Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
1998-10-16
2001-09-04
Phan, Tho (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S795000, C343S853000
Reexamination Certificate
active
06285323
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to flat plate antenna arrays and more particularly but not exclusively to flat plate antenna arrays for the transmission and reception of directional microwave communications.
BACKGROUND OF THE INVENTION
At microwave frequencies there is a range of antenna devices that can be used. These include slotted waveguide arrays, printed patch arrays, and reflector and lens systems. Above about 20 GHz slotted waveguide arrays require high tolerances and are thus expensive to manufacture in large quantities. For example at 20 GHz a large slotted waveguide array may need around 2000 slots, each of which must be individually machined to precise dimensions.
The aperture coupled patch array has all of the active elements of the antenna, radiating elements, transmission lines, coupled slots etc., on different layers of PCB. The elements are placed on the PCB using the conventional techniques of photo-lithography. In order for the device to work the layers must be very carefully lined up and must be carefully spaced apart. The tolerance limit for alignment and spacing between the layers is very tight and thus large arrays are difficult to mass produce.
Printed patch array antennae suffer from inferior efficiency due to high dissipative losses of transmission lines, particularly at high frequencies and for large arrays. In order to avoid radiation losses from the lines it is necessary to keep the spacings within the order of 0.01 &lgr;. Furthermore the restrictions on spacing mean that the transmission lines must be very thin. As they are thin they will have high losses and thus be inefficient for large arrays. Frequency bandwidths for such antennae are typically less than that which can be realized with slotted planar arrays, that is to say they are particularly narrow.
Reflector and lens arrays are generally employed in applications for which the additional bulk and weight of a reflector or lens system are deemed to be acceptable. The absence of discrete aperture excitation control in traditional reflector and lens antennae limit their effectiveness in low sidelobe and shaped beam applications.
Increasingly, as such antennae are becoming more widespread, and concern for the quality of the environment is growing, the use of lens or reflector systems is becoming less and less publicly acceptable. It is therefore desirable to provide a flat plate antenna array having the advantages of a lens or reflector but without the environmental impact.
SUMMARY OF THE INVENTION
It is therefore an aim of the present invention to provide a flat plate antennae for use in various parts of the 0.5-40 GHz range that is relatively easy to manufacture and has the qualities generally considered necessary for directional microwave transmission.
According to a first aspect of the present invention there is provided an antenna comprising at least one printed circuit board, and having active elements including radiating elements and transmission lines, and at least one ground plane for the radiating elements and at least one surface serving as a ground plane for the transmission lines, arranged such that the spacing between said radiating elements and said at least one groundplane therefor is independent of the spacing between said transmission lines and said at least one surface serving as a groundplane therefor.
In an embodiment the printed circuit board has a first face and a second, opposing, face and the active elements are located on both faces of said printed circuit board. The transmission lines of the first face may overlay the transmission lines of the second face.
In a preferred embodiment the transmission lines may extend outwardly from a central feed point. The radiating elements may extend from outward ends of the transmission lines. The electrical paths from the central feed point to each radiating element respectively through said transmission lines are preferably substantially the same, in terms of physical length and/or in terms of electrical impedance. Thus the antenna is electrically balanced. All the radiating elements are being fed with the same power and thus the antenna works with maximum bandwidth.
In an embodiment the radiating elements of each face extend at predetermined angles from ends of the transmission lines and a predetermined angle which is used primarily in the first face differs from the predetermined angle used primarily in the second face by 180°.
The printed circuit board may be of a predetermined thickness. The thickness of the PCB is at compromise between low loss, minimum extraneous radiation and cost. It is important for the correct interaction between the elements of the two faces that the thickness of the printed circuit board is made to within a certain tolerance.
Embodiments of the antenna may further comprise a polariser. The polarizer may be a grid polarizer.
The radiating elements may be arranged in rows about a central axis such that the rows are aligned parallel to the axis. The radiating elements may be aligned parallel to a second axis. The second axis may be offset from the central axis by substantially 45°. The antenna may be orientated such that the central axis is either +45° or −45° to the horizontal depending on the polarization required. Alternatively, if the presence of sidelobes is less critical, the radiating elements may be parallel to the central axis.
The number of radiating elements per row of the pattern is a function of the distance of each respective row from the central axis. That is to say each row may have a predetermined number of radiating elements and that predetermined number may increase with the proximity of each respective row to the central axis. Such an arrangement decreases the size of directional side lobes.
The antenna may further comprise a ground plate located at a predetermined distance from the printed circuit board. The predetermined distance would typically be less than a quarter of the wavelength of the signal.
In a preferred embodiment individual transmission lines split into two or more transmission lines at each of a plurality of branch points. The total impedance when taken in parallel, of the further lines following respective branch points is equal to the impedance of the individual transmission line preceding the respective branch point The impedance of the branches is seen as a parallel impedance by the central feed point and the intention is to keep the impedance constant along the length of the transmission lines.
An embodiment of the array has the elements fed in a series/parallel fashion. This is done to reduce further losses in the transmission lines and improve efficiency.
Embodiments of the antenna may be used for transmitting or receiving one or more wavebands within the 0.5-40 GHz range.
The antenna may typically be sealed from the environment by a radome. The radome may comprise a rigid polypropylene skin and a foamed polyethylene body, the body being comprised of approximately 80% cross-linked polymer, the skin preferably being UV protected, and both the skin and the body being held together, preferably by soldering.
According to a second aspect of the present invention there is provided an antenna comprising at least one printed circuit board, and having active elements including radiating elements and transmission lines, mounted on said printed circuit board, and at least one ground plane for the radiating elements and at least one surface serving as a ground plane for the transmission lines. The radiating elements are arranged in rows, which are parallel to a central axis of the antenna, and the radiating elements are elongated, and arranged with their elongated directions parallel to an axis offset from the central axis of the antenna. This aspect is particularly useful where low sidelobes are less important.
According to a third aspect of the invention there is provided an antenna comprising at least one printed circuit board having two oppositely facing printed surfaces, and having active elements including radiating elements and transmis
Darby & Darby
MTI Technology & Engineering (1993) Ltd.
Phan Tho
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