoles. In an array antenna a plurality of such aperture/element configurations are spaced at regular intervals co-linearly in the overall triplate structure. This antenna construction lends itself to a cheap yet effective construction for a linear array antenna such as may be utilised for a cellular telephone base station. Such an antenna is disclosed in our co-pending patent application No. EP-A-6B2261554B.
Another type of layered antenna array comprises a single aperture per radiating element. A further type of antenna comprises a primary aperture with two secondary apertures placed on opposite sides of the primary aperture. Such arrays may extend in a single direction (a linear array) or in two directions (a planar array). Alternatively, a number of linear arrays may be spaced apart to form a multi-antenna planar array.
Another type of linear array comprises apertures in both ground planes of each radiating element. An important factor in the design of an antenna is the gain of the antenna. In order to increase the gain from the antenna in a primary radiating direction, the antenna may further comprise a continuous (non-apertured) ground plane placed parallel with and spaced from one of the apertured ground planes to form a rear reflector for the antenna. Signals transmitted by the antenna towards the back plane are re-radiated in a forward direction. Provision of a reflector can increase the gain in front of the antenna whilst reducing the gain behind.
A further type of antenna with a single ground plane is the patch antenna which comprises a reflective ground plane and a dielectric sheet which is supported from the ground plane by a dielectric spacer and supports a microstrip pattern comprising printed patch radiating elements.
A further type of antenna is the patch antenna which comprises a reflective ground plane and a dielectric film or sheet which supports a microstrip pattern comprising printed patch radiating elements, which dielectric sheet is supported from the ground plane by a dielectric spacer.
A still further form of antenna is the dipole antenna in which a pair of colinear quarter wavelength radiators are fed in anti-phase to produce a substantially omni-directional radiation pattern in a plane normal to the axis of the radiators. If the radiators are placed parallel to and a quarter of a wavelength from a reflecting ground plane the radiation pattern similarly becomes substantially directional, see e.g. EP-B-054351 (Northern Telecom).
For modern telecommunications application at high frequencies, e.g. above 100 MHz, apart from the electrical performance of the antenna other factors need to be taken into account, such as size, weight, cost and ease of construction of the antenna. Depending on the requirements, an antenna can be either a single radiating element (e.g. one dipole) or an array of like radiating elements. With the increasing deployment of cellular radio, an increasing number of base stations which communicate with mobile handsets are required. Similarly an increasing number of antennas are required for the deployment of fixed radio access systems, both at the subscribers premises and base stations. Such antennas are required to be both inexpensive and easy to produce. A further requirement is that the antenna structures be of light weight yet of sufficient strength to be placed on the top of support poles, rooftops and similar places and maintain long term performance over environmental extremes.
The antennas described above have spacers to separate the various dielectric film or ground plane layers. Whilst some types of antennas have employed rigid dielectric boards, made from, for instance, FR4 or PTFE, mostly foamed dielectric has been employed. FR4 or PTFE circuit boards whilst having controllable dielectric permitivities, require solid supports to maintain their position relative to a ground plane, are expensive, both by way of raw material and in the manufacture of an antenna. Polystyrene is cheap, has a near unity dielectric constant, is of low loss and will support a dielectric sheet for circuitry, but presently cannot normally produced at a thickness of less than 4 mm by normal manufacturing techniques, in any case no thinner than 3 mm (when the tolerances have proved not to be good). It has not been possible to obtain thin polystyrene with repeatable tight tolerances cheaply. The dimensions of the mould allowing an even distribution of polystyrene to be expanded is one of reasons why this is so. In order to minimise circuit radiation losses, there is a requirement for thinner substrates; this would also reduce overmoding (which is the support of modes of propagation other than the intended mode(s). An acceptable range of characteristic impedances must also be realised for the transmission lines. Typically a thickness of 2 mm is sufficiently thin, with a tolerance of around ±5%. The Applicants have designed antennas with 1.6 mm spacing which was found to be impractical as tolerances of 10% to ensure good antenna return loss give rise to variations of ±0.16 mm; every antenna had to be individually matched with matching stubs and the like—economic production is not possible.
One part of an antenna which is particularly heavy are the grounding elements, comprising one or more ground planes. As described above, the ground planes may be several in number and must have an electrically conductive surface and be of a low resistivity per square whereby the microwave surface currents may propagate. Typically ground planes have been formed from a metal casting or sheet, typically from an aluminium alloy for reasons of weight or from plated steel for reasons of cost. Nevertheless, the weight of the ground planes, whether formed from an alloy or not, comprises a considerable amount of the overall weight of an antenna structure.
Dielectrics such as expanded polystyrene have not previously been successfully metallised and employed as ground planes, since the surface roughness of the polystyrene would result in any surface metallisation having a rough surface due to boundary interstitial voids (spaces between aggregated polystyrene groups at a surface). The technique of sputter coating or electroless plating of plastics such as ABS plastics is known, but this type of coating is not suitable for non-rigid plastics materials such as polystyrene, where cracking of the coating would result upon stressing or flexing. Dielectrics such as polystyrene as commonly produced have poor dimensional tolerances which prevents their use in the creation of structures other than that as a resilient spacer. EP-B-447 018 (Northern Telecom) provides an antenna wherein a dielectric film supporting an antenna feed and probe arrangement is maintained in a spaced apart relation with respect to two apertured ground planes by means of a flexible foamed dielectric either side of the film; the tolerances of the dielectric are not critical since the resilient foam urges the ground planes to a maximum spaced apart distance. If the foam was 15% oversize, then the ground plane separation will not vary; the foam spacer will be compressed further.
As is known, in the case of microwave propagation, the top surface (the boundary surface interfacing with the dielectric) carries the microwave signals by way of the skin effect. The lower frequency components flow in the bulk of the material. If the surface of a ground plane is rough, then it will not sustain controlled radio frequency propagation; a
Arnold Mark Hanley
McAlinden Michael John
McKinnon John Peter Bruce
Arbes Carl J.
Barnes & Thornburg
Nortel Networks Limited
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