Ultra wide band flat antenna

A flat, ultra wideband, unidirectional antenna is disclosed, the antenna may comprise a pair of active elements having the shape of substantially half-circles or half-ellipsoids made of thin conductive material and a ground element made of thin conductive material placed parallel and against to the active electrodes and spaced from them, the antenna having a nominal gain of at least 6 dbi and variations of gain in that range of +/−1.5 dbi at its bore sight.

BACKGROUND OF THE INVENTION

Several ultra wide band (UWB) antennas are known in the art, such as flat spiral, conical spiral, log periodic, Vivaldi-type, “horn”-type and dipole ‘bow tie’ antennas. These types of UWB flat antennas suffer from various drawbacks such as having an omni-directional radiation patterns, a low gain, or having a low-quality time response or combinations of the above. There is an ongoing demand for small dimensioned, relatively flat antenna with UWB response curve, a directional radiation pattern, a high gain and good time response over a wide angle of coverage.

DETAILED DESCRIPTION OF THE INVENTION

It should be understood that the present invention may be used in a variety of applications. Although the present invention is not limited in this respect, the antenna design disclosed herein may be used in many apparatuses in vide band or pulse type applications, such as wide band radar for ground penetration or looking through walls and the like.

Reference is made now toFIGS. 1A and 1B, which are schematic top and side views, respectively, of antenna10according to some embodiments of the present invention. Antenna10may be comprised of two co-planar flat elements12made of conductive material, a ground conductive plane14, an insulating layer15, feeding ports16, two resistors18and two auxiliary conductive planar elements19. For the sake of clarity the description of antenna10will be aided by the use of two symmetry lines A and B as inFIG. 1A. Elements12may each have a planar shape having a perimeter including a straight edge13and a remainder, which is typically shaped, as shown in shaped edge23so that the shaped edges of each of flat elements12are facing each other and arranged symmetrically with respect to symmetry line B. Planar elements12are further arranged so that their symmetry line coincides with symmetry line A. The straight edges13of the two elements12may be parallel to each other and the shaped edges23may be facing each other.

Shaped edges23may have at least one vertex, which may be for example, one or more points or a line, where the distance between the elements is at a minimum. Shaped edge23may have any shape, including a curve or a polygon or a combination of the two. Typically, the shape may be such that the length of cross-sections of each element transverse to the line of symmetry A decrease as the distance from the straight edge increases, until the vertex or vertices are reached. In some embodiments, the shape of shaped edge may be such that its cross-section tapers continuously, for example, in accordance with an equation or formula. Shaped edge23may be or include, for example but is not limited to, an arc, semi-circle, or other circular section, a semi-ellipsoid or other ellipsoid section, a polygon, or the like. For purposes of obtaining wide bandwidth, good VSWR, and fairly constant gain and beam width over a very wide band shaped edge23may preferably have the shape of a smoothly or continuously curved line such as a perimeter of a semi-circle or a semi-ellipsoid. In some embodiments, the contour of the shaped edge may include a notch, by which the contour of the notch section of the shaped edge is curved concave inwards towards the straight edge, for example, in order to filter out a sub-band frequency.

The points on the curved edges23most distal from the straight edges13, i.e., the vertices, may be proximal to each other with a small gap there between. Feeding port16may be placed symmetrically close to said small gap at or near the respective vertices of active elements12, to allow feeding of RF energy to active elements12. Ground conductive plane14may be mounted substantially parallel to the plane containing two active elements12, in a different plane, with a small gap between the planes.

In some embodiments of the invention, the typical size of the gap between the planes may be approximately 1/10 (one tenth) of the wavelength at low frequency end, yet this size may vary according to various engineering considerations, such as bandwidth or beamwidth requirements. Elements12may be co-planar, i.e., on the same flat plane, for example, both may be printed on the same single substrate board. An insulating layer15may be placed between the plane of the two active elements12and ground plane14. Insulation layer15may be realized using any kind of insulation material and preferably air, which may give better efficiency and bandwidth. Elements12,18and19may be supported by or installed on a substrate layer (not shown), which may be made of materials such as teflonglass, epoxyglass, polyesterene, polypropylene and materials for printed circuit board (PCB), etc.

The size and position of ground conductive plane14with respect to active elements12may vary according to engineering considerations. In the example depicted inFIGS. 1A-1Bground conductive plane14may be larger than that of a rectangle inscribing active elements12and it may be placed with its center point substantially opposite to the center point between two feeding ports16and to the cross of symmetry lines A and B. In another embodiment active elements12and ground plane14may be printed on two separate insulating boards spaced from each other with any kind of method to space between them.

The two main axes of antenna10are commonly marked H for the vertical axis and E for the horizontal axis, as marked by the respective double-headed arrows inFIG. 1A. Main axis E coincides with symmetry line A and main axis H coincides with symmetry line B. Antenna10has a boresight axis which is substantially perpendicular to the plane of the page ofFIG. 1Aand crosses substantially in the cross point of symmetry axes A and B. Reference planes H and E are defined so that they comprise the antenna boresight and either main axis H or E respectively.

Auxiliary conductive planar elements19may have substantially rectangular, circular, elliptical or other shapes, which substantially may be enclosed in a rectangle as depicted inFIG. 1A. Auxiliary elements19may be positioned symmetrically with respect to symmetry line B along symmetry line, spaced on the side of primary elements12proximal to the straight edge and at distance d4from the straight edge13of the respective active element12. Auxiliary elements19may be called also auxiliary active elements19. Impedance elements such as resistors18may be electrically connected at one end to one of active elements12substantially at a point most distal from its vertex, on its bisector. Resistors18may further be connected at its other end to auxiliary active element19. Two auxiliary active elements19may be placed in the plane of active elements14with one of their symmetrical axis coinciding with axis E of antenna20. This arrangement may provide forward flow path for RF energy fed to two active elements12and by this substantially minimize and even eliminate back-flow of such energy, thus enhancing the dispersion of the impulse response signal (by eliminating the trailing rings) of antenna10. Active elements12and auxiliary active elements19may be realized on a common PCB layer. It will be noted that impedance element may be a resistor, a capacitor or an inductor, or any suitable combination thereof.

The various parts of antenna10may have dimensions d1-d8(FIG. 1) as may be dictated by the performance required from it. Typical dimensions of the various parts of antenna10, which may allow the performances depicted in drawingsFIGS. 3A to 5Bmay be, as a non-limiting example, in fractions or multiples of the wavelength λ of the low-end of the working frequency band width of antenna10: d1=0.008, d2=0.27, d3=0.36, d4=0.02, d5=0.08, d6=0.07, d7=0.93 and d8=0.93. It would be apparent to a person with ordinary skill in the art that these typical dimensions may be varied so as to satisfy various engineering requirements without departing from the concept of the invention.

Feeding ports16may feed two active elements12allowing a balanced feed. Feeding lines (not shown) may be realized by two parallel printed lines on the opposite sides of a PCB being the substrate layer. According to yet another embodiment of the present invention feeding ports16may be fed from an unbalanced feeding line (such a coax cable) using any kind of balanced-to-unbalanced (“balun”) adaptor device.

Baluns of the known art may be used in connection with the antenna of the present invention; however, such known baluns may typically quite large and bulky with respect to typical dimensions of a flat antenna. For purposes of providing an antenna with a very low profile, a flat UWB balun is presented that may be used in connection with the antenna of the present invention. Attention is made now toFIGS. 2A-2C, which are a schematic top view with blow-up view, a positional view and partial side cross-section view respectively of a flat balun60according to some embodiments of the present invention. Flat balun60according to an embodiment of the present invention may be realized by removing part of conductive ground plane14, substantially shaped as an “H”, having two side legs and a middle leg, and centered at the crosspoint of symmetry lines A and B and placed with respect to active elements12as shown inFIG. 2B. Flat balun60may be achieved, for example, by removing a rectangle62having width e1and height h1+h2+ h3centered at the cross point of symmetry lines A and B, but leaving two non-removed strips63and64protruding from two opposite sides of perimeter of rectangular62into its center along symmetry line A, symmetrically with respect to both symmetry lines A and B, leaving a space e2between them.

Flat balun60may have balanced and unbalanced ports. The unbalanced port may be located at61and be between microstrip line66, which is a conducting strip on the underside of the ground plane substrate and ground plane14. Microstrip66may begin at a side of ground substrate proximal to strip63and on a side opposite the conducting side, extend underneath strip63, across the gap separating strips63and64and have its terminus at port68. The balanced port may be at edges67and68. The connection between the balanced side and unbalanced side may be via feed-through hole68. Thus, the ground plane may be common to both balanced and unbalanced ports.

RF energy emitted from the output of flat balun60may be conveyed to feeding ports16of antenna10by means of conductors69,70(shown inFIG. 2C), in a plane perpendicular to the plane shown inFIG. 2A. Conductors69,70may be printed on substrate. Accordingly, unbalanced RF energy may be provided to the system of antenna10via connector61and strip line66and converted to balanced energy to antenna10.

Installation of flat balun60made according to embodiments of the present invention may comprise feeding of RF energy in an unbalanced line66to unbalanced port68and feeding of RF energy to active elements12in balanced conductors69,70, where ground element14is realized on the top side of PCB65and strip line66on the lower side of it.

Typical dimensions of balun60that may provide for the performances described in this application may be, as a non-limiting example, in fractions of the wavelength λ of the low-end of the working frequency band width of antenna10: h1=h3=0.05, h2=0.04, e1=0.14 and e2=0.008.

Reference is made now toFIGS. 3A,3B,4,5A and5B which are diagrams of the electrical performance of antenna10according to some embodiments of the present invention.

An antenna made according to the present invention may have a UWB performance profile, a very low physical profile, high gain, low dispersion, high quality of impulse response and time response.

Reference is made now toFIGS. 3A and 3B, which are normalized impulse response diagrams of antenna10according to some embodiments of the present invention, given for seven different angles, substantially equally distributed off the bore sight from −30 degrees to +30 degrees, plotted on same graph.FIG. 3Adepicts normalized impulse response of antenna10for 0, +/−10, +/−20 and +/−30 degrees off bore sight line in the E plane andFIG. 3Bdepicts normalized impulse response of antenna10for 0, +/−10, +/−20 and +/−30 degrees off bore sight line in the H plane. As may be seen inFIGS. 3A and 3B, impulse response of antenna10exemplifies very low dispersion across the various angle of deviation from the bore sight line. The dispersion may be measured as the standard deviation between the graphs at every given point along the horizontal axis (time), averaged over time required for reception of 98% of the pulse energy. This mean deviation at any time taken over all time required for reception of 98% of the pulse energy may be denoted Arel—div—avg.

Preferably, in embodiments of the invention having the flat balun described above, Arel—div—avgmay be less than 4×10−4for each of the E and H planes. The graphs ofFIGS. 3A and 3Bshow the deviation in time domain for an antenna with the flat balun described herein with a 2 mm thick radome, having values 2.5×10−4and 3.7×10−4respectively for the E and H planes. It will be apparent to person with ordinary skill in the art that these values of Arel—div—avgindicate a very low dispersion in the angle of interest of antenna10. In another embodiment of the invention using a conventional or mechanical balun, Arel—div—avgmay be less than 3×10−4or more preferably less than 2.5×10−4. In one embodiment (graph not shown), Arel—div—avgmay have values of 1.4×10−4and 2.4×10−4respectively for the E and H planes.

Attention is made now toFIG. 4, which depicts the electrical gain of antenna10in varying frequencies at the boresight of the antenna.FIG. 4depicts results received in both E and H planes (also known as azimuth and elevation planes respectively). In one embodiment of the present invention, the antenna may have gain variation within limits of +/−1.5 dbi (decibels referenced to isotropic radiator) over a frequency range having a ratio of high end-to-low end higher than 3 and preferably 3.4 or higher, for example, from 3.1 to 10.6 GHz. The absolute nominal gain may generally be better than 6 dbi over the band 3.1 to 10.6 GHz, which is much higher than that of prior art UWB flat antennas. It would be noted that the gain of antenna10as depicted in graph ofFIG. 4complies with the definitions of an ultra wide band (UWB) antenna, as defined, for example, by the US Federal Communications Commission (FCC).

Attention is made now toFIGS. 5A and 5B, which depict normalized radiation curves of antenna10according to the spatial inclination angle from the boresight of the antennaFIG. 5Adepicts measurements taken in E plane andFIG. 5Bdepicts measurements taken in H plane, both with respect to boresight axis for 10 different frequencies in the range of 3.1 to 10.6 GHz.FIGS. 5A and 5Bexhibit the performance of antenna10with respect to beam width versus frequency exemplifying that its beam width is substantially constant over the bandwidth for beam angles in the range of −/+30° from boresight.

It will be appreciated by persons of ordinary skill in the art that according to some embodiments of the present invention other designs of flat antenna with substantially two circle-like conductive planes and a ground planes according to the principles of the present invention are possible and are in the scope of this application.