Communications: radio wave antennas – Antennas – Wave guide type
Patent
1995-12-14
1998-05-26
Hajed, Donald T.
Communications: radio wave antennas
Antennas
Wave guide type
343785, 343786, H01Q 1300
Patent
active
057573301
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an antenna for at least one of the transmission and the reception of electromagnetic radiation.
Conventional marine navigation radar antennas present a large area to wind because the radiating aperture is in the vertical plane. Such radar antennas comprise a horn arrangement as shown in FIG. 1 in cross-section. In such an arrangement a linear array antenna comprises a slotted waveguide 1 with slots 1a provided in a side wall coupled to a feedhorn 2. This configuration is termed "broadside fire" because the axis of the main beam is orthogonal to the plane of the aperture generally indicated by reference numeral 3.
The problem with such conventional radar antennas is that in order to provide the required beam width of the beam using such a "broadside fire" arrangement, the size of the aperture in the vertical direction is large, thus providing high wind resistance. Because of the poor aerodynamics of the structure, the antenna must be strongly fixed to a mounting which must be driven by a large and powerful motor to provide accurate rotation of the radar antenna so that its rotation is minimally effected by wind speed and direction.
It is therefore highly desirable to reduce the vertical dimensions of radar antennas. As an alternative to using the "broadside fire" configuration, the "end fire" configuration has been studied. One simple form of such an "end fire" antenna is the polythene rod or "polyrod" as shown in FIG. 2.
In the "polyrod" "end fire" configuration the radiating aperture is along the axial length of the rod and therefore directionality is increased by increasing the length of the aperture, i.e. the length of the polyethylene rod 6 in FIG. 2. The polyethylene rod 6 is coupled electromagnetically to the radiation provided from a waveguide 4 by a transition element 5. In the "polyrod" configuration the polyethylene forms a "leaky" antenna structure wherein energy "leaking" from the surface of the rod combines constructively in the direction the rod points, thus forming a beam.
The problem with using such an arrangement for a radar antenna is the weight of the polyethylene rod. To use such an arrangement would require strong fixings and a powerful drive motor, not only to cope with the weight, but also to cope with the fact that the "polyrod" would need to be quite long in order to provide the required beam width and therefore it would be susceptible to down draught and up draughts.
In order to overcome the weight disadvantage of the "polyrod" arrangement of FIG. 2, Japanese Patent No. 56-31205 has proposed the use of a hollow "polyrod" shown in FIG. 3 comprising a slotted waveguide 7 with slots 7a coupled to a hollow dielectric rod 9 via a transition element 8. However, whilst this arrangement overcomes the weight disadvantage of the "polyrod" arrangement this arrangement still suffers from the disadvantage of requiring a long axial length in order to provide sufficient beam width.
This problem has been further considered in Japanese Patent No. 62-171301 which discloses the arrangement disclosed in FIG. 4. In this arrangement a slotted waveguide 10 is coupled to a feedhorn 11 via a transition element 12. At the aperture of the feedhorn 11 is provided a double skin dielectric structure 13. The double skin dielectric stucture 13 is provided at the aperture of the feedhorn 11 to avoid having to increase the vertical aperture size of the feedhorn 11. However, whilst this arrangement reduces the length of the dielectric structure required to provide the necessary beam width, it still provides an arrangment which has a disadvantageous aerodynamic cross-section.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an antenna which has a reduced aerodynamic cross-section together with a reduced length whilst still providing the required beam width.
In one aspect the present invention provides an antenna for at least one of the transmission and the reception of electromagnetic radiation along an axis compri
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patent: 4841308 (1989-06-01), Terakawa
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"Microwave Antennas", Dielectric Antennaa, by A.Z. Fradin, published Pergamon, 1961, pp. 566-577, no month.
"Analysis of Propagating Modes in Dielectric Sheets" by leonard Hatkin, Proceedings of the I-R-E, Oct. 1954, pp. 1565-1568.
Hajed Donald T.
Ho Tan
Racal-Decca Marine Limited
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