US 2818565 A
Description (OCR text may contain errors)
Dec. 31,1957 .1. s. AJIOKA ETAL 5 SLAB EXCITED CONTINUOUS SLOT ANTENNA Filed Sept. 5. 1956 '3 Sheets-Sheet 1 JAMES 5. AJ/OKA GEORGE M. COLEMAN BY a l ATTORNEYS Fig.3
Dec. 31, 1957 J. 5. AJIOKA ETAL 2,818,565
SLAB EXCITED CONTINUOUS SLOT ANTENNA Filed Sept. 5, 1956 5 Sheets-Sheet 2 INVENTORT JAMES S. AJIOKA GEORGE M. COLEMAN i am Dec. 31, 1957 J. S. AJIOKA ETTAL SLAB EXCITED CONTINUOUS SLOT ANTENNA Filed Sept. 5, 1956 3 Sheets-Sheet 3 RELATIVE POWER ONE WAY (db) Fig. 8
IIOO MC I020 MC 2,818,565 SLAB EXCITED CONTINUOUS SLOT ANTENNA James S. Ajioka, Gardena, and George M. Coleman, San Diego, Calif., assignors to the United States of America as represented by the Secretary of the Navy Application September 5, 1956, Serial No. 608,174 9 Claims. (Cl. 343-7 67) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
This invention relates gnerally to wave guide structure for high frequency electromagnetic waves which are adapted for interchange of energy between a wave guide and surrounding space and more particularly to a slab excited antenna having a continuous slot cut longitudinally in a wave guide or coaxial transmission line. Slot excitation is accomplished by means of slabs or strips of conducting or dielectric material placed on alternate sides of the slot and protruding into the guide or transmission line.
One method of radiating or receiving electromagnetic energy is by means of a series of discrete resonant slots approximately one-half wave length long and separated by one-half wave guide or transmission line wave length. The slots are eitherv excited by thin conducting pins or probes extending into the field of the waveguide, or the slots are placed so that they are excited by interruption of the current flow in the walls of the guide. However, this requires precise machining of slot length and distance between slots, causing the entire antenna to be resonant and operable only in a relatively narrow frequency band. Because the slots are excited by thin pins or probes, field amplitude distribution along the slot with respect to the point of maximum field is substantially fixed and uncontrollable. Another method is to radiate or receive electromagnetic energy through a zigzag shaped slot alternating in position about a center line along the axis of the waveguide. The length of each portion of the slot is approximately one-half the guidewavelength. Precise machining is required in the production of the zigzag or off set slot. The field distribution across the slot is determined by the field within the guide and angle of the zig-zag slot. Because the length of each portion of the slot is dependent upon the wavelength within the guide, this type of antenna must be operated within a relatively narrow frequency band as imposed by the slot radiation pattern and impedance change with frequency.
The slab excited continuous slot antenna comprising the present invention may be made. in a waveguide or transmission line. The slot is located along the neutral transverse current or high electric field zone of the waveguide and parallel to the axis of propagation of energy within the guide. In the case of a coaxial transmission line a continuous longitudinal slot is cut parallel to the axis of the line and cut to a length greater than one-half wavelength in free-space. Such a slot does not appreciably interrupt the current flow in the wall of the line, and hence does not radiate unless excited by some type of probe. Instead of the conventional pin-type probes, this antenna is excited by long slabs or strips (%s to wavelength long) of conducting or dielectric material spaced a half wavelength apart and placed alternately United States PatentO 2,818,565 Patented Dec. 31, 1957 oneach side of .the slot for in-phase excitation. The slab lengths, depths, contour and locations with respect to the slot are adjusted for desired slot field and impedance distribution. The antenna can be centerfed or endfed, short circuited at the end or for such versions where its length is long with respect to a wavelength, the end can be either short circuited or open circuited. Through use of a continuous long slot (relative to a half wavelength) and slabs for excitation, a free selfadjustment of the electric field across and along the slot is realized. Because of this, a stable radiation pattern over a wide frequency range is achieved. Non-exciting slabs placed diametrically or directly opposite the slot can be used for further impedance control. Wings or chokes placed on the sides of the slot will shape the transverse beam and/or reduce back radiation. Because of its ability to handle broader frequency bands than waveguide antennae heretofore used, this antenna is especially useful when used with backward wave oscillators, jamming equipment, traveling wave tubes in which the frequency range is swept by changing the input voltage,
I and experimental equipment requiring the broader band.
An object of the present invention is the provision of a slab excited continuous slot antenna.
Another object is the provision of a continuous slot antenna which is simple to fabricate and which does not have the resonant effects of the half-wave length slots.
Another object is the provision of a continuous slot antenna excited by slabs alternately spaced on each side of the slot wherein their lengths, depths, contour and locations with respect to the slot determine slot field and impedance distribution.
A further object is the provision of a slab excited continuous slot antenna having radiation patterns which are substantially constant with frequency variations.
A still further object is the provision of a continuous slot slab excited antenna having non-exciting slabs placed diametrically opposite the slot for further impedance control.
Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
Fig. 1 shows a rectangular waveguide having a continuous slot and slabs spaced alternately therealong;
Fig. 2 shows a coaxial transmission line wherein the slabs connect the inner and outer conductors;
Fig. 3 shows a coaxial transmission line wherein the slabs are connected only to the inner conductor;
Fig. 4 shows a circular waveguide wherein the slabs are connected thereto adjacent the slot;
Fig. 5 shows the waveguide of Fig. 1 with non-exciting slabs placed opposite the slot;
Fig. 6 is a cross-sectional view taken along line 66 of Fig. 2;
Fig. 7 is a cross-sectional viewtaken along line 7-'7 of Fig. 2 to show the relationship of the slabs to the distribution of the electric fieldalong the line; and.
Fig. 8 is a chart depicting broad band response.
When a waveguide is excited in a suitable manner, electric waves will travel down the waveguide or coaxial transmission line in a uniform transverse distribut-ion pattern. If a longitudinal slot is cut in the outer conductor 11 near its junction with a longitudinal slab 12 as shown in Fig. 6 and Fig. 7, transverse currents of relatively great magnitude will be intercepted and an alternating electric field will be produced across the slot, so that the slot is adapted for interchange of energy between the waveguide and surrounding space. If, at the same longitudinal position in the waveguide, a slot were cut on the other side of the slab or the slab were moved to the other side of the slot, it will be seen that an electric field would be produced oppositely directed to that across the slot. By spacing a slab in Fig. 7 approximate ly one-half wave length (at the medium frequency of the desired band of operation) apart from the slab in Fig. 6, it is in that portion of the electric field 180 out of phase with the first. Thus, by placing this slab on the other side of the slot, it causes the field to radiate from the slot in the same direction as the radiation in Fig. 6 to effect radiation in the same direction along the entire length of the antenna.
Fig. 1 shows a slotted rectangular waveguide 14 operable in the H or TE mode wherein both longitudinal and transverse currents are found. The slabs 12 change the otherwise uniform distribution of the electric field intensity by concentrating the electric field in the neighborhood of the slabs, incidentally causing some transverse current to take place and consequently setting up horizontal electric fields across the slot. The slabs 12 are spaced to provide the desired phase difference, and if a zero phase difference is desired, the spacing between alternately positioned slabs should be one-half of the wave length in the waveguide. Because the slabs have length along the slot, thereby providing for a free selfadjustment of the electric field, the frequency sensitivity of the antenna system is reduced permitting operation over a broad band of frequencies such, for example, 930 to 1100 megacycles as shown in Fig. 8. As shown in Fig. 2, slab 12 extends from the outer conductor 11 to the inner conductor 13 and is spaced a small angular distance from the slot. Putting the slabs on an angularly adjustable bar 16 as in Fig. 3, makes their angular adjustment relative to the slot easier to accomplish. The adjustment of the slabs is for the purpose of adjusting the side lobes on the radiation pattern of the antenna.
The reflections toward the energization source due to the exciting slabs may be cancelled out by matching slabs 17 placed diametrically opposite the slot as shown in Fig. 4- and Fig. 5. These matching slabs will give reflections to cancel those of the exciting slabs. The amplitude of the reflections is controlled by the depth and shape of the matching slabs and the phase is controlled by the longitudinal placement of the matching slabs.
Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
What is claimed is:
l. A continuous slot antenna comprising a waveguide, 21 slot in the waveguide located along the high electric field zone thereof and parallel to the axis of propagation of the energy therewithin, said slot having a wave length greater than one-half the wave length in free space, a plurality of excitation slabs secured to the waveguide alternately on opposite sides of said slot and projecting into the waveguide, said slabs having geometrical configurations and locations with respect to the slot in accordance with the desired slot field and impedance distribution.
2. An antenna as in claim 1 in which a plurality of .4 additional non-exciting slabs are placed opposite the slot for further impedance control.
3. A continuous slot antenna comprising a coaxial transmission line, said line having an elongated slot parallel to the axis thereof, a plurality of excitation slabs secured to the outer conductor of said line alternately on opposite sides of said slot and projecting radially inwardly of the line, said slot having a wave length greater than one-half the wave length of free space, said slabs having a geometrical configuration and location with respect to the slot in accordance with the desired slot field and impedance distribution.
4. An antenna as in claim 3 further characterized in that the slabs connect the outer to the inner conductor of the coaxial line.
5. A broadband frequency continuous slot antenna comprising a waveguide having a slot therein, said slot having a length in excess of one-half wave length in space, exciting slabs spaced along said slot and extending. interiorly of said waveguide, said slabs being alternately positioned on either side of said slot, said slabs having a length parallel to said slot to permit self-adjustment of the operable wave length of said waveguide to accommodate changes in operating frequency.
6. A broadband continuous slot antenna comprising a waveguide having a slot therein of greater than one-half wave length of the middle operating frequency, exciting slabs having a length between one-eighth and one-fourth wave length parallel with said slot, said slabs being spaced in integral multiples of one-half wave length apart and positioned on alternate sides of said slot, said slabs being connected to the inner surface of said waveguide and extending inwardly therefrom, matching slabs having physical dimensions identical with said exciting slabs and positioned diametrically opposite therefrom, internally of said waveguide, said matching slabs being spaced onefourth wave length axially along said waveguide from said exciting slabs to minimize wave reflections toward the energizing source from said exciting slabs.
7. A broadband frequency continuous slot antenna comprising a circular waveguide having a slot axially cut therein, a center rod rotatably adjustable, matching slabs mounted on said rod in integral multiples of the middle frequency of the desired operating frequency range, said slabs being radially spaced consecutively and alternately in axial alignment, said slabs being radially adjustable relative to said slot upon rotation of said rod.
8. A broadband continuous slot antenna comprising a coaxial transmission line, an outer conductor thereof having a longitudinal slot cut therein of a length greater than a half-wave length of the desired. middle operating frequency in free space, exciting slabs spaced longitudinally and radially along said slot and connecting the inner conductor of said transmission line with the inner surface of said outer conductor.
9. A broadband continuous slot antenna as in claim 8 wherein. consecutive slabs are connected to. said inner surface on opposite sides of said slot, alternate of said slabs being in alignment parallel with said slot.
No references cited.