Polarization Converter Within Waveguide Feed for Dish Reflector

This invention is directed to a system and method for selectively converting electromagnetic waves (signals) from circular to linear polarization or vice versa.

The invention herein is particularly useful in converting commercial satellite communications transmitted as circular polarized signals to linearly polarized signals so that home TV receivers may conveniently receive such signals. With the polarization converter of this invention it is also possible to receive linearly polarized signals if desired.

The present system may be remotely operated by the home owner (operator) to switch between different polarized transmissions from different or the same satellites, for example, from a satellite in the U.S.A. eastern time zone transmitting left hand circularly polarized microwave TV signals to a satellite in the U.S.A. central time zone transmitting right hand circularly polarized microwave TV signals. The system is not only inexpensive but also it provides polarization conversion with only about 1/10 of a db losses at the primary frequencies of interest, 10,000 GHz to 15,000 GHz, but also unexpectedly provides reception separation between left hand and right hand circularly polarized signals of at least about 15 to 20 db received by the converter at the same frequency.

It is to be understood that the system of the invention will operate over a broad range of microwave frequencies.

The converter may also be used for transmitting both circular or linear polarized signals.

FIG. 1 is a side elevational view of the polarization converter supported by an antenna;

FIG. 2 is a sectional view of the converter taken from the top thereof;

FIG. 3 is a view taken along line 3--3 of FIG. 2;

FIG. 4 is a view taken along line 4--4 in FIG. 2;

FIG. 5 is a view taken along line 5--5 in FIG. 2;

FIGS. 6A to 6D are views showing various angular positions that the fin is movable about the axis of the circular waveguide with respect to the center line of the rectangular waveguide.

The polarization converter system of the invention includes a circular waveguide having a rectangular waveguide opening into the side thereof and forming a passage for microwave energy to pass between said waveguides. In proximity to said opening of said rectangular waveguide into said circular waveguide, there is positioned a metal fin and shorting member adjacent thereto for converting circularly polarized microwave energy entering said circular waveguide to linearly polarized microwave energy for transmission through the rectangular waveguide or converting linearly polarized microwave energy entering the rectangular waveguide and converting same to circularly polarized microwave energy for transmission from said circular waveguide.

The fin may if desired also be rotated to permit linearly polarized signals to be transmitted between waveguides without conversion.

Reference should now be had to FIGS. 1 to 5 which disclose a system for receiving microwave energy. At 20 there is shown an antenna system for receiving microwave energy transmitted, for example, by a communications satellite. The antenna 20 includes a dish 20-1 for collecting the transmitted microwave energy and focussing the collected energy at the horn 20-4. The horn 20-4 has a wide tapered mouth 20-4a gradually defining smaller interior circles and a narrower circular waveguide portion 20-4b. Both portions 20-4a and 20-4b have inner walls which are circularly shaped. The horn 20-4 is coupled by conventional flange connections to a circular waveguide 22 which is supported by members 20-3 coupled to the dish 20-1. The entire antenna complex is supported by a conventional tripod member 20-2 which may, e.g. be ground or roof-mounted. The circular waveguide 22 has a center line shown at 22-1. Extending from and opening into the circular waveguide 22 is a rectangular waveguide 23 (normally called a 90.degree. port) having a center line 23-1 which is substantially perpendicular to the circular waveguide center line 22-1.

Microwave energy entering the horn 20-4 travels into the circular waveguide 22 and leaves from the rectangular waveguide outlet 23-3. The outlet end of the rectangular waveguide is coupled to a conventional receiver-down converter 28 which in turn is connected via a coax cable 28-1 to the user's receiver.

The receiver-down converter 28 conventionally used requires linear polarized microwave signals for operation, whereas satellites often transmit signals as circularly polarized signals (either left or right hand polarized). Accordingly, to convert the circularly polarized signal to a linearly polarized signal, an assembly 26 including a fin 26-1 of electrically conductive material, e.g. copper or aluminum, is positioned for rotation about the center line (axis) 22-1 of the circular waveguide 22. The face of the fin 26-1a is substantially flat and is at substantially a right angle to the axis 22-1 of the circular waveguide 22. In the preferred embodiment the fin 26-1 is conveniently mounted in a di-electric material 26-2, e.g. polystyrene, etc. for rotation about the axis 22-1. The di-electric material support 26-2 is positioned on a coupling member 24-3 which is itself coupled to a shaft 24-2 driven by an electric motor 24 (e.g. step or continuous) to position the fin 26-1 about the axis 22-1.

In the preferred embodiment the fin edge 26-1a is positioned preferably at 1/4 (one quarter) of a wavelength from a circularly shaped metal shorting plate 27, e.g. copper, aluminum, etc. at the frequency to be converted to obtain the best conversion ratio. Where a band of frequencies is to be received, the 1/4 of a wavelength distance is usually based on the center frequency of the band. If the fin surface 26-1a is inwardly of the rectangular waveguide wall 23-5 i.e. to the left of FIG. 2, the bandwidth performance is unsatisfactory and if the fin 26-1a surface is positioned beyond the center line 23-1, i.e. to the right of FIG. 2, the incoming wave form is reflected to such a degree that the converter performance is unsatisfactory.

The fin 26-1 is positionable with respect to the shorting plate by adjusting the position of the coupler 24-3 on the shaft 24-2 by use of a set screw 24-3a. The shorting plate 27 as shown is mounted in a slidable manner in the circular waveguide portions 22-2 as shown by arrows 22-3 to position the shorting plate 27 and the fin 26-1 with respect to the inlet 23-4. In order to allow the flow of microwave energy without substantially reducing the amount of energy flow between waveguides, the fin face 26-1a is positioned between waveguide center line 23-1 and the proximal inner wall 23-5 of the rectangular waveguide 23.

The member 25 also supports the motor by way of flanges 25-1 and the coupling member is rotatable in a bearing sleeve 27-1. Thus in practice the fin surface 26-1a is always positioned between the center line 23-1 and the proximal wall 23-5 and the shorting plate 27 is preferably positioned to the left of the proximal wall 23-5 as shown in FIG. 2, however it may be positioned forward of the wall 23-5 and back of the center line 23-1. FIGS. 6A to 6D show four positions to which fin 26-1 is rotatably positionable about axis 22-1. In FIGS. 6A to 6D the fin 26-1 is 90.degree. to the center line 23-1 or parallel thereto and in such positions linearly polarized microwave energy entering the horn 20-4 will travel through rectangular waveguide 22 and be directed into rectangular wavguide 23 without having its polarization changed. With the fin 26-1 as shown in FIGS. 6A and 6D, circularly polarized signals entering the horn 20-4 will be substantially reflected back with the remainder entering the rectangular waveguide.

When the fin 26-1 is at .+-.45.degree. to the center line 23-1 as shown in FIGS. 6B and 6C, circularly polarized microwave energy signals (LH and RH) entering the horn 20-4 will be converted to linearly polarized microwave energy. With the fin 26-1 positioned as in FIG. 6B (45.degree. to the center line 23-1right hand (RH) circularly polarized microwave signals will be converted to linearly polarized microwave signals and the fin 26-1 as positioned in FIG. 6C will be converted to left hand (LH) circularly polarized microwave signals to linearly polarized signals, thus providing the linearly polarized signals which then pass through the rectangular waveguide and are accepted by the receiver 28. It should be understood that the converter of this invention may also be used in a transmitter mode by replacing the receiver 28 with a transmitter. Thus linearly polarized transmitted microwave signals now entering rectangular waveguide at 23-3 may be converted to circularly polarized microwave energy by positioning the fins as shown in either FIGS. 6B or 6C for transmission from the circular waveguide 22 through horn 20-4 against the dish 20-1 for beaming at e.g. a communications satellite.

Likewise, transmitted linearly polarized microwave signals entering the rectangular waveguide 23 may be transmitted as linearly polarized signals by the antenna without a change in polarization by positioning the fin as shown in either FIGS. 6A or 6D.

It has been found that for a bandwidth of 11.7-12.7 GHz with a center frequency of 12.2 GHz the fin face 26-1a should be 0.525 inches from the shorting plate 27 and it is preferred that the fin face by positioned to the right of wall 23-5 about 0.105" inches. It should also be understood that the fin may be directly driven by the coupling member by the shaft. The plate 27 may be of a thickness of 25 mils and a fin 26 having a width dimension (w) of 0.020 inches, a length (1) dimension of 0.605 inches and a thickness (t) of 0.225 inches has been found adequate using a circular waveguide 22 of an inner diameter 0.622 inches and a rectangular waveguide 23 of an inner length dimension (1) 0.750 inches between walls 23-5 and 23-6 and width (w) dimension 0.093 inches is satisfactory.

It should also be understood that the fin thickness as well as the other dimensions may be varied as will be appreciated, and it is understood that the dimensions given are only by way of example for the preferred embodiment at this time.