Low profile slot antenna using backside fed frequency selective surface

Low-profile slot antenna using the rear-side feeding frequency selection interface

Field of invention

The present invention relates to a slot antenna that can be flat-connected and provides good impedance matching for a transmitter and/or receiver coupled to the antenna.

Background of the invention

Previous technologies include D. Sievenpiper, E. Yablonovitch, "Circuit and Method for Eliminating Surface Currents on Metals" U.S. Patent Application Serial No. 60/079,953 application, The file was established on March 30, 1998, and its content is about a high impedance or Hi-Z interface, and its corresponding PCT application PCT/US99/06884, which was published in W099/50929 on October 7, 1999. This application Reveal a high impedance interface (here also called Hi-Z or frequency selection interface).

The Hi-Z interface (the subject of US Patent Application Serial No. 60/079,953) is shown in Figure 1a. This interface 10, also referred to as a frequency selective interface (FSS), includes an array of metal elements 12 arranged on a flat metal ground plane 14. The size of each element 12 is smaller than the operating wavelength of the antenna. The overall thickness of the structure is also smaller than the operating wavelength. The presence of the element 12 has the effect of changing the boundary condition of the surface, so it can be used as an artificial magnetic conductor instead of an electrical conductor. Depending on the thickness of the structure with respect to the operating wavelength (see Figure 1c), the properties of the element can range from a few percentages to almost an octave (Octave) band interval. The Hi-Z interface 10 can be made in various forms, including a multilayer structure with overlapping capacitor plates. The Hi-Z structure is preferably formed on the insulating substrate 16 of the printed circuit board (ignored in Figure 1 for brevity), and the component 12 is formed on one set of main interfaces and the ground plate 14 is formed on the other main interface. superior. The element 12 is preferably electrically coupled to the ground plane 14 through conductive through holes 18, and these through holes 18 can be formed by electroplating holes formed in the printed circuit board 16. The capacitive load allows the resonance frequency to be reduced for the given thickness. Operating frequencies ranging from hundreds of megahertz to tens of gigahertz have been demonstrated using a variety of Hi-Z interface geometries. In plan view, the shape of the element 12 can be a square, a hexagon (also shown in Figure 1a), or any other suitable and repeated geometric shape.

The prior art waveguide feed, aperture coupling slot or plug-in antenna is shown in Fig. 1d in a side view. The plug-in antenna element 8 is arranged on the rear plate 14 and has an opening or slot 9 directly coupled to the wall surface of the waveguide 22 in the plate. Although these antennas are flat, they also tend to have high Q values. That is, acceptable impedance matching between the waveguide 22 and the antenna 8 can only be achieved through a relatively narrow bandwidth, and a broadband impedance matching network cannot be used. Figure 1e shows the simulation results of the antenna type in Figure 1d over the frequency range of 11-16Ghz (graphic "A"). The high Q nature of this antenna is very obvious. The plug-in antenna is also quite large (its actual size is about 1/2 λ to achieve the relevant frequency), so that it is often difficult to configure such an antenna array in a confined space.

In the prior art, there are still other known techniques for coupling the waveguide to the antenna structure. However, these prior art structures are not planar. On the contrary, they have the characteristic of protruding away from the waveguide (in the direction of arrow A in Figure 1d). Therefore, in a side view, they have characteristics that make it difficult to use on surfaces that should be flat or flat, such as the surfaces of aircraft or ground vehicles. In the automotive market, antennas protruding from the surface of the vehicle can be considered quite unsightly. Therefore, a flat antenna (or a flat profile if necessary) is required. In addition, there is also a need for a technique for coupling the waveguide to a planar antenna structure (and preferably a flat profile when required) that has a relatively wide frequency band with acceptable impedance matching.

Summary of the invention

In one point, the present invention provides an antenna structure with a high impedance interface, which includes a set of conductive planes and an array of conductive elements, and the distance between the array of conductive elements and the conductive plane is lower than the operating frequency wavelength of the antenna structure. 25% (and preferably no more than 10% of the operating frequency wavelength of the antenna structure). The conductive plane has a driving opening, and an antenna driving element is arranged adjacent to the opening in the conductive plane. When the driving element is in operation, pumping RF energy is used to excite the antenna structure through the opening in the conductive plane.

In another argument, the present invention provides a method of fabricating a short, broadband antenna, which includes the steps of providing a high impedance interface, the high impedance interface has a conductive plane and a conductive element array, wherein the conductive element array and The conductive plane is separated by a distance of no more than 25% of the wavelength of the operating frequency of the antenna structure (and preferably no more than 10% of the wavelength of the operating frequency of the antenna structure), and the conductive plane has an opening; The line drive element is adjacent to the opening in the conductive plane.

Schematic description

Figure 1a is a perspective view of the Hi-Z interface; Figure 1b is a side view of the Hi-Z interface; Figure 1c is a chart of the frequency band spacing of the Hi-Z interface; Figure 1d is a waveguide feed, aperture coupling plug-in antenna Side view; Figure 1e shows the S of the antenna of Figure 1d_HFigure 2a is a plan view of the frequency selection or Hi-Z interface with an aperture in its ground plane; Figure 2b shows the side view of the frequency selection or Hi-Z interface of Figure 2a, the cut section It is taken along the line 2b-2b in Figure 2a; Figure 2c shows the side view of the frequency selection or Hi-Z interface in Figure 2a, and the section is taken along the line 2c-2c in Figure 2a ; Figure 2d shows the S of the antenna of Figure 2c_HThe polar coordinates diagram of the simulation results; Figure 2e is a plan view of another embodiment of the frequency selection or Hi-Z interface with an aperture in its ground plane. This embodiment is made after the frequency selection or Hi-Z interface Driven by the microstrip of the conduction interface.

Detailed description of the preferred embodiment

The Hi-Z or frequency selective interface (FSS) 10 is fed through an aperture 20 in the ground plane 14 on its back or rear interface. The aperture 20 is preferably fed by a waveguide 22 or a microstrip 24. The components 12 on the front interface of the Hi-Z interface 10 and the ground plane 14 on the rear interface of the Hi-Z interface 10 have electrical conductivity, and are preferably made of metal such as copper. In fact, the Hi-Z or frequency selection interface 10 is preferably made of the flat printed circuit board 16 described above.

Figures 2a to 2c show an embodiment of a slot antenna with a wave guide and a frequency-selective interface fed on the rear side. Figure 2a is a plan view of the antenna, Figure 2b is a cross-sectional view taken along the cross-sectional line 2b-2b shown in Figure 2a, and Figure 2c is a cross-sectional view taken along the cross-sectional line 2c- shown in Figure 2a. Cross-sectional view taken in 2c. Generally speaking, the Hi-Z interface in Figure 2a to Figure 2c is a traditional Hi-Z of the type discussed with reference to Figure 1a to Figure 1c. However, there are two important differences.

The first point, although not shown in Figure 1a or Figure 1b, in order for the Hi-Z interface of the prior art to work as part of the antenna, one or more antenna elements must be placed on it. In the embodiment disclosed here, such an antenna element is not required; in fact, adding an antenna element to the modified Hi-Z interface of Figures 2a to 2c will weaken the resulting antenna function (it may Will have a higher Q value).

The second point is that the rear side or ground plane 14 has an opening 20 therein, and in this embodiment, the opening will cooperate with the waveguide 22. In Figures 2a and 2c, two openings 20 and two sets of corresponding waveguides 22 are shown for display purposes only. In this embodiment, the ground plane may have a single opening 20 of a set of waveguides 22, or it may have a plurality of openings 20 of multiple sets of waveguides 22. In any case, the waveguide 22 will be aligned with the opening 20, and the aperture of the waveguide 22 should preferably match the size of the corresponding opening 20. To illustrate in sequence with reference to FIG. 2e, in another embodiment, the opening 20 in the rear side or the ground plane is driven by the micro-strip line 24 instead of the waveguide 22.

Each aperture of the waveguide 22 forms a rectangle. The longer side is preferably about 0.5 λ to 1 λ at the relevant frequency. The shorter side of the rectangle is shorter, and its range is preferably from (i) approximately equal to the width of the gap between the elements 12 (see the waveguide on the left in Figure 2c) to (ii) approximately equal to the gap between the elements 12 (See the waveguide on the right side of Figure 2c). The gap P in the center of the element 12 is less than 0.25 λ at the relevant frequency, and the range of the gap is preferably between about 1/8 λ and 1/10 λ at the relevant frequency. The distance or gap 9 between adjacent edges of the element 12 is smaller at the relevant frequency, generally about 0.01 λ.

The side surface of the waveguide 22 may indeed match the side surface of the corresponding opening 20, or in some embodiments, the opening may be smaller than the size of the waveguide 22.

Figure 2d is a polar coordinate diagram of the input reflection coefficient of the waveguide based on the computer simulation of Figures 2a to 2c (please refer to Figure B). This graphic covers frequencies from 11-16 GHz. For the simulation, use the following structural parameters: element 12 size = 124 mils squared (3.15mm on one side), element 12 pattern spacing (gap) = 125 mils (3.175mm), gap 9 width = 1 mil (0.025) mm), through hole 18 diameter = 4 mils (0.1 mm), substrate thickness = 20 mils (0.5 mm), substrate dielectric constant = 3, waveguide (slot) width = 40 mils. The graphic "C" in Figure 2d shows the effect of removing the Hi-Z interface 10; its impact is huge.

As shown in Figure 2d, this antenna embodiment is a set of effective transmitting antennas for RF energy in a very wide band of 11-16 GHz. If there is a usable bandwidth or interval of 5GHz and an operating frequency of up to 16GHz, this antenna design will have a bandwidth that exceeds the operating frequency by 30%! The antenna also has an extremely low height. The thickness of the insulating substrate 16 is only about 0.5 mm-even if a metal interface is used. The thickness of the Hi-Z interface should be less than 1mm, and the wavelength at 16GHz is about 19mm. The thickness of the antenna can be easily maintained in the range of 5 to 10% of the wavelength of the relevant frequency-of course, the thickness of the antenna can be easily maintained below 25% of the wavelength of the relevant frequency (the above-mentioned antenna is 11-16 GHz). Therefore, the disclosed antenna can have an extremely low height. It can be easily attached to the exterior surface of an aircraft or vehicle without causing unsightly or interfering with the operation of the aircraft/vehicle. Regarding the thickness of the exposed wire, if the antenna extends from the exterior surface of the aircraft/vehicle to the interior, it will not occupy much of the interior space of the aircraft/vehicle (if any).

Figure 2e shows another embodiment of the present invention. In this embodiment, the waveguide 22 is not used to drive the slot 20, but a micro strip 24 is used instead. The micro strip is separated from the rear or ground plane 14 by the second insulating substrate 28. In other respects, this embodiment is the same as the previously described embodiment. Of course, because this antenna has two sets of substrates 14 and 28, it will be thicker than the antenna of the above-mentioned embodiment. If the thickness of the second insul 10% of λ).

If it is in the relevant frequency range, the opening 20 in the rear side plane 14 is basically the same size as the waveguide feeding embodiment in FIG. 2c or the micro-strip line feeding embodiment in FIG. 2e.

For the computer mode of the waveguide feeding embodiment in FIG. 2c and the micro-strip line feeding embodiment in FIG. 2e, it is assumed that the Hi-Z or frequency selective interface (FSS) 10 is far away from the hole 20 and extends infinitely. If the extension distance of Hi-Z or frequency selection interface (FSS) 10 is approximately at least equal to 10 λ of the relevant frequency, then the Hi-Z or frequency selection interface (FSS) 10 will basically be compatible with the computer mode based on the infinite interface Works in the same way. However, when the size of Hi-Z or frequency selective interface (FSS) decreases relative to the relative frequency λ, its edge effect will start to impact the antenna, and the result will be larger than that of large Hi-Z or frequency selective interface (FSS). ) The result of the case of 10 is even more unsatisfactory. Therefore, the Hi-Z or frequency selective interface (FSS) 10 should extend at least several times the wavelength of the relevant frequency far away from the hole 20, and preferably should extend upwards from the hole 20 more than ten times the wavelength of the relevant frequency.

The present invention achieves a low-height antenna with excellent bandwidth characteristics. In addition, the structure of the antenna can be achieved using only standard printed circuit technology, so the disclosed antenna can be manufactured at very low cost. The Hi-Z interface disclosed here can be easily manufactured using printed circuit board technology to form a rectangular or square metal grid of the element 12 to print an appropriate dielectric material with a conductive back plane 14 on the bottom side 16, and there is a flat hole 18 (perforation) connecting each element 12 to the conductive back plane 14.

Both the waveguide embodiment and the microstrip embodiment provide an antenna drive that excites the antenna through the opening 20 in the back-side conductive plane 20. In this way, the present invention feeds the interface from the plane 14 side behind the Hi-Z interface 10 through the aperture or opening 20 in the conductive plane 14, thus combining the feeding circuit of the antenna with the radiating element on the front surface of the Hi-Z interface 10. Separated. The antenna has a low height, can be manufactured at low cost, and can be manufactured using a conductive plane 14 to isolate all feed electrons from the emitting area. The micro-strip antenna driver can also be easily manufactured using standard printed circuit board manufacturing techniques.

The electrical properties of the Hi-Z interface 10 provide an impedance conversion from a low (usually 50Ω) circuit or waveguide impedance to a high free space impedance. By appropriately selecting the scale of the Hi-Z interface 10, an excellent impedance matching can be achieved between the antenna feed and the free space.

Since the present invention has been described in conjunction with the preferred embodiments, those familiar with the related art will be able to inspire modifications accordingly. Therefore, the present invention is not limited to the disclosed embodiments unless as required by the scope of the appended patent application.

<p>8Plug in antenna element</p><p>9Gap</p><p>10Hi-Z interface</p><p>12Component</p><p>14Ground plane</p><p>16Printed Circuit Board</p><p>18Through hole</p><p>20Aperture</p><p>22waveguide</p><p>24Micro strip</p>

Figure 1a is a perspective view of the Hi-Z interface; Figure 1b is a side view of the Hi-Z interface; Figure 1c is a chart of the frequency band spacing of the Hi-Z interface; Figure 1d is a waveguide feed, aperture coupling plug-in antenna Side view; Figure 1e shows the S of the antenna of Figure 1d_HFigure 2a is a plan view of the frequency selection or Hi-Z interface with an aperture in its ground plane; Figure 2b shows the side view of the frequency selection or Hi-Z interface of Figure 2a, the cut section It is taken along the line 2b-2b in Figure 2a; Figure 2c shows the side view of the frequency selection or Hi-Z interface in Figure 2a, and the section is taken along the line 2c-2c in Figure 2a ; Figure 2d shows the S of the antenna of Figure 2c_HThe polar coordinates diagram of the simulation results; Figure 2e is a plan view of another embodiment of the frequency selection or Hi-Z interface with an aperture in its ground plane. This embodiment is made after the frequency selection or Hi-Z interface Driven by the microstrip of the conduction interface.