US2615132A - Directive broad band slot antenna system - Google Patents

Description

0a. 21, 1952 v, RUMSEY 2,615,132

DIRECTIVE BROAD BAND SLOT ANTENNA SYSTEM Filed June 5, 1946 3 Sheets-Sheet 1 FROM TRANSMITTER LIA 1s A= IMPEDANCE TRANSFORMER INVENITOR. VICTOR H. RUMSEY ATIQRNEY g BY OCLIZI, 1952 v, RUMSEY 2,615,132

DIRECTIVE BROAD BAND SLOT ANTENNA SYSTEM Filed June s, 1946 s'sheets-sneet 2 INVENTOR.

VICTOR HRUMSEY By E Oct. 21, 19 52 v. H. RUMSEY 2,515,132

DIRECTIVE BROAD BAND SLOT ANTENNA SYSTEM Filed June 5, 1946 3' Sheets-Sheet 3 H6 I20 me 7 III I I06 I Q K L3 w l o I M I 104 H5 H3 ILE'LQ INVENTOR.

VICTOR H. RUMSEY ATTORNEY Patented Oct. 21, 1952 UNITED S'iltld was a d FFECE DIRECTIVE BROAD BAND SLOT ANTENNA SYSTEM Application June 5, 1946, Serial No. 67 L437 This invention relates to antennas; in particular it is directed to providing a directive an tenna array possessing electrical stability and substantially uniform characteristics over a wide band of frequencies.

Antenna arrays normally comprise a plurality of substantially identical primary radiators oriented relative to one another according to a predetermined pattern. The primary radiators are excited by radio frequency energy fed to the individual radiators by a suitable feeder system. In some cases all the primary radiators in an array are excited by currents or fields which are uniform in magnitude and time-phase. Often, on the other hand, array design requires a non-' uniform distribution of energy among the various 4 Claims. (oi. zed-sass) primary radiators, or progressive change of time phase from one primary radiator to another. Conventional primary radiators, assembled in an array, are often difficult to excite according to a predetermined pattern of energy and phase distribution because of complications resulting from mutual electromagnetic coupling between radia tors within the array. An object of this invention is to provide an array wherein mutual electro-magnetic coupling between primary radiators is negligible.

A frequent requirement for antennas to be used in modern high-frequency applications is that the system possess substantially uniform radiation characteristics and input impedance over a relatively broad band of frequencies, percentage wise. It is accordingly another object of this invention to provide an arra which may be made to give substantially uniform performance over L a frequency band which may equal as much as twenty percent of the center frequency for which the system is designed.

A further object of this invention is to provide an antenna operative to radiate electromagnetic fields which are uniformly polarized in a desired plane.

The basic principle of the components of the invention is that an opening in a conducting surface, i. e., a slot, possesses radiating properties closely analogous to those of a dipole, with the important qualification that the spatial orientation of the electric and magnetic fields produced by a slot is at right angles to the orientation of the respective fields produced by a similarly situated dipole. Thus, whereas a horizontal dipole produces a wave wherein the electric field vector is polarized horizontally, a horizontal slot produces a wave having the electric field vector vertically polarized. From this fact it follows that an array for producing vertically polarized.

- mula;

2 i waves concentrated in a narrow beam in the horizontal plane can be made up of a plurality of horizontal slots placed end to end and separated one from another by conducting surfaces. individual radiating slots in such an array are almost perfectly isolated from one another, electromagnetically speaking, and mutual coupling is negligible. An array of dipoles operative to produce a similarly polarized beam in the horizontal plane must comprise a plurality of substantially parallel vertical dipoles. The tendency to mutual coupling between such dipoles is very strong, and shielding one from another is difiicult.

The particular embodiment of this invention which is herein shown and described is an array constructed according to the-slot principle and incorporating certain additional novel features which contribute to more efliective operation. The detailed description of this embodiment-of the invention will be made with reference to theappended drawings, of which:

Figure l is a showing in perspectiveof an illus trative array comprising four slot radiators," oriented end to end; e

Figure 2 is a schematic representation of the: array of Figure 1, together with afeeder sys-n tem, showing how the energy distribution among the component radiators of the array might be, tapered according to some predetermined for- Figure 3 shows in larger scale, in perspective, one o; the slot radiators from the array of Figure 1; an

Figure l, in cross section, shows constructional details of the slot radiator illustrated in Figure 3.

By referring to Figure 1 it may be seen that the array illustrated therein consists of four slot radiators, those to the left of thearray center. being denoted H and I2, and those to the right being denoted 2H and 22 respectively. In this embodiment each slot radiator has an aperture approximately one wavelength long. Figure 1 does not show details of assembly; those are illustrated more fully in Figures 3 and 4. Figure 1' does, however, show the general constructional features. A base plate Mil, a back plate I22, and

a top plate l2! run the entire length of the array and are, in this embodiment, integrally formed from a single piece of conducting material. A plurality of end plates H32, formed of conducting material, serve as partitions to form, incon-- junction with plates llll,.l2l, and M2, a series of open-faced rectangular boxes. Slot radiators ii, 52, 2t, and 22 consist respectively of the open faces of said rectangular boxes. 1 Base plate lill extends a substantial distance The 3 in front of the radiating slots II, I2, 2I, and 22, and is terminated by an extension I04 which is bent sharply backward to form an acute angle with the principal portion of base plate I I. This extension I04 functions as a choke to minimize radiation in the backward direction from the array. Another such choke is formed by an extension I03 on top plate I2I. This extension rises vertically directly above theradiating apertures or slots and is bent sharply backward at.

its extremity as shown in Figure 1. The function of this chokeis also to preventback-radiation.

The slot radiators which make up the array are supplied with radio frequency energy by currents fed to quarter-wave antennas I; oneof which is oriented vertically on the midline of each slot (Fig. 2). Antennas I05 and other'features of the individual slot radiators will be discussedin a subsequent paragraph with reference to Figures 3 and 4.-

Figure 2 shows schematically how the array of Figure 1 might be driven. from asingle; source were itldesired to' supply energy non-uniformly to the. various radiators. The radiators of the array in' Figure l are represented'by boxes having the same designating numerals as the corresponding radiators. Solid lines in Figure-2 represent transmission lines. Figure 2 illustrates'a feeder system appropriate to supply currents in uniform time phaseto all four radiators, with equal currents ofv one magnitude being fed to radiators I I and.2 I; and. equal currents of a different magnitude being fedtoradiators I2 and 22 Transmission-lines'from. radiators H and I2 are joined tothe respective branches of a T -junction II. In the line running to radiator I2 a nimpedance-transformer I3 is inserted; this transformer may be of. the'welll-known quarterwave-section type; its purpose, is tomake the ratio of impedances appearing at the two legs of T-junction I'I appropriate toeffect the desired division of power between radiator II and radiator I2. Transmission lines likewise join radiators 21 and 22 to therespective branches of a 'F-junction. I8, and anv impedance transformer I0, identical totransformer I3, is inserted in theline running to radiator 22.

The stems of T-junctions IT and I8 are connected by transmission lines to the respective branches of a third T-'-junction, designated IS on the drawing. The stem of junction I6 is connect'e'd through animpedance transformer: I5 to a line comin from the radio transmitter (not shown). The lengths of the various segments of transmission line may be proportioned so that the totallengthsof the feed paths from T-junction I6 to each of the radiators are equal. This will insure that the currents fed to the various radiators will have identical time phase. If it"be desiredto feed the radiators with currents Whose phases mutually differ, the lengths of the feed lines to the radiators may be adjusted in appropriate manner to achieve the required phase replifies the design of a feeder to supply currents of arbitrarily chosen phases and magnitudes to the various radiators.

In Figure 3 a single slot radiator is shown in perspective. Observe that the slot oropen radiating area is bisected by a. vertical: strut I01, formed of conducting material. As has been heretofore stated, the radiating slot in this embodimentofthe invention is about one wavelength long, and the radiation emanating from the slot has vertical electric field polarization. The presence, of strut I0'I, joining the base plate I01 and the top plate I2I at the midline of the slot, insures the'presence of an electric field null at the middleof=the slot and thereby compels the, slot to operate in the proper mode, with a half-Wavelength electric field configuration between one end plate and the strut and another such configuration between the strut and the opposite; end plate.v This strut has been found particularly-useful in achievingsubstantial uniformity of directive pattern over a band of operating frequencies.

The antenna I05, which provides electromagnetic excitation of the radiating slot, is located midway between-end plates I02, about a quarterwavelength in front of back plate I22. Antenna I05 is arelatively thick conducting tube about a quarter-wavelength long, mounted on a supporting insulator I05. A suitable coupling for connectingza' feeder to antenna I05 is providedon the underside of base plate IOI, it is shown in Figure 4 and will be described with reference to that drawing.

A pair of small parasitic radiators I00 are mounted: on baseiplate I01, symmetrically with They are situated slightly Figure l'is-a cross-section view of the radiator of Figure 3, the section being taken in the plane passing through the midline of strut I01 and the axis ofantenna: I05. Referring to Figure 4, note that a tubular coupling H3, externally threaded and'having a flange around. the upper portion thereof, is fitted into a circular aperture in base-plate I01 and heldsecurely in place by a circular nut I14. Resting on coupling H3 is insulator I05, which comprises a disk of insulating material having-a raised extension coaxial with the main disk but smaller in diameter. Insulator I06 has a central aperture concentric with the aperture in coupling II3- but substantially smaller in diameter. Insulator I06 is held rigidly in position over coupling II3 by means of machine screws III, by which insulator I06 is bolted to base plate, I01.

Antenna I 0.5, consistsof a lengthof hollow tubing made of conducting. material; its outer diameter-is substantially equal to the diameter of the raised portion I20 of insulator I 05, on which it rests. The inner diameter of antenna I05 is substantially greater, than that of the central aper-- ture in insulator I06. An insulating sleeve I59, having; outer diameterproportional to fit snugly within antenna I05 and'inner diameter equal to that of the aperture in insulator I06, is fitted within, the lowerportion of antenna I05 so as to rest. upon the. upper surface of extension I20.

The length of-sleeve H9 is about one-third the overall length of antenna I05.

i A conducting cylindrical rod I I5, having an enlarged cylindrical head, passes through sleeve l 59 and the aperture in'insulator I136. The enlarged head of rod H5 is proportioned to fit snugly within antenna Hi5 and it is securely affixed thereto by set screws H8. The principal portion of rod H5 has diameter appropriate to cause it to fit snugly within sleeve H9 and the aperture in insulator 5%. The length of rod H5 is such that its lower end extends beyond the insulator I66 to a point within and slightly above the lowest extension of coupling H3. A machine screw H5 passes transversely through raised extension I20 and rod 1 l5, suitable machined apertures being provided therefor. Nut Hl secures screw H6 and thereby holds rod l [5 rigidly in proper position.

Coupling H3, with rod H5 centered therein, affords a -means for coupling an unbalanced coaxial transmission line to the radiator unit. Antenna H05, acting as a quarter wave radiator in conjunction with base plate llll, excites the slot radiator with radiant energy. All the energy radiated by the unit is directed through the slot aperture, since the side, back, top, and bottom plates reflect substantially all the radiation which impinges upon them.

The impedance presented to the transmission line feeding the radiator depends to a large extent upon reflections from the walls and from the slot itself. A match is obtained when all the reflections are so phased as to cancel the existing mismatch between the antenna and the line. Under those conditions the energy in the transmission line is wholly radiated from the slot, and no standing waves are present on the transmission line. The major portion of the energy from antenna I05 reaches the slot aperture after reflecting from the walls of the cavity or from strut I01, only a small fraction of the energy proceeding directly to the slots. This state of affairs would normally result in a system highly sensitive to the frequency changes; in this invention frequency sensitivity due to multiple reflections is prevented by means of the parasites Hi3, heretofore described.

The action of parasites H18 may best be explained by regarding the transmission from antenna 595 to the radiating slot as transmission along a short length of waveguide, consisting of the enclosed cavity formed by the top, bottom, back, and side plates. This waveguide is of course loaded by the slot itself, which serves as a means of effecting energy transfer with outside space. An overall match between the slot, the waveguide, the antenna and the feeder can be achieved for any one frequency by proper choice of the slot dimensions, the antenna dimensions and the distance separating the antenna from the back plate. By adding the parasitic radiators I68, however, a good match can be obtained over a wide band of frequencies because the parasites act as capacitive loading stubs providing shunt reactance across the waveguide. The reactance variation of the parasites as a function of frequency is opposite in sign to that of the system proper and thus tends to maintain an essentially resonant condition over a wide band of frequencies. of the parasites depend upon their location and physical dimensions. In a practical construction it was found that by proper choice of stub shape, size and location, a good impedance match could The characteristics be obtained over a 20% frequency band and that moreover, the input resistance of the antenna I05 could be set at any desired value between and 70 ohms. Thus the parasites l08'not only broaden the usable frequency band but make unnecessary any impedance matching device to couple antenna I05 to its transmission line. The construction just referred to was designed for a center frequency of 1000'mc./s., and the voltage standing wave ratio on the feeder did not exceed 1.3 to 1 at any frequency between 900 mc./s. and 1150 mc./s. i

It will be understood that the embodiment of the invention herein shown and described is exemplary only, and that the scope of the invention is to be determined with reference to the appended claims.

What is claimed is:

l. A slot radiator comprising a conducting cavity substantially in the form of a rectangular box having a top, a base, and one open side forming the slot, a strut of conducting material bisecting the open side and extending between the top and base of the cavity to insure an electric field null at the center of the slot and preserve a uniform directivity pattern over a band of frequencies, an antenna substantially perpendicular to the base disposed within the cavity remote from the open face, feeder means coupled to the antenna, and a pair of capacity loading stubs much shorter than said strut and disposed parallel thereto, said stubs being mounted on the base on either side of the strut but within the cavity enclosure to provide a shunt reactance across the slot whose reactance variation as a function of frequency is opposite in sign to that of the cavity.

2. A slot radiator comprising a conducting cavitysubstantially in the form of a rectangular box having a top, a base, and one open side forming the slot, said open side measuring one wavelength at the center of the operating band in the dimension parallel to the top and the base,

a strut of conducting material bisecting the open side and extending between the top and base of thecavity to insure an electric field null at the center of the slot and preserve a uniform directivity pattern over a band of frequencies, an antenna substantially perpendicular to the base disposed within the cavity remote from the open face, feeder means coupled to the antenna, and a pair of capacity loading stubs much shorter than said strut and disposed parallel thereto, said stubs being mounted on the base on either side of the strut but within the cavity enclosure to provide a shunt reactance across the slot whose reactive variation as a function of frequency is opposite in sign to that of the cavity. a 3. A slot radiator comprising a conducting cavity substantially in the form of a rectangular box having a top, a base, and one open side forming the slot, said open side measuring one wavelength at the center of the operating band in the dimension parallel to the top and base of the cavity, a strut of conducting material bisecting the open side and extending between the top and base of the cavity to insure an electric field null at the center of the slot and preserve a uniform directivity pattern over a band of frequencies, an antenna substantially a' quarter wavelength at the center of the operating band mounted perpendicular to the base and disposed within the cavity remote from the open face, feeder means coupled.

to the antenna, and a pair of capacity loading stubs much shorter than said strut and disposed 7 parallel theretmzsaid stubs being mounted on the base on either :side of the strut but within the cavity enclosure to provide .a :shunt reactance across the slot whose reactance variation as a function .of rfrequency is opposite in sign to that ofthe cavity.

'4. An antenna array having a plurality of slot radiators formed from rectangular box-shaped conducting cavities disposed side by side comprising, three large conducting plates forming the top, the base, and the back respectively of all of said cavities, a plurality of-smaller conducting plates, forming side walls for said cavities, said smaller plates dividi'ngadj acent cavities and with the top and base plates defining open front-faces forming the slot for each cavity, each of said slots measuring one wavelength between the :side walls at the center of the operating band, a strut of conducting material bisecting the open face of each of said cavities and extending between its top and its base to insure an electric field null at the center of the slot and preserve a uniform directivity pattern over a band of frequencies, an antenna a quarter Wave in length at the center of the operating band vertically mounted on the base-of each of said cavities and disposed within its respective cavity remote from the open face, feeder means coupled to 'each of said antennas, and a pair of capacity loading stubs for each of said cavities, said stubs being much shorter than their respective struts and disposediparallel there- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,275,646 Peterson Mar. 10, 1942 2,283,935 King May 26, 1942 2,286,839 Schelkunoff June 16, 1942 2,400,867 Lindenblad May 21, 1946 2,408,435 Mason Oct. 1, 1946 2,414,266 Lindenblad Jan. 14, 1947 2,415,807 Barrow et a1 Feb. 18, 1947 2,516,921 Salinger Aug, 1, 1950 FOREIGN PATENTS Number Country Date 69,461 Norway July 23, .1945 696,986 Germany Oct. 3, 1940 OTHER REFERENCES Rectangular Hollow-Pipe Radiators by Barrow and Greene; Proceedings of the Institute of Radio Engineers, vol. 26, No. 12, December 1938.

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