US2762044A

US2762044A - Slot aerials

Info

Publication number: US2762044A
Application number: US305658A
Authority: US
Inventors: Boys Thomas Rex; Martin John Ernest
Original assignee: Marconi Co Ltd; Standard Telephone and Cables PLC
Current assignee: Marconis Wireless Telegraph Co Ltd; STC PLC; BAE Systems Electronics Ltd
Priority date: 1951-08-21
Filing date: 1952-08-21
Publication date: 1956-09-04
Anticipated expiration: 1973-09-04
Also published as: DE957857C; FR1061569A; BE513628A; GB737873A

Description

Sept. 4, 1956 T. R. BOYS ET AL 2,762,044

SLOT AERIALS Filed Aug. 21, 1952 i ZZ Inventor T. R J.

BOYS- United States Patent SLOT AERIALS Application August 21, 1952, Serial No. 305,658 Claims priority, application Great Britain August 21, 1951 2 Claims. Cl. 343-767) The present invention relates to slot aerials of the type comprising an elongated, hollow conducting body in which a slot is formed, the longitudinal axis of the slot being substantially parallel to the longitudinal axis of the hollow conducting body. The hollow conducting body is usually of circular cross section but it will be understood that the cross section of the conducting body may be of any one of a number of shapes such as square or rectangular. Aerials of the type specified are normally used to radiate horizontally polarised waves, in which case the hollow body is arranged with its longitudinal axis vertical. In one arrangement the electrical length of the slot is approximately half a wavelength long at the operating frequency and more than one such slot may be used, the several slots being arranged in a vertical line. The object in using more than one slot is to increase the power radiated in the horizontal directions and thus improve the aerial directivity. The same object can be achieved within limits by using a single longer slot but this is sometimes inconvenient from a mechanical point of view.

When the conducting body is a cylinder and it is required to produce a substantially circular radiationfpattern in the horizontal plane it is usual to make the diameter of the cylinder equal to a small fraction of a wavelength at the operating frequency. It is sometimes required, however, to provide a horizontal radiation pattern which is other than circular. For example Where two transmitters are operating on the same frequency or on closely spaced frequencies, it may become necessary, to avoid interference, to restrict the power radiated from at least one of the transmitters in the direction of the other without reducing the power radiated in other directions. Furthermore, when the radiated signals occupy a large frequency band as in television, it is necessary to maintain the aforesaid reduction in power in the same proportion over the whole of the requisite frequency band.

In order to obtain a non-uniform horizontal radiation pattern it has been proposed to make the diameter of the cylinder equal to a large fraction of the operating wave length. A disadvantage in using cylinders of large diameter is, however, that they are sometimes mechanically inconvenient and the number of variations obtainable in the radiation pattern is limited to a characteristic type. For example it is not possible to obtain a radiation pattern of approximately cardioid shape which may be required when two transmitters interfere with. one another as previously described. The term cardioid is used to describe the shape of a radiation pattern having a single sharply defined minimum value and not a zero value as is usually understood in the case of, say, a direction finding aerial system.

It has also been proposed to use a cylinder whose diameter is small compared with the wavelength in use and to add flanges to the edges of the slot or slots. This arrangement suffers from the disadvantage that the range of variations obtainable in the radiation pattern is small for flange dimensions which are mechanically convenient and the radiation pattern varies in shape considerably with change in frequency especially in the directions of minimum radiation, which would result in distortion of radiated signals which occupy a wide band of frequencies.

One object of the present invention is to provide an improved slot aerial of the type specified which can be arranged to provide a non-circular radiation pattern whose shape remains substantially unchanged over a substantial band of frequencies.

Another object of the an improved slot aerial of the type specified whose input admittance remains substantially constant over a substantial band of frequencies.

According to the present invention a slot aerial of the type specified, comprises two or more conductors disposed on or near the hollow conducting body, the longitudinal axis of the conductors being disposed substantially at right angles to the longitudinal axis of the conducting body. The number, length and spacing of the conductors from one another and from the hollow conducting body is dependent upon the shape of radiation pattern required. The conductors may be energised solely by incident radiation from the slot or they may be energised directly from a transmitter by way of a transmission line, the same transmitter being used to energise the slot.

The radiation will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic elevation of one embodiment of the invention,

Figure 2 is a cross-section taken at 22 in Figure 1,

Figure 3 is a diagrammatic elevation of a second embodiment of the invention, and

Figure 4 is a cross-section taken at 4-4 in Figure 3.

In Figures 1 and 2 avertical metal cylinder 10 whose diameter is approximately wavelength at the operating frequency has a straight slot 11 formed therein parallel to the axis of the cylinder and about two wave lengths long. A transmitter shown diagrammatically at 12 is concerned by a balanced feeder 13 to the centre of the slot, the two conductors of the feeder being connected to the centres of the two longitudinal edges respectively or" the slot.

Each of four horizontal rods 14 to 17 of insulating material has one of its ends fixed to the cylinder 10 and the other ends of the rods 14 to 17 carry four rods 18 to 21 respectively of conducting material. The axes of the four separating rods 14 to 17 lie in a common vertical plane which includes the axis of the cylinder 10 and the centre'line of the slot 11 and the rods are equally spaced in that vertical plane by half a wavelength at the operating frequency. The supporting rods are fixed to the cylinder at four points respectively along the line on the cylinder where the said vertical plane cuts the cylinder. Each of the supporting rods 14 to 17 is a quarter of a wavelength long at the operating frequency.

The axes of the rods 18 to 21 fixed to the supporting rods 14 to 17 are also horizontal and lie in a common vertical plane which is at right angles to the plane containing the axes of the supporting rods 14 to 17. The junctions between the rods 18 to 21 and the supporting rods 14 to 17 respectively are at the centres of the rods 18 to 21 respectively.

With this. arrangement the shape of the horizontal radiation pattern can be varied substantially by varying the length of the rods 18 to 21. When the length of the rods 18 to 21 is approximately equal to 0.6 of a wavelength the radiation pattern is of the aforesaid cardioid shape and the ratio of the maximum field strength of the present invention is to provide minimum, field strength may be about 4:1. Furthermore the radiation pattern remains of substantially con: stant shape and the input admittance of the aerial is of a form suitable for matching to a transmission line Within close. limits over a substantial band of frequencies for example. i4% of the mid-band frequency;

Referring now to the embodiment shown in Figures 3 and 4,. the cylinder 10, transmitter 12 and feeder 13 are identical with those shown inFigures l and 2.. The rods 14 to 21 of Figures 1 and 2 are, however, replaced by 8 rods of conducting material of which 4 are shown at 22 to 25 in Figure 3 and a fifth is shown at 23 in Figure. 4. The rods in Figures 3 and 4. are arranged in coaxial pairs such as 23, and 23' in Figure 4 and are fixed to and project from opposite sides? of the cylinder. The. aXes f. the rodsv are horizontal and lie in a common vertical plane which contains the. axis of. the cylinder and lies at. right angles to the plane containing the axis of the cylinder and the centre line of the slot. The rods on each Side of, the cylinder are. spaced by half a wavelength and; each rod may have a length of 0.2 of a wavelength. Such an arrangement is suitable for producing a horizontal radiation pattern of the same shape as that produced by a slottedcylinder whose diameter is a large fraction of the operating wavelength. The. inconvenience of having a; large mechanical structure is however avoided.

Furthermore variations of input admittance with change in frequency over a substantial band of frequencies can be made small whereby a substantially flat characteristic such as is required for television transmission can readily be achieved. 7

It will be understood that other dispositions of conducting rods may be used to obtain radiation patterns of other shapes and that the cross-sectional shape of the slotted hollow body may be other than circular. It is of course necessary to arrange the rods with their axes substantially at right angles to the centre line of the slot.

The invention may be used in both transmitting and receiving aerials.

If in the arrangement of Figs. 1 and 2 it is desired to utilize the conducting rods as active rather than as passive elements, they may be coupled to the same feeder as the slot provided that a. suitable impedance matching device is employed between the feeder and the rods. The construction of a suitable impedance matching device will be apparent to those skilled in the art.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What we claim is 1. An aerial comprising a plurality of conductors mounted ona hollow conducting body, including a slot extending along one surface of the body, the conductors being of equal lengths and insulated from the hollowbody, the mid points or the conductors lying along a line which is parallel to the longitudinal axis of the hollow body and is in a plane which includes the longitudinal axis of the hollow conducting body and the centre line of the slot, and is spaced from the hollow body in a direction proceeding from the centre line of the slot through the longitudinal axis of the hollow body, the

axes of the conductors being substantially at right angles 25 to the said plane.

2. A slotaerial according to claim 1, wherein the length of each of the said conductors is approximately 0.6 of a wavelength at the operating frequency, and the spacing from said hollow body is approximately 0.25 of said wavelength.

References Cited. in the. file of this patent UNITED STATES PATENTS 2,234,293 Usselman Mar. 11, 1941 2,510,290. Masters June 6, 1950 2,611,867 I Alford Sept. 23, 1952 OTHER REFERENCES Slot Antennas, by N. E. Lindenblad, Proceedings of the I. R. 13., vol. 35, No. 12, December 1947, pages 1472-3 relied on.