AU609602B2

AU609602B2 - A feed horn for a parabolic antenna

Info

Publication number: AU609602B2
Application number: AU33768/89A
Authority: AU
Inventors: Ronald Seaforth Weatherall
Original assignee: TANGLEWOOD HOLDINGS Pty LIMITE
Current assignee: TANGLEWOOD HOLDINGS Pty Ltd
Priority date: 1988-04-28
Filing date: 1989-04-28
Publication date: 1991-05-02
Anticipated expiration: 2009-04-28
Also published as: AU3376889A

Description

PATENTS ACT 1962 Forni COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: fThi; C.

Application Number: Lodged: PI 7946 28th April 1988 Complete Speciflcadorn-Lodged: Accepted: L-i'psed: Published: *PrioritV: Related Art: Name of Applicant: Adreess of Applicant: Actual Inventor: Addres.s for Service: TO BE COMPLETED BY APPLICANT TANGLEWOOD HOLDINGS PTY. LIMITED 19 Forrest Avenue, WAHROONGA, N.S.W., Australia 2076 Ronald S. Weatherall HALFORD AND COMPANY, Patent Trade Mark Attorneys, 49-51 York Street, SYDNEY, 2000 S007073 28/04/89 ComPlete SpeCl~ction for the invention entliced: "A FEED HORN FOR A PARABOLIC ANTENNA" The following statement is a full deicrlptlon of this Invontion, Including the best method of prforming It known to me:-

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-2- The present invention relates to a feed horn for a parabolic reflector antenna such as is used in satellite receiving antennas.

A feed horn is located at the focus of a paraboloid to receive radiation reflected from the surface of the paraboloid. Throughout this specification though the reference will be to the reception of electromagnetic radiation it is to be understood that the antenna is equally useful as a transmitting antenna.

With the advent of direct broadcast satellites where 1" the television signal is beamed directly at a local e9 receiving station there has been a need for an efficient s I and simple receiving antenna. Many designs for these antennas exists to operate in the K-Band and the Ku-Band of the microwave spectrum. These generally comprise a ofi cylindrical wave guide located at the focus of the paraboloid matching to a rectangular wave guide. The 00 rectangular wave guide feeds the signal to associated 0 o electronics to amplify and/or change the frequency of the received signal. Certain design parameters of the antenna are specified in terms of the ratio of the focal length to the diameter of the antenna with most parabolic reflector antennas having a ratio ranging from 0.3 to Known feed horns include a series of concentric rings coplanar with the focal point. These rings are designed to reject the radiation reaching the focal plane from outside the area of the paraboloid. As is well known in the art natural objects under ambient conditions produce Sinature, To: THE COMMISSIONER OF PATENTS, -3radiation in the microwave region. This radiation is a source of noise affecting the reception of the desired signal.

Feed horns are also good thermal conductors being made of metal. Thus the heat or infrared radiation collected by the surface of the paraboloid is also concentrated at the focus resulting in the feed horn receiving a large thermal loading. This thermal loading in prior art device is normally conducted to the low noise block down converter attached to the feed horn. This has adverse effects on the electronics of the converter.

o These disadvantages in the prior art the present invention seeks to overcome by providing a feed horn having an improved rejection of thermal noise and thermal 15 loading. According to the invention there is provided a feed horn for a parabolic antenna of the type having a cylindrical wave guide section with one end thereof connecting to a rectangular wave guide matching section, with a plurality of annular spaces arranged about the cylindrical wave guide section, characterised in that said annular spaces are defined by circular walls concentric with the cylindrical wave guide section the height of said It I *walls measured from a common plane located behind and parallel to the other end of the cylindrical wave guide section decreasing with increasing wall radius. A heat sink in heat conducting relationship with the cylindrical wave guide section and with the outermost of said walls is preferably provided. The heat sink is positioned nearer said common plane than the edge of the outermost of said -4-

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i 9 *0 1 9 9r 9 1 9 0 89 *0*9 o 99 9a 0 9 99:: walls. The heat sink preferably includes a series of discs or annuli spaced apart by collars.

The feed horn also preferably has an additional heat sink surrounding the rectangular wave guide matching section.

As is known in the art the feed horn also carries a means for attaching the feed horn at the focal point of the parabola. In the preferred form of the invention this constitutes a collar which is movable along the axis of the feed horn to enable the horn to be located at the correct focal point.

Preferably the rectangular wave guide section is rotatable with respect to the axis of the cylindrical wave guide section so as to allow adjustment of the 15 polarisation that is received and/or to maximise the received signal. This can be done by having the rectangular waveguide section screw threaded onto the cylindrical waveguide section.

A preferred embodiment of the invention will now be 20 described with respect to the following drawings in which: Figure 1 shows a parabolic reflector antenna having a feed horn according to the invention shown in exploded view; and Figure 2 shows the feed horn according to the invention in its separate parts; and Figure 3 shows an isometric view of the feed horn of Fig. 2 when assembled.

The preferred embodiment is designed to operate in the Ku-Band of the microwave spectrum. In particular, the embodiment is designed for operation at a centre frequency of 12.5 GHz. This frequency is one of the down-link frequencies used in the AUSSAT satellite communications system. Other satellite broadcasing systems throughout the world employ other microwave frequency ranges such as K-Band. The invention is generally applicable over the frequency band of approximately 11-14 GHz.

As shown in Fig. 1, microwave radiation from the geostationary satellite is collected by the surface of the paraboloid 10 and concentrated at the focus 12 where the feed horn 14 is located. The feed horn is supported at 0 t the focal point by the struts 16. A low noise block down 0 0 t" converter 18 is attached to the rear of the feed horn to I t allow amplification and/or frequency changing of the received signal.

As the surface of the paraboloid 10 is also a good 0nc reflector of other electromagnetic signals, heat and light ,o are also concentrated at the focal point 12. This heat is S0 o conducted by the metal structure of the feed horn to the 20 electronics of the converter 18. This thermal loading of the electronics can adversely affect the performance of the electronics and requires special consideration in the design thereof.

As shown in Figure 2 the feed horn comprises a cylindrical sleeve 20 having at the end 22 a circular aperture 24. The rear end of the cylindrical sleeve 26 is provided with a screw thread connection by which the rectangular wave guide adaptor 28 is attached. A collar 32 is provided to fit about the cylindrical sleeve 20 to

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i i J I i secure the feed horn at the focus 12 of the paraboloid and is fixed to the struts 16 of the antenna by nuts 34.

An optional collar 36 can also be used to provide focal adjustment for the horn.

The interior 38 of the cylindrical wave guide at end 26 is tapered to match the circularly polarised cylindrical wave guide to the rectangular wave guide adaptor 28. The interface between the cylindrical wave guide and the rectangular wave guide is also provided with tapered cut outs 40 and 42 in the X and Y direction respectively of adaptor 28.

o° Threaded holes 44 are provided in collar 32 through 0 a o which nuts 34 secure the collar 32 to the struts 16.

o o o Sano The inner diameter of the collars 32 and 36 is 0 0 0 1 0I 15 slightly greater than the outer diameter of the o 0 cylindrical sleeve 20 such that the collars 32 and 36 can 0 ::slide along the sleeve. This allows the focal point of 0: the feed horn to be adjusted. Threaded holes 46 with associated nuts then secure the collars 32 and 36 to the 20 cylindrical sleeve 20 at the desired position.

The aperture 24 is surrounded by a set of rings OoQ ooO 62 and 64., These are used in feed horns of this type to o.

0• prevent or reduce over-illumination of the cylindrical wave guide. We have found that substantial improvement in performance occurs if the height of the rings decreases from the innermost ring 60 to the outermost ring 64 as this arrangement results in improved rejection of the environmental noise which would otherwise interfere with the desired signal.

Cooling fins 48 and 50 in the form of spaced discs or annuli act as heat sinks and are located at either end of the feed horn, that is, about the focal point 12 and about the matching portion 28. Heat tending to be conducted toward the rear of the feed horn and thus towards the electronics attached thereto is dissipated by these heat sinks improving the efficiency and performance of the electronics. In addition the foremost disc 52 of the heat sink 50 contributes to the diffusion of the ambient noise background.

The rectangular wave guide matching section 28 of the feed horn is attached to the cylindrical sleeve by a screw threaded portion 70. This threaded portion 70 in turn allows the orthogonal axes of the rectangular wave 15 guide to be rotated in order to give a maximum signal for 4 a particular polarisation, or by rotating through 900 to receive the orthogonal polarisation. A spectrum analyser, a digital volt meter or other suitable instrument can be o 0 employed to detect the maximum received signal to thereby 20 fine tune the antenna. Similarly this instrumentation can be used to adjust the collar to position the feed horn at °1 the focus of the paraboloid.

t The feed horn according to the preferred embodiment will accept F/D (focus/diameter) ratios from 0.3 to 0.45 at a mid range frequency of 12.5 GHz. The feed horn will also work at a frequency of 11.7 GHz in a range of F/D ratios of 0.33 to 0.45. The illumination taper of the feed horn is designed to illuminate the parabolic antenna from zero to 2 dB noise figure at the centre to i rl -l 1 l-rr~r;-tu-,~ii, approximately 13 dB noise figure at the outer edge of the antenna. The stepped rings 60, 62 and 64 and the flat 52 at the outermost radius provide improved noise rejection for the feed horn. In addition the heat sinks at the focal end and at the coupling end of the feed horn provide improved heat rejection and minimise the thermal loading and design constraints on the attached electronics. The collar in turn provides in conjunction with the rotatability of the rectangular wave guide matching section simple means to enable the antenna gain to be optimised without over illumination that would otherwise C ol increase the noise factor.

Though the invention has been described above with a C C Coo respect to a preferred embodiment it is to be understood 15 that other variations are possible in the construction of 0o 0 the device. For example, the preferred embodiment has 0090 been described with respect to a microwave frequency of o o 12.5 GHz. There is no restriction on the feed horn being o e9 constructed at some other frequency range for example Keaa 20 Band. The size of the heat sinks can be adjusted depending upon the environment in which the feed horn is C00 to be used. For example, in an Australian environment 6 where large temperatures at the feed horn are produced by the ambient conditions larger cooling capacity for the heat sink would be necessary compared to a cooler Northern latitude environment. The heat sink also has the added advantage that in a northern environment the deposition of snow which is a strong attenuator of the microwave signal can be melted and evaporated from the feed horn by heat -9conducted from the electronics through the metal structure of the feed horn. The material of the feed horn is preferably aluminium though other materials can also be used. The rotation of the rectangular wave guide matching section could be effected remotely and automatically with suitable electric motors and control circuitry as is well known. Equally the collar could be provided with a worm drive gear engaging the sleeve to provide automatic adjustment of the focal point.

ao 4 0 o 0 4 *4 do 04

Claims

2. A feed horn for a parabolic antenna as claimed in claim 1 further including a first heat sink in heat conducting relationship with said cylindrical wave guide section and with the outermost of said walls.
3. A feed horn for a parabolic antenna as claimed in claim 2 wherein said first heat sink is positioned below the height of the outermost of said walls.
4. A feed horn for a parabolic antenna as claimed in claim 3 wherein said first heat sink includes a series of discs spaced apart by collars along the axis of said cylindrical wave guide section. A feed horn for a parabolic antenna as claimed in claim 4 further including a second heat sink surrounding the rectangular wave guide matching section.
6. A feed horn for a parabolic antenna as claimed in claim 5 further including means for positioning the :u c 'it Ir i -11- rc r C S C tie1 C t C Ct C C CC IC 1 C C rC mouth of said feed horn at the focal point of a parabolic antenna.
7. A feed horn for a parabolic antenna as claimed in claim 6 wherein said positioning means comprises a collar removably securable along the axis of the feed horn and a flange for holding said feed horn once said mouth has been positioned at the focal point of the antenna.
8. A feed horn for a parabolic antenna as claimed in any one of the previous claims wherein said rectangular wave guide section is rotatably secured to said cylindrical wave guide section by a screw threaded connection.
9. A feed horn for a parabolic antenna substantially as hereinbefore described with reference to the accompanying drawings. A microwave antenna including, a paraboloidal surface, means for supporting the parboloidal surface in azimuth and elevation, means for supporting a feed horn at a focal region of said paraboloidal surface, said feed horn being as defined in any one of the previous claims. DATED this 25th day of January 1991 TANGLEWOOD HOLDINGS PTY. LIMITED Patent Attorneys for the Applicant HALFORD AND COMPANY g