US2937373A - Slotted waveguide aerials - Google Patents

Description

May 17, 1960 A. w. H. CARTER 2,937,313

sLoTTED wAvEGUIDE AERIALS Filed Nov. 26. 1957 -Jnitecl States Patent() SLOTTED WAVEGUIDE AERIALS Application November 26, 1957, Serial No. 699,038

Claims priority, application Great Britain November 27, 1956 7 Claims. (Cl. 343-771) This invention relates to slotted waveguide aerials.

'One form of slotted waveguide aerial which has been proposed for use at microwave frequencies comprises a rectangular waveguide having one pair of faces substantially broader than the other pair. On each side of a centre line of one of the broader faces of the waveguide a series of slots is positioned in longitudinal manner along the face, the slots being of the order of one half wavelength long and the slots on one side of the centre line being staggered in relation to those on the other side of the centre line. Each slot operates as an elementary radiating ldipole and a radiation pattern can be produced comprising one main lobe. However, itis also found that minor lobes are produced, concentrated into portions of two small cones of radiation on either side of the plane normal to the `face of the guide and including the axis of the guide. These lobes `because they are of the form of butterfly Wings, are termed buttery lobes, one pair of lobes appearing on either side of the main lobe of the radiation pattern. The butterfly lobes arise because there are phase errors in planes parallel to the slotted face of the waveguide due to the varying projected distances travelled by the wave components from the various slots. In certain applications butterfly lobes may not be objectionable but in other applications where it is desirable to provide an aerial having a single main lobe in a given direction in relation to the waveguide, butterfly lobes can introduce substantial difficulties if the aerial is to be employed, for example in a direction finding arrangement.

The object of the present invention is to provide an improved slotted waveguide aerial such that the radiated energy is confined substantially into a single main lobe without the formation of buttery lobes.

According to the present invention there is provided a slotted waveguide aerial comprising a waveguide having an array of slots arranged at least partially in longitudinal manner on one side of the centre line of a face of said waveguide and the ratio of the wavelength of signals of a given frequency in free space to the wavelength of signals of the same frequency in said waveguide is determined in relation to the longitudinal spacing of said slots to provide a radiation pattern having substantially a single main lobe.

In a preferred aspectof the invention said waveguide is lled with a solid fdielectric so that said ratio is not less than unity.

In order that the invention may be clearly understood and readily carried into effect, the invention will be described with reference to Ythe accompanying drawing, the single figure of which illustrates one example of a waveguide aerial according to the present invention.

Referring to the drawing, reference 1 is a rectangular waveguide for radiating electromagnetic energy of a desired microwave frequency band and which may be fed with microwavesignals by vmeans of an oscillator connected via the base 2 of the guide. According to the particular application of the waveguide aerial moreover "ice suitable switching means may be included in the connection between the oscillator and the base of the waveguide but this switching means is not relevant to the present invention. Thedotted line 3 illustrates the position of the central line of a broader face 4 of the Waveguide 1 and serves to indicate the relative proximity of the slots S1 to S11 on one side of the centre line 3 in this face of the waveguide. The interior of 1 is completely filled with a ydielectric material one such suitable material being polystyrene, which has a dielectric constant of approximately 2.6. The dielectric material also lls the slots. S1 to S11 so that the face 4 which contains them is substantially uninterrupted. In practice the aerial may be of substantial length and the number of slots may be much greater than indicated. For example the broad face of the waveguide may contain between 20 and 30 slots. The lateral displacement between adjacent slots is relatively small but is predetermined in known manner to give a desired power 'distribution amongst the slots along the length of the waveguide aerial, for the purpose of reducing the formation of the normal side lobes.

In operation of the aerial the longitudinal distance between the mid points of successive slots is of the order of the wavelength )tg at the centre frequency of the desired frequency band for the aerial, but choice of this displacement depends upon the desired direction of the main beam in the radiation pattern. The length of each slot is approximately equal to hal-f the wavelength of waves of the above frequency in free space. Each slot may be considered to operate as an elementary radiating dipole and the energy radiation or received by a slot is proportional to its lateral displacement from the central line 3.

It can be shown that the direction of a main lobe in the radiation pattern from a slotted waveguide aerial is given by the equation In this equation 0 is the angle of the main lobe with the direction of a forward going wave in the guide, measured in the plane containing the axis of the guide and normal to the face containing the slots. The symbol )t denotes the wavelength of the electromagnetic wave in free space, Ag denotes the wavelength in the guide, and s denotes the longitudinal spacing between successive slots. The term K is required if there are slots on alternate sides of the centre line to take account of the phase reversal between adjacent slots,land in this case the term K is 1/2. The term k is either zero or an integer such as il, i2, i3 and so on.

In a waveguide aerial according to the invention the slots are arranged on one side of the centre line of the slotted face. The lateral displacement between adjacent slots, employed to give a desired power distribution, is then always relatively small so that there is small tendency for the formation of butterfly lobes. However since the slots are on one side of the centre line, the term K in the foregoing equation isnow zero, and a waveguide aerial which is iilled with the same material as that which makes up free space exhibits the property that the wavelength within the waveguide is substantially greater than lthe wavelength of the same frequency in free space.

As a consequence the ratio lt/Ag is less than unity and there is a solution for the' above equation corresponding to a real value for 0 when k is zero; There is therefore a main lobe in the radiation pattern which is independent of the longitudinal spacings of the slots. Moreover in waveguides as hitherto proposed, there would be at least onev other solution of the equation giving a real value of 0, corresponding to a different value of k, therefore if the slots on one side of the centre line be removed the radiated energy would be split into two or more main lobes, with undesirable effect. However, such undesirable elect is avoided in accordance with the present invention by adjusting the ratio Mag and the value of s relative to one another so that although the slots are on one side of the centre line, the radiated energy is nevertheless substantially confined to a single main lobe.

In the example of the invention which has been described the formation of a main lobe corresponding to k=zero has been prevented by filling the waveguide with a dielectric of such dielectric constant that the term Mag is greater than unity and there is no real solution to the equation for k=zero. It is then readily possible to choose a value of s so as to confine the radiated er1- ergy to a single main lobe directed in a desired direction 0. The dielectric material lling the guide has the eifect of reducing the velocity of propagation of the electro-magnetic energy in the guide thereby reducing the wavelength Ag, the reduction being in proportion to the square root of the dielectric constant, assuming the dimensions of the guide are adjusted to preserve the cutoff conditions unaltered. In a practical example, using polystyrene as dielectric material, Mag has a value of 1.1 and s has a value of 0.67\. K= as indicated above. There is no solution to the above equation when k=0, but k=1 gives a solution for 0 of the order of 120. Clearly if desired parameters may be chosen for which there is a virtually single main lobelat substantially 90 to the axis of the waveguide and moreover virtually any other required angle for the main lobe can be produced.

When the angle 0 is 90 the loads on the waveguide produced by the slots are all co-phasal and therefore constitute a resonant load but when 0 is substantially different from 90 this is not so since the loads are spaced at distances which are dilierent from M2 and the load is non-resonant. For a resonantly loaded arrangement the operational bandwidth may be limited but for a nonresonant load the advantages of increased bandwidth are obtained so that it may be desirable to produce a radiation pattern for the aerial which has a main lobe at an angle which is substantially removed from 90. In practice this may also for'other reasons be the more desirable form of radiation pattern.

Clearly the choice of s for a particular radiation pattern is dependent upon the choice of dielectric constant for the material which is used to ll the waveguide. Thus for a given main lobe angle an increase in dielectric constant permits of a reduction in s.

An aerial according to the present invention may be constructed by moulding a solid dielectric material into the desired shape for the waveguide cavity and having small protrusions corresponding to slots. The surface of the moulded dielectric can then be metallised after which a metal casing can be built up thereon by electro-plating. When a sufficient metallic skin has been formed, metal on the protrusions corresponding to the slots can be removed by suitable machining thereby leaving the slots. The protrusions can moreover be faced oli ush with the slotted face of the waveguide.

The present invention is not limited to the above described method of reducing the velocity of propagation within the waveguide, and thereby reducing xg. For example lumped reactive loads consisting of irises or other periodic disturbances within the waveguide may be employed to alter its electrical dimensions. Alternatively the physical form of the waveguide can be modified. In all cases however it may be necessary to ascertain that the modification of the waveguide which is made is so chosen that no higher order propagation modes are included in the transmissible bandwidth for the aerial. Thus when a dielectric filling is used it may be desirable also to vary the dimensions of the waveguide to exclude undesired higher order propagation modes.

Although moreover, the example of the invention described herein is a non-resonant type of waveguide aerial,

slotted waveguide aerials of the resonant type can also, by virtue of the invention, be made substantially free from buttery lobes. By introduction of a short circuit at the top of the guide a main beam at 1r/2 radians to the axis of the guide is enhanced by the reflected component of microwave energy. This is due to the boundary condition which introduces a phase change of 1r/2 radians at the end of the guide thereby tending to establish a standing wave. A suitable ferrite insulator may be employed in the feeder to the aerial, to reduce the effects of variation in the impedance of the aerial with frequency.

What l claim is:

1. A slotted waveguide aerial comprising a waveguide having an array of slots arranged at least partially in longitudinal manner in a face of said waveguide at least some of said slots being wholly on one side of the centre line of said face and no slots being wholly on the other side of the centre line of said face and the ratio of the wavelength of signals of a given frequency in free space to the wavelength of signals of the same frequency in said waveguide is determined in relation to the longitudinal spacing of said slots to provide a radiation pattern having substantially a single main lobe.

2. A slotted waveguide aerial according to claim 1 wherein said ratio is not less than unity.

3. A slotted waveguide aerial according to claim 2 wherein the waveguide contains solid dielectric material so as to reduce the velocity of propagation of electromagnetic energy within the waveguide.

4. A slotted waveguide aerial according to claim 2 comprising lumped reactive loads within the waveguide to reduce the velocity of propagation of electromagnetic energy within the waveguide.

5. A slotted waveguide aerial according to claim 1 wherein the longitudinal spacing of the slots is less than the wavelength of signals of said given frequency in free space.

6. A slotted waveguide aerial comprising a waveguide having an array of slots arranged at least partially in longitudinal manner in a face of said waveguide, at least some of said slots being wholly on one side of the centre line of said face and no slots being wholly on the other side of the centre line of said face, whereby radiation lobes are produced at angles determined by the equation:

where:

0 represents the angle of a radiation lobe with the direction of a forward going wave in the waveguide in the plane including the axis of the wave guide and the normal to said face,

x represents the wavelength of the electro magnetic wave in free space,

7\g represents the wavelength of the electro magnetic wave in the waveguide,

s represents the longitudinal spacing between the slots and k represents an integer which takes the value of zero or a positive or negative whole number,

said waveguide being constructed to produce a relationship for Mag and Ms giving a realA solution for said equation for only one value of k.

7. A slotted waveguide aerial according to claim 6, said slots being all confined to one side of said centre line.

References Cited in the le of this patent UNITED STATES PATENTS 2,433,368 Johnson et al. Dec. 30, 1947 2,629,052 Iams Feb. 17, 1953 2,761,137 Van Atta et al Aug. 28, 1956 FOREIGN PATENTS 760,388 Great Britain Oct. 31, 1956

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