US2942263A - Antennas - Google Patents

Description

June 21, 1960 D. BALDWIN ANTENNAS 2 Sheets-Sheet 1 Filed Feb. 25, 1957 FIG. 2

FIG. 3

FIG. I PRIOR ART FIG. 4

V/A/ V/ /////////////////a INVENTOR LEROY D. BALDWIN lob I; I I I I' BY (Z1 ATTORNEY FIG.6

June 21, 1960 Filed Feb.

VOLTAGE STANDING WAVE RATIO L. D. BALDWIN ANTENNAS 25, 1957 2 Sheets-Sheet 2 H PLANE RADIATION PATERN OF QUARTER WAVE SLOT ANTENNA DECIBELS PROBE DETAIL INVENTOR. FREQUENCY (MC) LEROY o. BALDWIN FIG. 8

A AT RNEY United States, Patent 4 ANTENNAS Leroy D. Baldwin, Rochester, N.Y., assignor to General Dynamics Corporation, Rochester, N.Y., a corporation of Delaware Filed Feb. 25, 1957, Ser. No. 642,039

3 Claims. (Cl. 343-767) This invention relates to slot antennas and is particularly directed to means for shortening the length of the slot without loss of radiating efiiciency and, at the same time, improving the directional characteristics of the antenna.

The slot antenna of the type treated here may be mathematically analyzed by an elongated opening or slot in a plateof extended area and good electrical conductivity, as shown in Figure lof the accompanying drawing. The slot is analogous to the arms of a'dipole antenna, each arm of which is one-quarter ofthe operating wave length. This requires the dipole or the slot to be at least one-half wave length long. By impressing the high frequency voltage on opposite sides of the slot midway between its ends, maximum impedance can be presented to the transmission line feeding the energy to be radiated. Since the high frequency current disperses into the ground plate, the mean electrical path between the points to which the energy isapplied becomes reasonably long and radiation reasonably efiicient. Heretofore, it has not been possible to construct the slot less than the-quarter wave length on either side of the transmission line else the line becomes short-circuited and the antenna fails.

For various electrical and mechanical reasons it is desirable "that slot 'lengthsless thanone-half wave length be employed. For exampleflthe size of the ground plate containing the slot may be limited in size for mechanical reasons and the operating frequency may be low, 'thus making it unfeasible to use the half wave length slot. In cases where broad bands of frequencies. are to be radiated or received, the quarter wave dimension of the slot must be taken for the lowest frequency of the band.

The object of this invention is' to provide? an improved slot antenna less than one-half wave length long yet having eflicient radiating characteristics and good directionability over a broad band of frequencies.

The object of this invention is attained by forming a slot in a ground plate of extended area inward from the edge of the plate a distance corresponding to about onequarter of a wave length at the lowest operating frequency. Means is provided for energizing the slot near its open end. A housing may be placed over the slot to prevent its radiation from one plane of the slot, and means is provided for establishing a high impedance between the sides of the slot adjacent said open end and for obviating short-circuiting effects of the housing, said means comprising resonant stubs on either side of the slot within the housing to present a high impedance load to the slot so that high frequency energy may be transferred efiiciently to the open end of the slot.

Other objects and features of this invention will become apparent to those skilled in the art by referring to the specific embodiments of the invention described in the following specification and shown in the accompanying drawing in which:

Figure 1 is a plan view of a conventional slot antenna of the prior art,

Figure 2 is a plan view of a slot antenna embodying this invention,

2,942,263 Patented June 21, 1960 one slot antenna'emvention,

Figure 7 is an enlarged detail sectional taken on line 7--7 of Figure 6,

Figure 8 is a graph of the standing wave ratio of a broad band antenna of this invention, and Figure 9 is a polar diagram of the field strength quarter wave antenna of this invention. According to this invention, the ground plate 1, Figure 2, is slotted inwardly as at 2 from one edge of the ground plate, the length of the slot being about one-quarter of a wave length in extent as contrasted to the one-half wave length for slots of the prior art; such as shown in Figure 1. The transmission line 3 maybe thus coupled directly to opposite sides of the slot adjacent the edge of the plate. The transmission line may comprise the two-wire line of Figure 2, or the coaxial line for Figure 3, or the special transmission line of Figures 4, 5, and 6. Further,

of the the two sides of the line either may be attached directly to opposite sides of the slot as in Figure 2, or coupled to the space therebetween by an irradiating probe in the slot as suggested at 5 in Figure 3. it has been found that such a slot will radiate perpendicularly to either side to the ground plate 1, and that negligible radiation occurs from the end of the slot in the plane of the ground plate 1.

A conventional, cup-shaped housing cannot be .placed over the slot to form a cavity and to confine radiation to one side of the ground plate because the housing would short-circuit the transmission line and reduce the impedance of the open end of the slot to zero. According to an important feature of this invention, the housing 10' is placed over the slot and is attached to the ground plate 1, but the housing is open-ended at the openend of I the'slot and the side panels of the housing are spaced outwardly from the side edges of the slot. Conveniently, 'three flanges 10a, 10b, 100 are formed on the edges of the housing and'are soldered, brazed, or otherwise electrically and mechanically attached to the ground plate. Preferably, the inside length of'the housing 'is the same as the length of the slot. Such a housingwill prevent radiation from the slot toward the rear of theground plate, but, as thus far described, will cause strong radiation from the end of the slot in the plane of the ground plate. I

are placed along either side of the slot and are extended into the housing a distance which is a significant fraction of a' 'wave length. That is, the foldedcav'ities formed by the housing and flanges are so proportioned as to produce an impedance at the slot, which impedance is matched to the transmission line impedance. Ideally, electric waves created between the edges of the slot by the transmission line will see two stubs or cavities exactly onequarter wave length long when measured from this slot toward the bottom wall of the housing, around the outer edges of the flanges 11 and 12, andrhence to the shortcircuited end of the stub between the flanges and housing at the backof the ground plate 1. This quarter wave dimension must, however, be compromised to minimize radiation from the ends of the cavities at the open'end of the housing. Extensive experimentation with the cavities of the type shown in Figure 3 shows that a good impedance match and minimum end radiation can be achieved for broad band operation when the mean electrical length of the stubs are each less than one-quarter of the longest wave of the operating band. Hence, the flanges 11 and 3 12 extend from-the-edges 'ofthe slot into the housing a distance-corresponding to, or less than, one-eighth of the longest wave of the operating band.

Figures 4, 5 and 6 show the top, edge, and bottom views,-respectively,' of one particularly effective antenna of this invention. In this embodiment, the transmission line '-is printedon one side of a thin sheet 19 of a high grade insulating material.- The printed transmission line comprises the printed strip terminating at one end in a radiating probe 15a of measured area. 'To the other side of the insulating sheet is attached a thin foil 17 of extended area: compared to'the thickness of the insulating sheet and to the width-of the printed -transmission line 15. 50 called microstrip" laminates are commercially obtainable, comprising a thin sheet of Tefion-impregnated'fiberglass board-clad on both sides with copper foil. Insconstructing the-antenna of this invention, the foil on one side is coated with a photo-resist and is selectively irradiated by a strong light through a ncgative of the pat tern ofthe printed circuitry desired. In this case, the negative is prepared to show the exact outline of the transmission line 15 and its probe end 15a. Then the 4 Figure 9, the principal lobe of the antenna pattern being outward from one side of the quarter wave slot with little radiation from the rear or open end of the housing.

Many mechanical modifications may be made in the specific structure shown without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

l. A slot-type antenna comprising a ground plate, the plate having a window therethrough adjacent one edge of the plate, said window extending inward from said edge a distance approximately one-quarter of a wave length at the lowest operating frequency of the antenna, means for activating said window with energy of said operating frequency, a housing attached -to-one side' of said plate and enclosing said window to confine radiation from the other side of said plate, said housing being open at the end adjacent said edge to prevent short-circuiting said window at said edge, and means to prevent radiation from the open end of said housing comprising flanges along the side edges of said window and extending into the cavity of said housing toform short-circuited stubs along either side of the window.

unwanted portionof the foil is selectively removed in an etchingsolution. The window 16 in the ground plate 17, .on the other side of the sheet,.as.best shown Figure 7, is removed also by the etching :technique if desired, :the removed portion being at'least as large as the desired slot. The window .16 may be :formed :entirely within the boundaries of the ground plate, as shown, or along one edge thereof. 1 he window is formed .to underlie, the antenna-probe 15a and .is formed to the exact size and shape of .the desired slot. Since the sandwich comprising the insulating sheet, ground plate, transmission line, and probe are mechanically bonded together and are .incapable of relative movement, the precise position ofithe window slot with respect to the probe .is easily established and reproducible in manufacture. It has been found-desirabe for good impedance matching and for efiicientradiation to extend the area of the probe, as shown :atlSa in Figure 4. vThe printed circuit board is then bolted or otherwise fastened to the metal chassis 18 in which the slot 16a has been cut. The edges of the window 16 of the printed circuit board are made to register with the edges of the slot 16a in the chassis. The metal angles 11 and 12 are soldered, brazed, or otherwise affixed along the edges of window 16a, and the housing 10 conveniently formed from sheet metal and with flanges 10a. 10b and 10c, is placed over the angles as shown and fastened to the chassis. The depth and width of the housing 10 is'selected -to provide the stub cavities around the angles 11 and .12. The open end .ofthehou'sing .ispIaced flush with the edge .of the ground plate. The details of the slot structure are exaggerated .in Figure 7 to better show the arrangement of parts.

Figure 8 is adiagram of the voltage standing wave ratio of'the quarter wave slot antenna of Figures 4, 5 and 6 for variousfrequencies, when the probe and .slothas the specific dimensions shown in Figure 8. .Alteration of any of the dimensions will, of ocurse, alter different small portions of the voltage wave pattern but will not materially change the broad outline of the wave pattern. Where the dimensions of the probe end of the printed transmission line are of the specific values shown in .Figure 8, it is seen that the antenna has a remarkable broad band characteristic, the response being substantially uniform from 1800 to 3600 megacycles. The H plane radiation pattern of the quarter wave slot antenna for frequencies of the order of 2000 megacycles is shown in 2. In a unitary slot antenna assembly, aground plate of extended surface area, a'slotin said plate, .a transmission line for energizing said slot at an operating 'frequency, means for matching the impedance of said slot at said frequency :to the impedance of said transmission line comprising a rectangular-type housing affixed to said plate along lines "parallel to and spaced from the side edges of .said slot, .flangesalong and coextensive with the side edges of said .slot and extending into said housing to form folded cavities, said cavities being open at .the :slot end andshort-circuited at the other end by said plate.

3. In combination in an antenna structure, a sheet of insulating materialzof good insulating properties at microwave frequencies, a strip transmission 'lineaffixed to one surface of said sheet, a ground plate of extended area against the othersurface of ,said sheet, an elongated slot in said plate, said transmission line terminatingopposite said slot, said slot being about one-quarter wave length long and being cfiectively open circuited atone end for the operating frequency,- fianges. along the side edges of said slot, and a housing attached to said ground plate and enclosing" said slot and flanges, said flanges having such width and said housing being so spaced from said flanges as to form elongated impedance matching stubs along opposite edges of said-slot;

ReferenceslCited in the file-of this patent UNITED STATES PATENTS OTHER REFERENCES Journal of Institution of ElectricalEngineers, Bailey: Slot Feeders and Slot Aerials," vol. 93, part .IIIA, No.

4, May 1946, page 617.

v The proceedingsof the Institution of Electrical En ineers (British), vol. 97, ,part llL No. 5.0, November 1950, pp. 414-417, page 415.

Images