Communications: radio wave antennas – Antennas – Wave guide type
Patent
1988-03-07
1990-10-16
Wimer, Michael C.
Communications: radio wave antennas
Antennas
Wave guide type
343786, 343840, H01Q 1919
Patent
active
049638785
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention consists of a reflector antenna with a self-supported feed for the transmission or reception of polarized electromagnetic waves. The antenna is principally intended for the reception of TV signals from sattelites, however it can be used as a radio link, and as a ground station for sattelite communications.
BACKGROUND OF THE INVENTION
These types of reflector antennas are chiefly used because they are straightforward and inexpensive to manufacture. They also provide greater antenna efficiency and lower side lobes in the radiaton pattern than is the case when the feed element has to be supported by diagonal struts. The drawback with the latter configuration is that the main reflector becomes blocked. A self-supported feed is also easely accessible from the back of the reflector, thus is frequently selected when it is best to locate the transmitter and/or the receiver there. This also reduces the loss that occurs when the waves have to be led in a cable along one at the support struts.
A. Chlavin, "A New Antenna Feed Having Equal E and H-Plane Patterns", IRE Trans. Antennas Propagat., Vol. AP-2, pp. 113-119, Jul. 1954, describes a reflector antenna with a self-supporting feed. However since this antenna uses a waveguide with a rectanguler cross-section, it can only transmit or receive waves with one particular linear polarization.
C. C-Cutler, "Parabolic-antenna design for microwaves", Proc. IRE, Vol. 35, pp. 1284-1294, Nov. 1947, describes a dual polarized reflector antenna with two variants of a self-supporting feed, called the "ring focus" and the "waveguide cup" feeds respectively. A circular waveguide is used in these two feeds with a reflecting object in front of the waveguide opening, this reflector being respectively shaped like a flat disc and a cup. Both of these feeds unfortunately produce high cross-polarization within the main lobe the radiation pattern.
SUMMARY OF THE INVENTION
The main purpose of the present invention is to design a reflector antenna which has dual polarization with low cross-polarization within the main lobe of the radiation pattern. Dual polarization means that the antenna is capable of receiving or transmitting two waves with orthogonal linear or circular polarization simultaneously. The waveguide must have an almost circular or quadratic cross-section.
This objective can be achieved by a design which is in accordance with the charaterizing part of Claim 1. Further details about the invention are given in Claims 2-10.
The surface of the subreflector is treated so that the electromagnetic waves are reflected from and propagate along the surface in approximately the same way regardless of whether the electric field is normally on the surface or is tangential to it. Furthermore, the design of the other geometries of the feed ensures that the cross-polarization remains low within the main lobe of the radiaton pattern.
It should be mentioned that a dual polarized reflector antenna with a self-supporting feed is already known from among other sources such as P. Newham, "The Search for an Efficient Splashplate Feed", Proceedings of the Third International Conference on Antennas and Propagation (ICAP 83), IEE Conference Publication No. 219, pp. 348-352, Apr. 1983, and in previous publications by the same author. In this design the subreflector has a smooth surface. However, it is also possible to obtain low cross-polarization when the subreflector is positioned at a distance from the waveguide aperture so that the waves are prevented from becoming radial and cannot propagate along the surface of the subreflector. This avoids the polarization-dependent reflection coefficient for radial waves found in the smooth subreflector. The present invention, on the other hand, has conceived of an antenna where this distance is so small that some of the waves are able to propagate along the surface of the subreflector. Low cross-polarization is then only ensured by a surface where the reflection coefficient for radial waves is independent of the polari
REFERENCES:
patent: 2698901 (1955-01-01), Wilkes
patent: 2829366 (1958-04-01), Armstrong et al.
patent: 3055004 (1962-09-01), Cutler
patent: 3162858 (1964-12-01), Cutler
patent: 3983560 (1976-09-01), MacDougall
patent: 4188632 (1980-02-01), Knox
patent: 4673945 (1987-06-01), Syrigos
patent: 4673947 (1987-06-01), Newham
Pittenger James E.
Wimer Michael C.
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