US2990547A - Antenna structure - Google Patents

Description

June 27, 1961 c o ANTENNA STRUCTURE 2 Sheets-Sheet 1 Filed July 28, 1959 INVENTOR. v James R M flouya/ BY .5: @MM GM/MK June 27, 1961 J. R. M DOUGAL 2,990,547

ANTENNA STRUCTURE Filed July 28, 1959 2 Sheets-Sheet 2 Q a, 26 30 9 7 Mk 9/ =fl f t t in INVENTOR.

v 2- v 3= C 1, 11 $5 James A? M flue/ya 2y 8 M MW United States Patent This invention relates to antennas for electromagnetic wave propagation and more particularly concerns a dual dipole flush-mounted antenna structure particularly suited forandUHF operation. The invention is herein illu'strat ivelydescribed by reference to thepresently preferred form; however, it will be recognized that certain modifications and changes therein withrespect to details may be made without departing from'the underlying essentials.

While the invention is especially suited for aircraft applications requiring. flush-mounted antennas, it has a wide variety of other-applications as well. One of its "special characteristics is the capacityto operate either with .dualpolarization or with singular polarization and as a directive radiator in either mode. Another object visito provide such an antenna which may be flush-mounted, which will have. a relatively wide -band width and which,

for its degree of gain or directionality, may be made I comparatively small and light in weight.

3 A further object is to provide a simple antenna of these characteristics which is relatively inexpensive to manufacture and islent to mass production including production by printed circuit techniques if desired.

' Still another object is a flush-mounted antenna suitable 7 for high speed aircraft skin installations which may be ,relatively shallow in its depth perpendicular to the plane of the skinand which may be made impervious to humidity, temperature, altitude (i.e. atmospheric pressure), sand, -dust, etc. In order to achieve the objective of a :shallow flush mounted construction, the novel antenna structure is readily adapted to be dielectrically loaded in .an artificial manner which adds inappreciable weight to the structure,.- this purpose being achievable by artificial dielectric loadingusingmetal elements in a relatively .simple arrangement.

Still another object is ;a relatively versatileantenna which may be energized at any of different feed points as a convenientmeans to-adjust inputimpedance thereof. By simple switching arrangements the versatility of vthe antenna as an alternatively dually or, singularly polar- .ized device is realized with no structural modifications orja djustments of the antenna itself for either mode of operation.

One feature of the invention resides in the antenna Isjtructure combining a conductive radiation surface havingan elongated slot therein with a zig-zag' conductive elementrnounted in the slot and presenting a broadside radiation face in substantially coplanar relation to the merit-may be energized to produce singular polarization of iface of theconductive radiation surface, Either or both v .the conductive radiation surface and the conductive eleminimum transverse width between its ends and'in- .creases in width exponentially toward both ends, the slot preferably having a similar configuration of somewhat Ilargenoutline; Because of its shape, this antenna has {been referred to as a bow-tie dual dipole antenna. The

ing-from the point of minimum transverse width toward successively adjacent and interconnected transverse mem-. bers of the,zigzag element should be designed with I logarithmically increasing thickness and spacing progress ice As a further feature, the slot. is backed by a reflective cavity less than a quarter wave length in depth, the same being minimized in depth preferably by the use of artificial dielectric loading achieved by metallic posts projecting from the zig-zag element, towards the base of the cavity but not electrical contact with such base.

These and other features, objects and advantages of the invention will become more fully evident from. the following description thereof based on the accompanying drawings. 7

FIGURE 1 is a front perspective view of the antenna.

FIGURE Z is a front perspective view of the zig-zag element with metal posts for artificial dielectric loading. FIGURE 3 is a front perspective view of the cavitybacked slotted conductive radiation surface.

FIGURE: 4 is a transverse sectional view taken 0 line 4-'4 in FIGURE 1.

FIGURE 5 is a view similar to FIGURE 4, illustrating a modified construction.

FIGURE 6 is a front perspective view illustrating one mode of antenna energization for dually polarized operation'.

FIGURE 7 is a front perspective view of the antenna with a different mode of energization for dually polarized operation.

FIGURE 8 is a diagram illustrating the design theory applicable to the zig-zag element.

FIGURE 9 is a transverse sectional view indicating printed circuit or equivalent technique for forming the zigzag element and the conductive radiation surface slot configuration.

Referring to the first embodiment shown in FIGURES 1 to 4, inclusive, the conductive radiation surface 10 is formed by means which may, in a practical case, com- 'prise a rectangular metal sheet or block or may actually comprise the skin of an aircraft or other conductive support. A slot 12 of elongated proportions is formed in the radiation surface 10 and an elongated zig-zag ele- -rnent' 14 is mounted in the slot and presents a broadside radiation face which is substantially in coplanar relation to the surface 10. This zig-zag element 14 comprises a plurality of transversely. extending members 14a which transverse width intermediate its ends (i.e. preferably midway) and increases in width exponentially toward those ends as shown. Moreover, the thickness of and spacing between the successively adjacent transverse members 14a should be minimum substantially where the transverse width of the element is minimum and should increase logarithmically toward opposite ends of the zigzag element. While the spacing between the peripheral outline of the zigzag element and the edges of the slot is not critical, this spacing should be quite small at the ends in order to provide a high capacitive loading on the zig-zag element. The slot preferably has the same exponential taper as the element, although this is not strictly compulsory.

For broadside radiation in one direction, the slot 12 should, in the preferred construction, be backed by a conductive cavity 16 comprising the rear wall 16a and the peripheral side walls 16b which extend around the slot edge. This cavity should have a depth less than a 0 quarter wave length, which in physical dimensions may be minimized by filling the cavity with a dielectric material 18 as shown in FIGURE 5 or, preferably (because of the lesser weight involved), by using an artificial dielectric loading technique involving metal rods or posts 20 mounted on certain of the transverse members 14a at intervalsnalong the longitudinal axis of the zig-zag ele'r'nent'14; These electrically conductive members 20 project-from the backside of the zig-zag element toward th'ecavity surface 16a, but do not contact such surface. If de'sired these posts may be supported by stand-oh insulators or dielectric footings 22 secured to the cavity surface 16a and may actually serve as a means to physically support the zig-zag element centered in the slot.

Preferably, the slot is sealed and the zig-zag element is supported by and embedded in a dielectric panel or sheet-24 which closes the slot. Such an arrangement presents minimum-weight and also minimizes the cavity depth so that the antenna structure may be llush mounted in= 'an-' aircraft skin, for example, and will .not project far into the interior of the aircraft. The solid dielectric filling technique shown in FIGURE 5 adds weight but is otherwise satisfactory. In any case, the dielectric rnaterial used should be a low-loss dielectric.

The overall length of the zigzag element should be equal to or slightly less than one-half wave length at the'ceriter frequency of the operating band to which it is applied. Various modes of energization for such an antenna structure may be employed. In FIGURE 6 both theslot, or'more correctly the surrounding conductive radiation surface in which the slot is formed, and the zigzag element itself are separately energized in order to form a dual dipole arrangement with dual polarization. Theslotted radiation surface is connected to be energized by a coaxial transmission line 26, one conductor of which is connected to the surface at a point 26a and the other conductor of which is connected to the same at a point 26b, such points being located at corresponding positions alongthe length of the slot at opposite sides thereof. Nominally these locations are at the longitudinal midpoint of the slot, but as a matter of practice their position may be shifted along the length of the slot in order to obtain different input impedances suitable for matching the energy source used in different cases. Likewise,

the zigzag element in this example is energized from a coaxial transmission line 28, one conductor of which is connected to the end transverse member of the zig-zag element at point 28a and the other conductor of which is -connected to the adjacent point 28b of the slot edge, nominally midway between the transverse side of the slot. However, as in the case of the slot itself, impedance match may be achieved by adjusting the position of these connecting points along the end of the structure, i.e. perpendicular to its length. Suitable switching means .30 may be interposed in thetransmission lines 26 and 28, between the antenna structure and the energy source (not shown) in order to connect either or both transmission lines to the respective antenna elements at a given time.

.In this way it is possible to produce a dually polarized radiation pattern or a singularly polarized radiation pattern withone direction of polarization or the other.

In the modification shown in FIGURE 7 the zig-zag element 14' is formed in two parts separated at the longitudinal midpoint (i.e. 14s). The transmission line 28' in this instance has one conductor connected to the inner end of one-half of the Zig-zag element at point 28a and its other conductor connected to the adjacent inner end of the other half of the zig-zag element at point 28'b. In this case the transmission line 26' is connected to the slotted reflective surface in the same manner as in FIG- URE 6.

FIGURE 9 illustrates the application of printed circuit techniques to forming the antenna cavity and the zig-zag element. The zig-zag element in this case is designated 140 and the slotted conductive surface 100.

, Turningnow to FIGURE 8, a portion of the zig-zag element 14 is" shown, particularly an end :portion, to

illustrate the theoretical design considerations for optimum performance of the antenna element as a dipole device having the characteristics described. The thickness of the successive transversely extending members 14a and the intervening slots 14b is designated 1. with appropriate subscripts to indicate serial relationship. These slots represent dielectric surfaces which intervene between the conductive surfaces represented by the members 14a. The distance from the longitudinal center axis xx transversely to the outside extremities of the slots or gaps formed between successively adjacent elements and also the corresponding outer extremities of the interconnecting portions of conductor which join'together successively adjacent elements on either side of an individual slot are designated y with appropriate subscripts and superscripts to designate serial relationship, as shown in the view. The dimensions t t t t are related tothe ordinate by the relationship:

where n denotes the numerical order of the surfaces (i.e. conductive or dielectric) as counted from the origin of thecoordinate system. The interval t should decrease in logarithmic fashion progressing from the origin (where the origin is at the end of the zigzag element) as may bedemonstrated by solving. the above equation for x as follows:

1 A-B 1 A-B x Log e( tn or x= n3Log e( in The constants A, B, a, the dielectric constant of the material used 'in the sheet 24, the number of intervals t the type of feed selected and the size and shape of the reflecting cavity are all design parameters which in any particular instance will affect the impedance, pattern, power handling capacity and other characteristics of the antenna. The complete zig-zag configuration is obtained by detemiim'ng the design parameters for onehalf of the element, duplicating the curve thus obtained, for the other half, and connecting the two halves together in the center as shown in FIGURES 1 and 2 or with a 'gap between the two halves as indicated in FIGURE 7.

The y dimensions are defined by the exponential functions indicated in the diagram in FIGURE 8, from which the bow-tie designation is obviously derived.

It has been found that the radiation pattern of such an antenna is essentially independent of frequency over as much as a fifty percent band width variation and that the impedance is such as to be inherently compensated over a thirty percent band width variation, depending upon the standards used, when the backing cavity 16 is approximately 0.08.wave length deep, measured at the center frequency of the hand. If the reflecting cavity is omitted, the resulting radiation pattern is, of course, different. It is found as a matter of interest, that the slot 12 should be cut only slightly longer and wider than the zig-zag element and, as previously mentioned, may be although is not necessarily of the same exponential taper. This is another design parameter which is subjectto some variation, with consequent eflfect on the impedance characteristics and radiation characteristics of the device. As previously stated, the gap between the slotand the-zig-zag element should be quite narrow at the ends in order to provide capacitive loading to the zig-zag element suitable for proper operation thereof as a dipole.

It should be noted that present-day theories applicable to antenna devices, specifically the log periodic theory, do not hold in this case since the dimensions of this antenna structure are such that the interval t in the mid region especially, is so small as to appear almost negligible when compared with wave lengths even at the highest operating frequency of the antenna. A more applicable analysis is obtained by comparison with an array of infinitesimally thin; connected V-shapedelements with an assumed sinusoidal current distribution and with mode of operation herein disclosed ,S uch theory proves to ,be. true in practice, indicating an extended range of "usefulness for the antenna.

It should beiffnoted that the optimum operation for broadside radiation [from the zig-zag element requires that the currents be essentiallyin phase a'longits length. condition is met in practice by capacitivelygloading the zig-zag element by use of relat" ly narrow gaps between itscnd' members and'the a 'd i 1 tli number of transverse members 1411 is increased, to the point where it is no longer possible to maintain the inphase relationship of the currents. It is found that the isotropic pattern directivity or gain of the zig-zag element is approximately 8 decibels when the antenna has optimum design and a cavity depth of approximately 0.08 wave length.

It should also be noted that the zig-zag element may be operated independently in space (i.e., without a surrounding slotted radiation surface), particularly when such element is backed by a reflective cavity and when it is connected for energization in shunt with such a cavity. Under these conditions the zig-zag element has a very workable input impedance.

It is recognized that slot antennas as such are well known in the art.

These and other aspects of the invention will be evident to those skilled in the art based on the foregoing disclosure of the preferred form and mode of operation of the invention.

I claim as my invention:

1. Electromagnetic antenna structure comprising two radiating elements, one a means presenting a conductive radiation surface having an elongated slot therein, and the other a zig-zag conductive element mounted in said slot and presenting a broadside radiation face in substantially coplanar relation to said surface, said zig-zag element having a plurality of members extending transversely to the legnth of said slot and successively interconnected at alternately opposite ends to define an outline configuration contained within the outline of said slot and all portions of which. are marginally spaced inwardly from the edge of said slot on all sides, and energy transmission means electrically connected to at least one of said elements to energize said antenna.

2. The anenna structure defined in claim 1, wherein the transverse width of the zig-zag conductive element is minimum intermediate its ends and increases exponentially to its respective ends.

3. The antenna structure defined in claim 2, wherein the thickness of and spacing between the successively adjacent transverse members of the zig-zag element is minimum substantially Where the transverse width of the element is minimum, and increases logarithmically toward opposite ends of said element.

4. The antenna structure defined in claim 3, wherein the spacing between the slot and the zig-za-g element at the respective ends thereof is relatively small and is materially less than the spacing therebetween at the sides, whereby the zigzag element has a high capacitive loading producing a broadside radiation pattern.

5. The antenna structure defined in claim 4, and means defining a shallow conductive cavity behind the slot and substantially enclosed on all sides extending around the slot and on the back side extending generally parallel to the slot, said cavity having less than a quarter-wave length depth, transverse to the general plane of the slot.

6. The antenna structure defined in claim 5, further comprising means electrically loading the antenna including conductive rod-like members projecting from selected transverse members of the zigzag element toward the back; side of the cavity but terminating shortfof ,making electrical contact therewith, therebyto efiect artificial dielectric loading of said cavity.

defined in claim .3, wherein '7. The antenna structure the zig-zag element is formed in two partsseparated from each other substantially midway between the ends,

each part being electrically connected with opposing polarity to the energy transmission means. I? 1 8. The antenna structure defined in claim 3, and refiector means comprising a shallow conductivesurface mounted behind theslot at less than aquarter-wavelength spacing. r

The antenna structure defined in claim 18,;f urther comprising means electrically loading the antenna including conductive rod-like members projecting from se lected transverse members of the zig-zag element toward the reflector means surface but terminating short of making electrical contact therewith, thereby to effect artificial dielectric loading of said cavity.

10. The antenna structure defined in claim 3, wherein the zig-zag element is energized by the energy transmission means connected to one end thereof and to an adjacent location on the conductive surface element.

11. The antenna structure defined in claim 10, wherein the slotted conductive surface element is also energized by the energy transmission means connected to the same at corresponding locations along the longitudinally extending slot edges, thereby to dually polarize the antenna structure.

12. The antenna structure defined in claim 3, wherein the slotted conductive surface element is energized by the energy transmission means connected to the same at corresponding locations along the longitudinally extending slot edges.

13. Electromagnetic antenna structure comprising two radiating elements, one a means presenting a conductive radiation surface having an elongated slot therein, and the other a zig-zag conductive element mounted in said slot and presenting a broadside radiation face in substantially coplanar relation to said surface, said zig-zag element having a plurality of members extending transversely to the length of said slot and successively interconnected at alternately opposite ends to define an outline configuration contained wholly within the outline of said slot and all portions of which are marginally spaced inwardly from the edge of said slot on all sides, and energy transmission means electrically connected to both of said elements to energize said antenna for dual polarization.

14. The antenna structure defined in claim 13, wherein the zigzag element is formed in two parts separated from each other substantially midway between the ends, each part being electrically connected with opposing polarity to the energy transmission means, and wherein the slotted conductive surface element is also energized by the energy transmission means connected to the same at corresponding locations along the longitudinally extending slot edges, thereby to dually polarize the antenna structure.

15. Electromagnetic antenna structure comprising two radiating elements, one a means presenting a conductive radiation surface having an elongated slot therein, and the other a relatively thin sheet-like zig-zag conductive element positioned in said slot and presenting a broadside radiation face in substantially coplanar relation to said surface, said zig-zag element having a plurality of members extending transversely to the length of said slot and successively interconnected at alternately opposite ends to define an outline configuration contained wholly within the outline of said slot and all portions of which are marginally spaced inwardly from the edges of said slot on all sides, said slot being occupied by a body of dielectric material across its breadth and width supporting said zig-zag element so positioned therein, and energy transmission means electrically connected to at least one of said elements to energize said antenna.

16. Electromagnetic antenna structure comprising an elongated zig-zag conductive element having a'iplurality of members extending transversely of the length of said element and successively interconnected at alternately'oppos'ite ends, the transverse width of said element being minimum intermediate its ends and increasing exponen- -tially'to its respective ends, and the thickness of and spacing between the successively adjacent transverse members being minimum substantially where the transverse width-of the element is minimum and increasing "logarithmicallytoward opposite ends of said element, and

means electricallyconnected to said element for energizing the same.

. References Cited in the file of this patent UNITED STATES PATENTS 2,75 1,589

OTHER REFERENCES 'Kraus: Antennas, McGraw-Hill Book Co., NewYork,

1950, page .368.

"Du'Hamel and Isbell: Broadband Logarithmically Periodic Antenna Structures, March 1957, I.R.E. National Convention Record, ,part I. pp. 119-128.

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