GB2027558A - Improvements in or relating to waveguide radiators - Google Patents

Abstract

The invention relates to a waveguide radiator in which a directional beam of electromagnetic radiation is formed by a hollow waveguide having an array of slot pairs 8, 9 formed in one of its broad walls, the slots of a pair radiating in anti-phase. The slot pairs 8, 9 are provided in at least those longitudinal regions of the waveguide radiator where a relatively low amplitude radiation is required, single slots being provided where higher amplitudes are required, so as to produce by amplitude- tapering a directional beam having only low amplitude sidelobes. By providing a pair of slots they can be positioned at regions of the waveguide where the change in amplitude or phase of the electrical field is not critically dependent on position. This reduces the manufacturing tolerances involved in producing a waveguide radiator. Asymmetry of the feed conditions of the two slots of a pair determines their net radiation amplitude and can be controlled by screws 10-13 or by the slot positioning. <IMAGE>

Description

SPECIFICATION Improvements in or relating to waveguide radiators This invention relates to waveguide radiators of the kind in which a directional beam of electromagnetic radiation is formed by a hollow waveguide structure having an array of slots formed in one of its broad walls. The positon of the slots affects the profile of the beam so formed and it is important to precisely locate the slots at predetermined positions in order to minimise the generation of undesirable sidelobes.

It is extremely difficult to manufacture waveguides having slots positioned to the required degree of accuracy and the present invention seeks to provide a waveguide radiator in which this difficulty is reduced.

According to this invention a waveguide radiator includes a length of rectangularly sectioned waveguide having slots formed in one broad wall thereof so as to be capable of producing a directional electromagnetic radiation beam, the slots being off-set from the centre line of the broad wall to produce a required beam profile, and the slots being positioned in pairs in at least those portions of the radiator in which a relatively low amplitude radiation is required, the resultant amplitude of the energy radiated being dependent on the difference in the individual energy provided by each slot of a pair.

Preferably the two slots of a given pair are off-set by the same amount and means are provided to produce an asymmetry in the electrical field within the waveguide at the pair of slots so as to cause the required resultant amplitude to be radiated by the pairofslots.

If no asymmetry were provided in the electrical field within the waveguide, the provision of two slots which are off-set by equal and opposite amounts from the centre line of the broad wall would cause the pairto emit no resultant radiation, since the energy from each slot of the pair would completely cancel that from the other slot. Thus by providing a small asymmetry precise control of the resultant amplitude can be achieved.

Preferably a conductive member is arranged to protrude into the internal cavity of the waveguide adjacent to a pair of slots, the depth of insertion producing a corresponding degree of asymmetry in the electric field.

Preferably again the conductive member is an adjustable screw-threaded conductor mounted in a wall of the waveguide. The conductive member may be mounted in a broad wall or a narrow wall of the waveguide, and to provide a finer degree of adjustment more than one conductive member can be provided for each pair of slots.

Instead of providing a pair of slots having the same off-set, it is alternatively possible to provide a pair of slots which are off-set by different amounts from and on opposite sides of the centre line of the waveguide, the difference in the off-set distance producing the required degree of asymmetry in the amplitude or phase of the energy produced at each slot of a given pair.

It is most advantageous to provide those slots in pairs which determine the low amplitude characteristics of the radiation pattern. However, if desired all slots in the waveguide radiator may be provided as pairs.

Preferably in order to suppress undesired difference mode radiation in the electric plane which can arise from a pair of spaced slots, a parallel plate waveguide section is mounted on the broad wall of said rectangularly sectioned waveguide so as to be fed via said slots.

Preferably again the length of the parallel plate waveguide section exceeds one half wavelength.

The end of the parallel plate waveguide section remote from said slots may with advantage be flared in the electric plane.

The invention is further described by way of example with reference to the accompanying drawings in which, Figure 1 shows a known form of waveguide radiator and, Figure 2 illustrates part of a waveguide radiator in accordance with the present invention and Figure 3 illustrates a section view of the waveguide radiator shown in Figure 2.

Referring to Figure 1, a waveguide radiator is in the form of an elongate section of hollow rectangu larwaveguide having a number of slots 1 formed in a broad wall thereof. The waveguide is formed from a hollow thin walled section and the slots completely pierce the wall so as to provide communication with the interior. One end of the waveguide radiator is fed from a transmitter by means of a feed section 2, and the other end is terminated by a matched load 3. The waveguide is such as to support waveguide modes in which the strength of the electrical field increases progressively from a minimum at a centre line 4 towards the side walls 5 and 6. In accordance with known practice the slots 1 are positioned at half wavelength intervals along the waveguide radiator and are positioned on alternate sides of the centre line 4 as shown.Each slot is approximately a half wave length long. It should be noted that the wavelength used to determine the slot spacing is the wavelength of the radiation within the guide, whereas the wavelength which determines the length of the slot is that of the radiation in free space. Since the wavelength in free space is slightly shorter than that in the waveguide, the end of one slot is slightly spaced apart in a longitudinal direction from the beginning of adjacent slots. The slots are positioned so that a symmetrical electromagnetic beam is radiated having its greatest amplitude at the centre.

The beam tapers off at each side by an amount determined by the extent to which the slots at the extremities of the waveguide radiator are off-set from the centre line 4.

The use of an almost symmetrical pair of slots with substantially larger off-sets significantly reduces the reactance presented to the waveguide by the radiator, so that the phase of the radiation from the pair deviates from the nominal much less than that of a single slot having the same conductance.

For waveguide radiators designed to produce very low sidelobe levels the conductance of the outer slots needs to be small, requiring correspondingly small slot off-set distances. When the conductance is very low the reactive component of the admittance of the slot predominates and the radiated phase of the radiation from the slot changes rapidly as the off-set distance is reduced towards zero. Thus the positioning of the slots towards the extremities of the ends of the waveguide radiator is very critical.

Figure 2 shows part of a waveguide radiator in accordance with the present invention in which the need to position a slot at a precise off-set distance is avoided. The broad wall of the waveguide is provided with a pair of slots 8 and 9, which are positioned at equal distance from and on opposite sides of the centre line 4. Since the electric field distribution within the waveguide is inherently symmetrical, the electromagnetic energy which is radiated by the slots 8 and 9 will be mutually in anti-phase and will cancel. A pair of adjusting conductive screws 10 and 11 are provided in the broad wall of the waveguide and by suitable adjustment of these an asymmetry in the electrical field can be produced so that the pair of slots radiates with a corresponding controllable low level of radiation.Instead of screws 10 and 11 being placed in the broad wall they may be instead mounted in a narrow wall as indicated by screws 12 and 13.

Figure 3 shows a section view taken on the line X-Y of Figure 2 and indicates more clearly the internal shape of the waveguide section, and it also shows a parallel plate waveguide 14which is mounted on the slotted broad wall of the rectangular waveguide. The waveguide 14 suppresses the difference mode radiation in the electric plane which arises from a pair of anti-phase slots. The waveguide 14 is typically greater than one half-wavelength long, as shown, and its open end 15 is flared in the electric plane. The plate spacing is such that the odd mode of propagation excited by a pair of slots is below cut-off and is therefore greatly attenuated being it reaches the final radiating aperture at 15.

As with the waveguide radiator shown in Figure 1, each slot has a length which is half a wavelength of the radiation in free space and the longitudinal spacing of one pair of slots from the next is half a wavelength of the radiation in the waveguide. Since the phase of the energy radiated is most critical for slots near the ends of the waveguide, it may not be necessary to provide all slots in pairs. Where a number of pairs of slots are provided, each pair of slots may have a different off-set distance or they may instead all be provided with the same off-set distance. This latter alternative simplifies the manufacturing process, but for pairs of slots located towards the centre of the waveguide radiator a great deal of asymmetry must be introduced by means of adjustable conductive screws if a sufficiently high amplitude beam is to be radiated.

It will be noticed in Figure 1 that the slots are positioned on alternate sides of the centre line 4. If in Figure 2 adjacent pairs of slots are provided, the asymmetry in the electrical field within the waveguide is adjusted such that more radiation is produced from slots on alternate sides of the centre line 4. The form of waveguide radiator shown is intended for use at high frequencies, typically in the range 2 to 10 GHz and allows a directional beam having low sidelobes to be generated in a simple yet precise manner.

Instead of the arrangement shown in Figure 2 in which each slot of a pair is off-set from the centre line 4 by equal distances, the slots can be positioned at different off-set distances. In this case the difference in the off-set distances determines the degree of asymmetry, and by placing the slots at positions in which the phase of the wave within the waveguide does not change too rapidly with off-set distance the actual positioning of the slots becomes less critical.

As before the degree of asymmetry can be adjusted by means of adjustable conductive members which protrude into the interior of the waveguide cavity.

Claims (8)

1. Awaveguide radiator including a length of rectangularly sectioned waveguides having slots formed in one broad wall thereof so as to be capable of producing a directional electromagnetic radiation beam, the slots being off-set from the centre line of the broad wall to produce a required beam profile, and the slots being positioned in pairs in at least those portions of the radiator in which a relatively low amplitude radiation is required, the resultant amplitude of the energy radiated being dependent on the difference in the individual energy provided by each slot of a pair.

2. A waveguide radiator as claimed in claim 1 and wherein the two slots of a given pair are off-set by the same amount and means are provided to produce an asymmetry in the electrical field within the waveguide at the pair of slots so as to cause the required resultant amplitude to be radiated by the pair of slots.

3. A waveguide radiator as claimed in claim 2 and wherein a conductive member is arranged to protrude into the internal cavity of the waveguide adjacent to a pair of slots, the depth of insertion producing a corresponding degree of asymmetry in the electric field.

4. Awaveguide radiator as claimed in claim 3 and wherein the conductive member is an adjustable screw-threaded conductor mounted in a wall of the waveguide.

5. A waveguide radiator as claimed in claim 1 and wherein there is provided a pair of slots which are off-set by different amounts from and on opposite sides of the centre line of the waveguide.

6. A waveguide radiator as claimed in any of the preceding claims and wherein a parallel plate waveguide section is mounted on the broad wall of said rectangularly sectioned waveguide so as to be fed via said slots.

7. A waveguide radiator as claimed in claim 6 and wherein the length of the parallel plate waveguide section exceeds one half wavelength.

8. A waveguide radiator substantially as illustrated in and described with reference to Figures 2 and 3 of the accompanying drawings.