Reduced cavity wideband multi polar spiral antenna

A wideband antenna with backlobe elimination in a confined space is disclosed. Backlobe elimination is by an array of electromagnetically conductive members disposed between the spiral element and the ground plane and almost contacting the ground plane so as to bleed a backlobe away from the spiral antenna to the ground plane. In one embodiment, the members are conical elements disposed between the spiral element and the ground plane of a spiral antenna such that the apexes of the conical elements almost contact the spiral element and the bases of the conical elements contact the ground plane. In some embodiments, other means may be employed to perform this function. In operation, conical elements bleed the backlobe of the signal away from the spiral element to the ground plane.

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to very wideband spiral antennas and, more particularly, to backlobe elimination in such spiral antennas in small, confined spaces.

2. Description of the Related Art

This section of this document introduces various aspects of the art that may be related to various aspects of the present invention described and/or claimed below. It provides background information to facilitate a better understanding of the various aspects of the present invention. As the section's title implies, this is a discussion of “related” art. That such art is related in no way implies that it is also “prior” art. The related art may or may not be prior art. The discussion in this section of this document is to be read in this light, and not as admissions of prior art.

One particular type of antenna is known as a planar “spiral antenna”. Spiral antennas take their name from the spiral shape of their radiating/receiving elements. As with most antennas, the planar spiral antenna radiates or receives from all directions, but primarily from the front side and the back side, more commonly known at the “frontlobe” and the “backlobe”. The backlobe is commonly undesirable, and the art has proposed a number of techniques for eliminating, or at least mitigating, such backlobes.

The classic technique absorbs the backlobe in a deep cavity (e.g., a depth that is twice the width of the antenna; for example, 3″) filled with a honeycombed dielectric material. Another approach is disclosed in U.S. Pat. No. 6,407,721, which we have not seen reproduced in the laboratory. However, all of the techniques proposed by the art have drawbacks associated with their benefits. The art therefore continues to seek alternative backlobe elimination techniques in reduced volume space and by simple designs allowing low cost embodiments.

The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.

SUMMARY OF THE INVENTION

In a first aspect, a spiral antenna, comprising a substrate; a spiral element formed upon the substrate; a ground plane behind the spiral element; and an array of electromagnetically conductive members disposed between and almost contacting the spiral element and the ground plane so as to bleed a backlobe of a signal away from the spiral element to the ground plane.

In a third aspect, the invention includes a method of eliminating a backlobe associated with a signal transmitted or received through a spiral antenna, comprising bleeding the backlobe of the signal away from a spiral element to a ground plane through a plurality of electromagnetically conductive conical elements disposed therebetween.

In a fourth aspect, the invention includes a spiral antenna, comprising: a spiral element; a ground plane behind the spiral element; and means for bleeding the backlobe of the signal away from the spiral element to the ground plane.

While the invention is susceptible to various modifications and alternative forms, the drawings illustrate specific embodiments herein described in detail by way of example. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to miniaturizing wideband multipolar antennas that have backlobe elimination. In the embodiments disclosed herein, the spiral antennas are deployed on midsized missiles, for example, those that are 5″ diameter. On missiles of this size, it is difficult to package multiple wideband antennas due to surface area and depth limitations. The goal for this particular embodiment is to package multiple separate antennas circumferentially about the missile surface at the nose and as described herein. Each antenna's performance is very wideband (approximately 2 Ghz to 18 Ghz); left and right hand circular polarization sensitive; and horizontal (“H”) H-H, vertical (“V”)-V, and V-H sensitive. Each antenna's size is not more than 2.4″ in diameter and 0.4″ deep.

More particularly, a four (or eight) arm natural log growth spiral with members is used as the primary antenna with center feed. The side of the antenna (i.e., the backlobe) requiring rejection contains an array of three-dimensional (“3D”) electrically conductive, geometrically shaped members approximately ⅜″ tall. The geometric shape of the members in the illustrated embodiment is conical. The conical elements are “pointy” or “narrow” where it is is closest, or proximate, to the spiral element of the antenna—that is, the apex of the conical elements almost contacts the plane in which the spiral elements reside. The conical elements are “broad” or “spread” where they are furthest, or most distal, from the antenna—that is, the base of the conical elements contact the ground plane. This allows for a gradual spatial transition into a conductive zone where the back lobe waves are neutralized. In the illustrated embodiment, the members are arranged in a diamond-shaped array.

The present invention will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present invention with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present invention.

FIG. 1A-FIG.1C illustrate one particular embodiment of a spiral antenna100constructed and operated in accordance with several aspects of the present invention.FIG. 1Ais an assembled, perspective view of the spiral antenna100.FIG. 1Bis the spiral antenna100ofFIG. 1Awith the face102removed.FIG. 1Cis partially sectioned, fragmented side view of the spriral antenna100illustrate certain structural and functional relationships described further below.

Referring now toFIG. 1A-FIG.1C collectively, the spiral antenna comprises a spiral element105, a ground plane110behind the spiral element105, and an array115of electromagnetically conductive members120(only one indicated). The ground plane110is “behind” the spiral element105in the sense that it is positioned on the side opposite that in which the signal propagates in its intended direction. Stated alternatively, it is “behind” the spiral element105in the sense that it is positioned on the side of the spiral element105where the backlobe of any such signal will be found.

The members120are disposed between and almost contacting the spiral element105and the ground plane110so as to bleed a backlobe of a signal away from the spiral element105to the ground plane110. In the illustrated embodiment, the array115of members120comprises a plurality of conical elements disposed between the spiral element105and the ground plane120such that the apexes125of the conical elements almost contact the spiral element105and the bases130of the conical elements contact the ground plane110.

The conical form of the members120in the illustrated embodiment is desirable because they provide a pathway that channels the backlobe to the ground plane110, thereby allowing it to dissipate, or “bleed” away, without short circuiting the spiral element105to the ground plane110. Note, however, that the invention is not necessarily limited to the members120shown in the illustrated embodiment. These are, by way of example and illustration, but one means for bleeding a backlobe of a signal away from the spiral element to the ground plane. Other embodiments may alternatively employ equivalent structures performing that same function. For example, other embodiments may employ members that are spires, pyramids, needles, or are irregularly shaped.

In the illustrated embodiment, the array of the members120is arranged in a triangular, or a diamond, pattern. However, computer simulations have established that the technique will also work with lesser efficacy with other array patterns. Such other arrays may include, but are not limited to, circular, spiral (e.g., circular spiral, rose spiral), rectangular, square patterns.

The spiral element105can similarly be implemented in alternative spiral designs. The design of the spiral element105illustrated inFIG. 1A-FIG.1C is what is known as a “modulated arm width” as a “natural log growth” spiral since it embodies both those characteristics. Other suitable spiral designs include, but are not limited to, Archimedean (FIG. 2A), logarithmic (or, log base 10) (FIG. 2B), rose cone (FIG. 2C), cone, Mehen, single, hybrid, and natural spiral designs. The “Mehen” spiral design is the one disclosed in U.S. Pat. No. 6,407,721.

Techniques for designing and fabricating spiral elements such as the spiral element105are known to the art. For example, U.S. Pat. No. 6,407,721 provides details for both design and fabrication of a Mehen spiral element. Any suitable technique known to the art for any given spiral design that is desired may be employed. The present invention therefore admits wide variation in design and fabrication considerations such as materials selection. In the illustrated embodiment, the spiral element105is fabricated from a metal material such as a copper chase on a substrate of dielectric material140such as Kapton (a polyimide plastic).

The members120are, as noted above, electromagnetically conductive. They are therefore fabricated from a ferromagnetic metal such as Nickel, Aluminum, or Zinc, or some other electrically conductive material. They alternatively may be fabricated of a material that is not ferromagnetic so long as that material is electromagnetically conductive or they are coated in an electromagnetically conductive material.

In the illustrated embodiment, the ground plane110is fabricated in the shape of a flanged cup. The bowl of the cup design provides a convenient area in which to dispose the array115of members120and the flange provides a convenient point at which to fasten the facing101to enclose the bowl. The ground plane110is fabricated from a ferromagnetic metal whose selection can be governed by principles well known to those in the art. The ground plane110may be fabricated from the same material as the members120.

The illustrated embodiment fabricates both the members120and the ground plane110from the same ferromagnetic material using a casting process. That is, the members120and the ground plane110are cast as a single piece. The members120and ground plane110may therefore, in this embodiment, be considered a single, particularly shaped, ground plane. However, this is not necessary to the practice of the invention. The members120and ground plane110may be assembled as separate pieces in some embodiments. Packaging considerations may also be of interest. For example, when the members120and the ground plane110are assembled as separate pieces, a packing dielectric material may be used to fill the ullage of the bowl in the cup of the ground plane110. In the illustrated embodiment, the ullage is filled with ambient atmosphere.

The entire antenna may also, in some alternative embodiments, be manufactured to be conformal to a surface. This is a common practice in shallow depth antennas packaged on aircraft surfaces. Multiple configurations of producing the members with a spiral antenna element in a surface conformal profile are possible.

Those in the art having the benefit of this disclosure will appreciate that the spiral antenna may be used to transmit, receive, or both transmit and receive signals. To this end, the spiral antenna100includes access to a plurality of signal leads145. In the illustrated embodiment, bores for four leads145, one for each arm, brought in through the ground plane110and the central one of the members120and electrically connected to the backside of the arms of the spiral element115. The leads145are implemented in micro-coaxial cables, but any suitable lead known to the art may be used. This will typically be an implementation specific detail depending on design constraints of the individual implementation.

The electrical connection of one lead145is more particularly shown inFIG. 3. As noted above, the lead145is a micro-coaxial cable. The lead145therefore comprises an inner conductor300electromagnetically shielded by an outer conductor303, for which reason the outer conductor303is sometimes referred to as the “shield”. The lead145is threaded through a via306in the grounding plane110and a bore309in the member120. The outer conductor303is electromagnetically connected—e.g., soldered—to the ground plane at one or more points312. It is similarly electromagnetically connected to the member120at one or more points315. The inner conductor300extends through the bore309and the ullage318, through a via321in the substrate140, whereupon it is electromagnetically connected to one arm324of the spiral105at one or more points327.

The embodiments illustrated herein disclose the present invention in the context of a missile application. The invention is not so limited. For example, the present invention may find applicability is many applications where a wideband, small size (e.g., 56 mm diameter) multi-polar synthetic aperture radar (“SAR”) antenna is desirable. SAR antennas are used in, for example, protected communications, imaging, mapping, seeking, and still other uses. Those in the art having the benefit of this disclosure may appreciate still other applications for the present invention.

The presently disclosed technique therefore provides a low profile antenna with a simple, low cost means for bleeding the backlobe away into the ground plane without affecting the frontlobe. More particularly, the illustrated embodiments comprise a simple antenna system providing wideband coverage over multiple polarizations while eliminating the parasitic backlobe within a confined space at a low cost.

The phrase “capable of” as used herein is a recognition of the fact that some functions described for the various parts of the disclosed apparatus are performed only when the apparatus is powered and/or in operation. Those in the art having the benefit of this disclosure will appreciate that the embodiments illustrated herein include a number of electronic or electromechanical parts that, to operate, require electrical power. Even when provided with power, some functions described herein only occur when in operation. Thus, at times, some embodiments of the apparatus of the invention are “capable of” performing the recited functions even when they are not actually performing them—i.e., when there is no power or when they are powered but not in operation.

The following documents are hereby incorporated into this specification by reference as is set forth verbatim herein for purposes noted:U.S. Pat. No. 6,407,721, entitled, “Super Thin, Cavity Free Spiral Antenna”, issued Jun. 18, 2002, to Raytheon Company as assignee of the inventors Mike Mehen et al. for its teachings as comprising a part of the prior art, as well as its teachings regarding the design and fabrication of the spiral radiating element disclosed therein to the exclusion of the backlobe mitigation technique;U.S. Pat. No. 5,990,849, entitled, “Compact Spiral Antenna”, issued Nov. 23, 1999, to Raytheon Company as assignee of the inventors Gary Salvall et al. for its teachings as comprising a part of the prior art, as well as its teachings regarding the design and fabrication of the spiral radiating element disclosed therein to the exclusion of the backlobe mitigation technique;