US2684444A - Pocket antenna - Google Patents

Description

D. FALES lll POCKET ANTENNA July 20, 1954 Filed Aug. 15 1950 FIG. 3

DAVID FALES III INVENTOR Patented July 20, 1954 POCKET ANTENNA David Fales III, Baltimore County, Md., assignor to Bendix Aviation Corporation, Towson, Md., a corporation of Delaware Application August 15, 1950, Serial No. 179,617

11 Claims.

This invention relates to antennas and more particularly to antennas employing a slot radiator.

Due to the recentrapid increase in the speed of aircraft, it has become a matter of great importance in their design to reduce the size of all protuberances from the-surfaces of the craft or to eliminate them entirely, if possible. The solution of this problem with respect to antennas has resulted in several arrangements in which the antenna is reduced in size or incorporated within the body of the aircraft. ihese arrangements have not completely solved the problem such an antenna which may be disposed within or without an aircraft in a manner to consume a minimum of useful space Without loss of radiation efficiency with respect to previously known types.

It is another object of the invention to provide a recessed type of antenna which will strengthen rather than weaken the skin of an aircraft.

It is a still further object of the invention to provide an antenna which may be wrapped around cylindrical or spherical surfaces.

It is still another object the invention to provide an antenna which may at lower frequencies utilize the side of a building, the deck of a ship or other large surfaces as a portion of its radiating structure.

It is yet another object of the invention that lends itself to use for broadcast and very low frequencies, eliminating the necessity for towers of high altitude.

It is a further object of the invention to provide an antenna having high radiation efficiencies at very low frequencies.

These and other objects and advantages of the invention are realized by an antenna construction in which a resonant cavity lies fiat upon a metal surface. The open side of the resonant cavity is excited and forms a slot radiator. Various modifications of this form are available.

Referring now to the drawings:

Fig. .1 .is a perspective view of a slot radiator opening on one side into a resonant cavity;

Fig. 2 is a perspective view of an antenna formed in accordance with the invention;

Fig. 3 is a cross-sectional view of an antenna formed in accordance with the invention;

Fig. 4 is a perspective view-of another form of antenna embodying the invention;

Fig. 5 is a cross-sectional view of the antenna of Fig. 4 taken along the line 5-5 of that figure;

Fig. 6 is a perspective view of an antenna of the type of Fig. 4, showing a feeding arrangement;

Fig. 7 is a perspective view of superimposed antennas of the type of Figs. 4 and 6;

Fig. 8 is a perspective view of an array of superimposed antennas, the upper antenna being offset; and

Figure 9 is a perspective view of an antenna of the general type of the antenna of Figure 4 but utilizing a wafer of solid dielectric material.

There is shown in Fig. 1 a conducting plate or sheet it), having formed therein a rectangular slot i 8 having side edges l2 and end edges [3. It is known in the art to utilize, as an antenna or radiating structure, a conducting plate having a rectangular slot of this configuration formed therein. When a' slotted plate of this'sort is excited radiation will be emitted from both sides of the plate. It is possible to limit the radiation to one side of an infinite sheet by the addition to the radiator of a resonant cavity as shown in Fig. 1. The cavity is shown as formed by a rectangular flattened box havingparallel side walls It and I6 and parallel end walls 15. The end of this structure opposite the slot is likewise closed,this well being invisible in the drawing of Fig. 1. The depth of the box should be a quarter wavelength or an odd multiple thereof as calculated for a wave guide of the same crosssectional dimensions, while the distance between the side edges l3 of the slot should be a half wavelength or multiple thereof as measured in an.

While this type of antenna might be useful for aircraft installations, it nevertheless projects inwardly into the aircraft taking up valuable space and forming an obstruction which is objectionable.

Fig. 2 illustrates an antenna constructed in accordance with the invention which.v retains the radiation efiiciency of the antenna of Fig. 1 Without the objectionable space consuming feature of that configuration; The same slot H is formed in plate '10 as in-Fig. l. The box forming the resonant cavity has, however, in effect been bent downwardly until it now lies against the inner side or" the plate ill. The lower it of the box can be dispensed with, the side walls is being joined directly to the inner surface of the plate la. The wall it has been elongated and curved around with a constant radius of curvature to join the upper ide edge iii of the slot. The wall l6 need not be curved, how ever. This end of the box may be made identical with the opposite end. It can be seen that the antenna of Figs. 2 and 3, by lying flat against the outer surface of the aircraft, could be made to take up very little space within, and it has been discovered that no loss in radiation eniciency occurs by reason of the change in configuration of the resonant cavity from that shown in Fig. 1.

Another form of the invention which is readily adaptable to aircraft use is that shown in Figs. 4 and 5. In these figures the plate it is not provided with an opening. A resonant cavity which may be of similar dimensions to that of the foregoing figures is formed by a box or pocket on the exterior surface of the plate E8. The plate it itself may form one side of the box, taking the place, for example, of the side wall it. The remaining side wall i l, the end walls l5, and the end wall l? are retained. The end opposite the end wall ll is left open and this opening acts as a slot radiator. No alteration in the electrical characteristics, over the previously described; antennas, occurs in the present form if the width of the end walls L5 is small compared to the wavelength. This form of antenna may made very thin, being attached to the surface of the aircraft by screws or rivets. In this manner it can be made virtually dragless, the major part of the drag being caused by the screws or rivets. The antenna may also be formed of paint or similar materials, the box being formed of conductive paint applied over a dielectric film in a manner to contact the surface of the craft on three sides. The film may be a dielectric paint or a wafer of other dielectric material. All forms of the antenna may of course utilize other dielectric materials than air within the pocket.

Figure 4 illustrates an antenna of the type described above in which a wafer of solid dielectric material has replaced the air Within the box of Figure i. The walls it are formed of conductive paint.

Fig. 6 illustrates a method of feeding an antenna of the type shown in Figs. 4 and 5, the feeding being accomplished by means of a coaxial line it, the inner conductor it of which is connected to the open edge of the wall it, while the outer conductor 2% is secured within an opening in the plate id. The plate it? is grounded, thus grounding the three sides l5 and ll of the antenna. The cavity is made resonant at the operating frequency by making the distance from the open end to the end wall ll equal to a quarter wavelength or odd multiple thereof as measured within the cavity and the length of the free edge of the side M equal to a half wavelength as measured in air.

When the distance from the side is to the plate iii is much less than the wavelength, the radiation from this system is largely from currents circulating in the conducting surfaces round the opening and very little from the opening itself. For this reason the pattern of an antenna of this form is the same as that from the slot of Fig. 1, even though the plane of the opening in Fig. 6 is perpendicular to the conducting surface.

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Fig. '7 illustrates an antenna formed by two superimposed pockets of the type shown in Fig. 6. This has the effect of conserving space and of increasing the bandwidth of this system. In addition to being superimposed, as illustrated in Fig. 7, pocket antennas of the types illustrated may be wrapped around cylindrical or spherical surfaces without ill effects. This expedient is particularly useful in the case of low frequency pockets where it may be desirable to wrap the resonant cavity around the fuselage. The band width is increased in the configuration of Fig. 7 by tuning the two pockets to difierent, but adjacent, frequencies. This is accomplished in the system illustrated in Fig. 7 by means of con densers 2i and 22 connected between the walls M and the plate l9. Such condensers may also be used to provide individual spot frequency response from the two cavities of Fig. 7 or to tune a single cavity as illustrated in Figs. 2, 4 and 6.

Fig. 8 illustrates a system in which the two pockets of Fig. '7 are offset from one another in stacking. This has the effect of reducing the Q of the antenna because of the larger area from which radiation takes place. The broad banding effect of Fig. '7 is retained.

Where external cavities of the type shown in Figs. 4, 5, and 6 are utilized for aircraft installations, it is advantageous to have the pocket thin in order to limit the extent to which it disturbs the air stream. In theory the antenna may be made as thin as desired; practically, however, there are limiting features.

An external pocket antenna divides basically into two parts, the resonant cavity and the slot opening. The opennig behaves exactly like other slots and when thin serves only to excite currents in the conducting sheet surrounding it. The resonant cavity is in parallel with this opening and acts as a parallel resonant circuit across it. The impedance of the shunting resonant cavity varies with its thickness, therefore, as the cavity becomes thinner, even though its Q remains the same, the loss conductance paralleling the slot increases and the efficiency of radiation is reduced. A slot of .08 inch thickness has been built with an efficiency of 5 or 6 db below a free space dipole. A thick pocket may have a gain of 3 db over such a dipole.

The impedance of the thin cavity can be increased by making its walls of a wire grid instead of solid conducting sheets. The finer the wire and the coarser the mesh of the grid, the greator the resulting impedance will be. The emciency of a thin pocket can be increased by making the top of the resonant cavity structure of wire screen.

When the mesh of the wire screen is made too coarse, however, there is leakage of energy from the inside of the cavity to the outside through the screen. This is a disadvantage since the operation of the pocket antenna depends upon the slot opening being surrounded on all sides by a solid conducting surface. When this is not true, an increase of radiation resistance at the mouth of the pocket results. An increase of radiation resistance relative to loss resistance decreases the eiiiciency of the antenna. While this is not true of all antennas, it is true of the present antenna because of the fact that the loss resistance is in parallel with the radiation resistance.

The usable bandwidth of any pocket antenna increases with its thickness. The superposition of one pocket upon another, as illustrated in Figs. 7. and 8, each of the pockets being tuned to a slightly different. frequency, will give. a broader pass. band. Bandwiolths of up. to 15% have been obtained by this means without. difficulty. The resonant, frequency of any pocket may of course be determined by selection of dimensions rather than by tuning.

Spot frequencies over a wide range may be obtained by varying a single tuning control at the feed point.

The amount of power that a thin pocket antenna can handle is limited by dielectric break down within the resonant cavity. At the resonant frequency the impedance at the center of the slot is between 500 and 1000 ohms. It is across this impedance that the highest voltage in the antenna develops and it is at this point that break down occurs.

The pocket antenna lends itself very readily to use at low frequencies, including the broadcast frequencies. For some applications at these frequencies it offers striking advantages. One advantage is the elimination of costly towers. At these frequencies the pocket antenna may, for example, be used to excite the decks of ships or the roofs or sides of buildings. An antenna of the form of Fig. 6 may be built up of a grid of wires supported by poles, the poles extending along the sides [5 and I1, being conductive, while those supporting the wire grid forming the side I4 are provided with insulators. By these means costly towers are eliminated.

At very low frequencies eificient radiation is impossible to obtain by the use of standard types of radiators because of the impracticability of elevating the radiating element to a suiiicient height. However, a pocket antenna operates here just as it does at any other frequency and should provide radiation efficiencies far in excess of anything heretofore available for these frequencies.

While in the foregoing description and in the following claims reference is made only to the radiation of energy from the antenna, it should be understood that the invention is equally adaptable to the reception of radiant energy.

What is claimed is:

1. A resonant cavity type antenna comprising a rectangular wafer of dielectric material, two sheets of conductive material each in face to face contacting relationship with a respective one of the major sides of said wafer, said sheets being conductively joined along three edges of said wafer and being separated in a conductive sense by the thickness of said wafer along the remaining edge thereof, one of said sheets extending beyond the boundaries of the other, said conductive material thus constituting a fiat rectangular box having one open end, and means coupling into said box electromagnetic energy of a frequency at which said box constitutes a resonant cavity and the open end thereof constitutes a radiating slot.

2. A radiating system comprisin an extended sheet of conductive material, a conductive body having the shape of a flat rectangular box having two parallel side walls and three end walls, the remaining end of said box being open, one of said side walls being constituted by a portion of said extended sheet, means coupling to the interior of said body electromagnetic energy of a frequency to which said body is resonant, the distance from said open end to the opposite end of said body being equal to an odd integral multiple of a quarterwavelengthv of said frequency as measured within said body and. the distance across said open end parallel to said side walls being equal to an in-- tegral multiple of a half-wavelength of said frequency as measured in freeair.

3. An aircraft radiating system asset forth in claim 2, said extended sheet of conductivev material being constituted by the skin of said aircraft.

4,. Aradiating system, as, set forth in claim 2, said conductive body being filled with a solid dielectric material.

5. An antenna system comprising an extended sheet of conductive material, means forming with a portion of said sheet a first fiat rectangular box of conductive material having two parallel side walls and three end walls, the remaining end of said box being open, said portion of said sheet constituting one of said parallel side walls, means forming with the other of said side walls a second rectangular box of conductive materials having similar proportions to said first box, the said other side of said first box constituting one of the parallel sides of said second box, the open ends of said boxes being similarly orientated, said boxes being resonant at different, but adjacent, frequencies, and means coupling into each of said boxes electromagnetic energy of the frequency to which it is resonant, said open ends of said boxes constituting radiating slots at said frequencies.

6. An antenna system as claimed in claim 5, characterized in that a respective tuning reactor is connected in shunt with the open end of each of said boxes.

7. An antenna system comprising an extended sheet of conductive material, means forming with a portion of said sheet a first fiat rectangular box of conductive material having two parallel side walls and three end walls, the remaining end of said box being open, said portion of said sheet constituting one of said parallel side walls, means forming a second fiat rectangular box of similar proportions to said first box and superimposed on the other side wall of said first box in conductive relation thereto, said second box being laterally offset with respect to said first box, and means coupling into each of said boxes electromagnetic energy at a frequency at which it constitutes a resonant cavity and the open end thereof constitutes a radiating slot.

8. An antenna system as claimed in claim 7, characterized in that said boxes are resonant at different, but adjacent, frequencies.

9. An antenna system as claimed in claim 7, said boxes being resonant at different frequencies.

10. An antenna comprising an extended sheet of conductive material, a rectangular layer of dielectric material applied to and covering a portion of said sheet, a layer of conductive paint applied to and covering the free side and three of the edge surfaces of said layer of dielectric material and extending into contact with said conductive sheet at said three edges of said layer of dielectric material, said layer of conductive paint thus defining with said portion of said sheet a fiat rectangular box having one open end, and means coupling into said box electromagnetic energy of a, frequency at which said box constitutes a resonant cavity and the open end thereof constitutes a radiating slot.

11. An antenna as claimed in claim 10, the distance between said open end of said box and the opposite end thereof being an odd integral multiple of a quarter-wavelength of said frequency of said electromagnetic energy and the length of said open end being an integral multiple of a Number half-Wavelength of said frequency as measured 2,425,488 in free air. 2,488,419 2,508,085 References Cited in the file of this patent 5 2,579,315

UNITED STATES PATENTS Number Name Date Number 2,129,712 Southworth Sept. 13, 1938 502 4 0 2,400,867 Lindenblad May 21, 1946 25,413 2,414,266 Lindenblad Jan. 14, 1947 Name Date Peterson Aug. 12, 1947 Lindenblad Nov. 15, 1949 Alford May 16, 1950 Gurewitsch Dec. 18, 1951 FOREIGN PATENTS Country Date Great Britain Mar. 17, 1939 Great Britain June 28, 1949

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