GB2173646A - Compound horn antenna - Google Patents

Abstract

In a broadband compound horn antenna it is desirable to suppress the TE30 mode whilst allowing the propagation of the fundamental TE10 mode. A broadband compound horn antenna is disclosed having opposed ridges 15 and 16 extending, in the E plane, centrally from opposite major faces. The spacing d varies with the axial distance L along the tapered transition 12 as the fourth or greater power of L. Such a profile has been found to gradually reduce the cut off frequency for the unwanted TE30 mode and thus finally suppress the propagation of this mode. <IMAGE>

Description

SPECIFICATION Compound horn antenna The present invention relates to horn antennas and particularly to a broadband compound horn antenna.

Horn antennas are typically fed from a section of waveguide at the base of the horn and have a maximum usable bandwidth corresponding to frequencies between the cut off frequency for the desired TE,o mode and the cut off frequency of the unwanted TE30 mode in the waveguide. The TE20 mode is normally suppressed by a suitable arrangement for launching the energy into the waveguide. The above implies a maximum usabie bandwidth of 3:1 for an ordinary rectangular section wave guide.

It is know to use ridges extending in the E plane of the waveguide centrally from opposite major faces of the waveguide, in order to extend the bandwidth. Such ridges of appropriate dimensions have the effect of substantially reducing the cut off frequency for the TE,o mode whilst causing little change to the cut off frequency of the To20. The use of ridges in a broadband horn is discussed in an article by K. L. Walton and V.C. Sundberg in Microwave Journal, volume 7, pages 96-101, March 1964. Figure 1 of that article shows a flared horn fed from a short length of rectangular waveguide.

The waveguide has opposed ridges designed to extend the bandwidth capabilities of the waveguide and these ridges are extended into the flared part of the horn. The article indicates that the ridges must be shaped within the tapered horn section to ensure that the cut off frequency for the desired TE,o mode at all positions along the length of the flare of the horn remains below the bottom end of the operating band of the antenna. However it is also desired that the height of the ridges becomes zero at or before the aperture of the horn.

The article is unclear on what is the desired profile of the ridges within the horn flare except that there should be apparently a smooth curve from the ridges in the waveguide feed section to the ends of the ridges at the aperture of the horn. As is apparent from the set of design curves illustrated on pages 110 and 111 of the article, the ridges also have a substantial effect on the characteristic impedance in the waveguide and horn flare. The ridges are used to provide a smooth transition from the ridge impedance of the feeding waveguide (50 ohms or less) to the impedance of free space at the aperture (377 ohms). The articles say that an exponential impedance taper has been found quite satisfactory. It is not clear what implication this has for the profile of the ridges themselves.

According to the present invention, a broadband compound horn antenna has a flare and aperture selected to provide a predetermined beamwidth in the H plane over a predetermined bandwidth, and has a double ridged tapered transition providing bandwidth extension to longer wavelengths, the separation d of the ridges varying in the transition as a function F(Ln) of the axial distance L along the transition towards the aperture, where the power n is 4 or more.

The present invention is concerned with the double ridge arrangement with symmetrical ridges extending in the E plane of the waveguide and horn taper and centrally of the wave guide or horn cross section. Furthermore, the invention is concerned only with such ridges which has a constant width, i.e. dimension in the H plane. The selection of the spacing between the ridges in the waveguide section feeding the tapered transition of the horn, together with the width of these ridges is dependent on the desired bandwidth performance of the resultant antenna.

It has been found that the use in the tapered transition of a ridge profile which provides a spacing varying with the fourth or greater power of the distance along the transition towards the aperture provides substantially improved performance for the antenna to be made as compact as possibie, i.e. having the smallest aperture relative to the longest wavelength in the operating band, whilst maintaining reasonably constant beamwidth over the band and minimising the production of sidelobes. At the same time, the profile shape provides reasonably good impedance taper between the waveguide feed and free space.

Preferably, the power n is equal to 5. The advantages of using a fifth power ridge profile cannot be predicted satisfactorily by applying the design curves in the above mentioned articles by Walton and Sundberg. The step discontinuity associated with the fifth power ridge profile introduces a susceptance in the tapered transition which lowers the cut off frequency for the unwanted TE20 mode "gradually" into the upper end of the operating band of the antenna. By "gradually" it is meant that there is a substantial axial length of the tapered transition over which the cut off frequency of the TE20 mode is close to the maximum operating frequency at the upper end of the operating band.From another point of view, a graphical representation of the TE20 cut off frequency with distance axially along the tapered transition shows the curve of the cut off frequency crossing the maximum operating frequency at a relatively acute angle, i.e. with a relatively small negative gradient.

This "smooth" introduction of the TE20 cut off frequency has been found to suppress its propagation in the tapered transition. This ensures a broadband principal mode only performance of the resultant horn antenna.

In this connection, it will be appreciated that the unwanted TE20 mode is readily suppressed by appropriate design of the arrangement for launching energy into the waveguide section feeding the horn.

By comparison, in prior art ridged horn designs, the TE20 mode cut off frequency descends relatively abruptly into the operating band of the antenna with the result that propagation within the tapered part of the horn may take place resulting in the generation of unwanted sidelobes in the radiation pattern of the antenna.

In a preferred example of the present invention, in the H plane, the flare angle of the antenna is between 85 and 105 (preferably 95 ) and the operating aperture is from about 1.7 to about 5.2 wave lengths over the bandwidth of the antenna.

The present invention will now be described in greater detail and in relation to a particular example, with reference to the accompanying drawings in which: Figure 1 is a cross sectional view in the E plane of a compound horn antenna embodying the present invention and illustrating the double ridged arrangement in the tapered transition of the horn; Figure 2 is a cross sectional view of the waveguide feed section at the base of the horn; Figure 3 is a cross sectional view in the H plane illustrating the flare and aperture of the horn; Figure 4 is a cross sectional view in the E plane of a particular embodiment of the invention having a test probe in the throat of the horn; Figure 5 is a graphical representation of a set of curves of the variation of beamwidth in the H plane with electrical aperture for various large flare angles;; Figure 6 is a graphical representation illustrating the variation of the cut off frequency for the wanted and unwanted modes of propagation along the length of the tapered transition; and Figure 7 is a graphical representation of the variation of the return loss of the antenna against operating wavelength.

Referring to Figures 1, 2 and 3, the illustrated horn antenna has a coax to waveguide transformer 10 at the base of the horn, which includes a short section of ridged waveguide 11. The waveguide 11 is connected to a tapered transition or feed portion 12 of the horn. The waveguide section 11 and the tapered transition 12 are of generally rectangular cross section having a major dimension in the H plane of the antenna. The tapered transition 12 flares outwardly in both the H and E planes with a total included angle of about 20 . In the H plane, as illustrated in Figure 3, additional flare plates 13 and 14 are provided providing a greater flare angle of 95" in the H plane at the aperture of the horn.

As can be seen in Figure 2, the waveguide section 1 of the coax to waveguide transformer has a double ridged arrangement comprising opposed ridges 15 and 16 extending in the E plane of the waveguide, centrally from opposite major faces.

The width (in the H plane direction) and spacing between the ridges 15 and 16 in the waveguide is selected to provide the desired bandwidth extension, i.e. reduction in the cut off frequency for TE,o mode propagation. the design curves shown in the above mentioned article by Walton and Sundberg may be used for appropriate selection of the dimensions of the ridges.

The ridges 15 and 16 continue into the tapered feed 12 as illustrated in Figure 1. The width of the ridges remains constant along the axial length L of the tapered transition but the spacing d between the ridges becomes progressively larger until, at the outer end of the transition 12 the spacing d is the same as the dimension in the E plane of the horn itself i.e. the ridges terminate.

It is an essential characteristic of the present invention that the spacing d varies with the axial distance L along the tapered transition in accordance with the fourth or greater power of L. In the present example, d varies with L5. With such an arrangement, as has been mentioned before, the propagation of the unwanted TE20 mode in the tapered transition is suppressed permitting optimum performance of the resulting horn over the greatest bandwidth.

Referring now to the particular embodiment illustrated in Figure 4, a test probe 17 commonly known as a BITE (Built In Test Equipment) probe is positioned in the wave guide 11 close to the tapered transition or feed portion 12 of the horn. The probe 17 comprises an input 18 connected to a cylindrical attenuator means 19 and further connected to one end of a cylindrical low loss spacer 20. An output 21 is connected to the other end of the spacer 20 and positioned coaxially with it. Test signals are fed to the input 18 and are then attenuated, generally by a small amount such as a factor of 2, as they pass through the attenuator means 19. The signals continue through the spacer 20 to the output 21, from which they are transmitted to simulate radiation entering the horn of the antenna: this allows the correct operation of the antenna and associated equipment to be tested.

It can be seen from the family of curves shown in Figure 5, that the beamwidth of the horn in the H plane is relatively constant over a range of electrical apertures for larger apertures and with horn flare angles of at least about 50". However, in order to keep the horn antenna as compact as possible the electrical aperture must be kept as small as possible relative to the longest wavelength in the operating band. If a flare angle of about 95" is selected in the H plane, a relatively constant beamwidth of about 45" plus or minus 5 is obtained over an operating region in which the minimum electrical aperture is about 1.75 wavelengths.

Referring to Figure 6, the operating bandwidth of the antenna is shown extending between a minimum frequency f0 and a maximum frequency 4f0.

The graph shows the variation of the cut off frequencies f, for the TE10 and TE20 propagation modes over the axial length of the tapered transition.

It will be seen firstly that the ridges both in the waveguide feed section to the tapered transition and throughtout the length of the transition are arranged to keep f,(TE,,) below f0 along the length of the transition. At the outer end of the transition, the spacing between the ridges becomes equal to the spacing between the walls of the horn which must be in excess of 0.5 x the longest wavelength in the operating band.

The use of the fifth power profile for the ridges in the tapered transition causes the TE30 cut off frequency to decline gently making a "smooth" crossing of the upper end of the operating band 4F0. This is representated in Figure 5 by the relatively small negative gradient of the curve of f,(TE 30) as it crosses the 4f0 line. Also, the ridge profile causes this cut off frequency to drop below the upper end of the operating band relatively far back in the tapered transition of the horn, i.e. relatively close to the launching end of the horn. It has been found that this arrangement has the effect of suppressing the propagation of the unwanted TE20 mode in the horn so that principal mode only performance is obtained over the entire operating band of the antenna.

The fifth power ridge profile has also been found to present a reasonably good impedance taper between the impedance of the ridged waveguide 11 and the impedance of free space at the aperture of the horn. The return loss for the antenna has been found to be better than 10 dB over a 3:1 bandwidth, as illustrated in Figure 6.

The waveguide to coax transformer employed in the described example of this invention is similar to that described in the above articles by Walton and Sundberg and as illustrated therein in Figure 9 on page 111. A turnable shorting plate is employed to optimise the V.S.W.R. performance over the desired operating band.

Claims (6)

1. A broad band compound horn antenna having a flare and aperture selected to provide a predetermined beamwidth in the H plane over a predetermined bandwidth, and having a double ridged tapered transition providing bandwidth extension to longer wavelengths, the separation d of the ridges varying in the transition as a function F(Ln) of the axial distance L along the transition towards the aperture, where the power n is 4 or more.

2. A horn antenna as claimed in Claim 1 wherein n is equal to 5.

3. A horn antenna as claimed in either of Claims 1 and 2 wherein in the H plane the flare angle is between 85 and 105 and the operating aperture is from about 1.7 to about 5.2 wavelengths over the bandwidth of the antenna.

4. A horn antenna as claimed in Claim 3 wherein the flare angle is 95 .

5. A horn antenna as claimed in any preceeding claim having a means for injecting signals into the antenna for simulating the receipt of reflected radiation.

6. A horn antenna substantially as hereinbefore described with reference to the accompanying drawings.