Abstract:
A dielectric loaded microstrip patch antenna is provided for delivering relatively wide operational bandwidth dual-band, with relatively good isolation and flexibility for circular polarization, while having a compact design and lightweight to suit mobile and wireless applications. The microstrip patch antenna has a conducting ground plane and a patch radiator. The patch radiator is spaced from the ground plane by a substantial distance having a first dielectric material therein. A slot feed in the ground plane provides the patch radiator with radio signal energy across the space having the first dielectric material therein. A piece of a second dielectric material is disposed adjacent the slot feed between the patch radiator and the ground plane. The second dielectric material has a dielectric constant that is higher than the dielectric constant of the first dielectric material. The piece of the second dielectric material acts to load the feed in order to improve coupling between the slot and the patch. The piece has a dimension along one of the x and y axes smaller than a dimension of the patch along a same axis. Since the piece is situated between the ground plane and the patch it determines operational characteristics of the microstrip patch antenna.

Description:
FIELD OF THE INVENTION 
     The invention relates to microstrip patch antennas and more particularly to a microstrip patch antenna spaced from a ground plane by a substance having a very low dielectric constant such as air. 
     BACKGROUND OF THE INVENTION 
     The performance of an antenna is determined by several parameters, one of which is efficiency. For a microstrip antenna, “efficiency” is defined as the power radiated divided by the power received by the input to the antenna. A one-hundred percent efficient antenna has zero power loss between the received power input and the radiated power output. In the design and construction of microstrip antennas it is desirable to produce antennas having a relatively high efficiency rating, preferably in the range of 95 to 99 percent. 
     One factor in constructing a high efficiency microstrip antenna is minimizing power loss, which may be caused by several factors including dielectric loss. Dielectric loss is due to the imperfect behavior of bound charges, and exists whenever a dielectric material is located in a time varying electrical field. Moreover, because dielectric loss increases with operating frequency, the problem of dielectric loss is aggravated when operating at higher frequencies. 
     The extent of dielectric loss for a particular microstrip antenna is determined by, inter alia, the permittivity, ∈, expressed in units of farads/meter (F/m), of the dielectric space between the radiator and the ground plane which varies somewhat with the operating frequency of the antenna system. As a more convenient alternative to permittivity, the relative dielectric constant, ∈ r , of the dielectric space may be used. The relative dielectric constant is defined by the equation: 
      ∈ r =∈/∈ o   
     where ∈is the permittivity of the dielectric space and ∈ o  is the permittivity of free space (8.854.times.10.sup.−12 F/m). It is apparent from this equation that free space, or air for most purposes, has a relative dielectric constant approximately equal to unity. 
     A dielectric material having a relative dielectric constant close to one is considered a “good” dielectric material—that is, the dielectric material exhibits low dielectric loss at the operating frequency of interest. When a dielectric material having a relative dielectric constant equal to unity is used, dielectric loss is effectively eliminated. Therefore, one method for maintaining high efficiency in a microstrip antenna system involves the use of a material having a low relative dielectric constant in the dielectric space between the radiator patch and the ground plane. 
     Furthermore, the use of a material with a lower relative dielectric constant permits the use of wider transmission lines that, in turn, reduce conductor losses and further improve the efficiency of the microstrip antenna. 
     The use of a material with a low dielectric constant, however, is not without drawbacks. One typical drawback is that it is difficult to produce high-speed compact patch antennas spaced from a ground plane by a “good” dielectric. When a dielectric material disposed between a patch and a ground plane has a low dielectric constant (about 1), the resulting patch size is large (for example at 3.6 GHz patches of about 1550 mm 2  result). For mobile applications and for use in arrays, such a patch size is often problematic. 
     Another problem with antennas as described above is that the feed efficiency often degrades substantially as the patch is spaced further away from the ground plane. That said, more spacing of the patch from the ground plane is often advantageous and, as such, is usually accommodated using dielectric material with a higher dielectric constant to fill the space between the patch and the ground plane. Unfortunately, efficiency is substantially compromised in order to meet other design parameters. 
     It would be advantageous to provide a patch antenna that is spaced a distance from a ground plane and efficiently coupled to a feed absent a substrate having a high dielectric constant filling the space therebetween. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a patch antenna comprising a ground plane, a feed and a patch spaced from the ground plane by a predetermined distance. A dielectric material having a low dielectric constant is disposed therebetween. This dielectric material could be air, foam, or the like. In order to improve coupling efficiency between the patch and the feed, a piece of second dielectric material having a higher dielectric constant than the dielectric material is inserted between the patch and the ground plane in order to load the feed and thereby improve coupling efficiency between the feed and the patch. Exact placement of the piece of the second dielectric material is important for optimising antenna performance. 
     Further pieces of another dielectric material are optionally disposed between the ground plane and the patch to shape the radiation fed to the patch. This permits a smaller size patch than would be possible in a conventional air spaced patch antenna. 
     In accordance with the invention there is provided a microstrip patch antenna comprising: 
     a patch radiator; 
     a conducting ground plane spaced from the patch radiator by a first predetermined distance along a z-axis; 
     a first dielectric material having a low dielectric constant and disposed between the ground plane and the patch radiator; 
     a feed for providing the patch radiator with radio signal energy; and, 
     a second dielectric material having a relative dielectric constant greater than that of the first dielectric material for loading the feed and having a dimension along an axis orthogonal to the z-axis smaller than a dimension along a same axis of the patch and disposed between said patch radiator and said ground plane for determining operational characteristics of said microstrip patch antenna. 
     In accordance with another embodiment of the invention there is provided a microstrip patch antenna comprising: 
     a conducting ground plane having a thickness along a z axis and dimensions along an x and y axis orthogonal to the z axis; 
     a patch radiator spaced by a first dielectric material having a low dielectric constant from the ground plane along the z-axis orthogonal; 
     a slot feed for providing the patch radiator with radio signal energy across the space containing the first dielectric material; and, 
     a piece of second dielectric material adjacent the slot feed between the patch radiator and the ground plane for loading the feed and having a dimension along one of the x and y axes smaller than a dimension of the patch along a same axis, wherein the piece of second dielectric material determines operational characteristics of the microstrip patch antenna, the second dielectric material having a dielectric constant that is higher than the dielectric constant of the first dielectric material. 
     In accordance with another aspect of the present invention there is provided a method of designing a microstrip patch antenna comprising the steps of: 
     providing a design of a patch radiator; 
     providing a design of a conducting ground plane spaced from the patch radiator by a first predetermined distance along a z-axis; 
     providing a design for a feed for providing the patch radiator with radio signal energy; and, 
     providing a design for a second dielectric for loading the feed and having a dimension along an axis orthogonal to the z-axis smaller than a dimension along a same axis of the patch and disposed between said patch radiator and said ground plane for determining operational characteristics of said microstrip patch antenna simulating the provided designs; and, 
     adjusting the design of the second dielectric until a desired radiation pattern from the microstrip patch results. 
     Advantageously, an antenna according to the invention provides high speed, high efficiency, and reasonable bandwidth with reduced size over prior art air gap patch antennas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in conjunction with the following drawings: 
     FIG. 1 is a side view of the inverted microstrip antenna structure according to the prior art; 
     FIG. 2 is a side view of the inverted microstrip antenna structure similar to the antenna structure of FIG. 1 with a piece of dielectric material for loading the patch according to the invention; 
     FIGS. 3 a  and  3   b  illustrate in a top view a dielectric loaded microstrip patch configuration in accordance with an embodiment of the present invention; 
     FIGS. 3 c  and  3   d  illustrate in a front cross-sectional view a dielectric loaded microstrip patch configuration in accordance with an embodiment of the present invention; 
     FIG. 4 illustrates in a graph, a reduction of resonant frequency by dielectric loading obtained with a first embodiment of the present invention; 
     FIG. 5 illustrates in a graph, an increase in bandwidth by dielectric loading obtained with a second embodiment of the invention; 
     FIG. 6 a  illustrates in a top view and FIG. 6 b  illustrates in a front cross-sectional view a dielectric loaded microstrip patch configuration in accordance with an embodiment of the present invention; 
     FIG. 7 illustrates two graphs for measured return loss and impedance locus of a linear-polarised dielectric loaded patch; 
     FIGS. 8 a ,  8   b  and  8   c  illustrate graphically a measured radiation patterns of a circularly polarised dielectric loaded patch; and, 
     FIG. 9 illustrates graphically measured radiation patterns of a linear-polarised dielectric loaded patch. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a prior art air spaced patch radiator is shown. An inverted microstrip antenna structure is indicated generally at  101 . In its simplest form, a microstrip antenna comprises a radiator patch that is separated from a ground plane by a dielectric space. 
     In the prior art shown, the inverted microstrip antenna  101  comprises a radiator layer  106  that includes a thin substrate layer  107  made of a dielectric material having suitable dielectric and rigidity properties. Affixed to a bottom face of the substrate layer  107  is a radiator patch  109 , made of electrically conductive material. The radiator patch  109  is made by appropriate etching of the thin substrate layer  107  having one or both faces entirely coated with the conductive material. Alternatively, the radiator patch is affixed by one of several available means; for example, an elastic adhesive or glue is applied to the surface area formed by the contact of the substrate layer  107  and the radiator patch  109  to hold the radiator patch  109  securely in place. 
     As an alternative to etching and affixing, the radiator patch  109  may be formed directly on the substrate layer  107  using one of several different methods including mirror metallizing techniques, decal transfer techniques, silk screening, or other printed circuit techniques. 
     Supporting the radiator layer  106  is a ground plane  103  made of electrically conductive material having a plurality of integral support posts or dimples  105  extending substantially perpendicularly from one face of the ground plane  103 . 
     The sides of the inverted microstrip antenna  101  are not covered and, as a consequence, leave the space between the ground plane  103  and the radiator layer  106  exposed to the external environment. This can serve, at least in terrestrial applications, to reduce side wind loading and promote the drainage or evaporation of moisture located in the space. Similarly, one or more holes  108  can be established in the ground plane  103  and/or radiator layer  106  to reduce frontal and back wind loading on the antenna  101  or promote evaporation or drainage of moisture. Any holes  108  established in the ground plane  103  should be located and of a dimension that avoids producing a resonant structure with the radiator patch  109  that substantially reduces the efficiency of the antenna  101 . 
     The prior art antenna design is excellent when a patch is very closely spaced from the ground plane. Unfortunately, as frequency of operation increases, optimal spacing between the ground plane and the radiator increases. This results in coupling inefficiencies and trade-offs are made in antenna design to balance these trade-offs. 
     According to the present invention, an antenna design is presented that provides for a patch spaced from the ground plane for improved high frequency operation and having more efficient coupling with a feed than the previously described antenna with similar spacing. This is achieved by loading the patch antenna using a piece of dielectric material in order to improve coupling between a feed and the patch. 
     Referring to FIG. 2, a simple embodiment of the invention is presented. The embodiment has similar elements to the prior art antenna of FIG. 1 with the addition of a piece of dielectric material  115  having a higher dielectric constant than the dielectric material in the gap between the patch and the ground plane. The piece is shown in the form of a block. The dielectric block  115  is shown adjacent a feed  116  in the form of a slot fed by a microstripline  117 . The slot feed  116  is loaded by the dielectric block  115  which effects the field radiated from the slot. Careful selection of the dielectric block material, size, shape, and location results in an improved coupling between the slot  116  and the patch  109  even with substantial distances therebetween. By properly loading a patch, its operational characteristics including resonating frequency and its quality factor which is related to operational bandwidth are modified. This provides substantial control over coupling efficiency in a controlled geometric environment. 
     Referring to FIGS. 3 a ,  3   b ,  3   c , and  3   d , an embodiment of the present invention is shown including further pieces of second dielectric material  113  for effecting the Q factor further to shape radiation fed to the patch  109  in order to meet design criteria with a smaller radiating patch. 
     Loading of the patch is achieved using dielectric means in the form of dielectric strips  115  and  113 , which are strategically placed between the microstrip patch radiator  109  and the ground plane  103 . The antenna geometry shown comprises a conducting microstrip patch  109  suspended in air above a ground plane  103  at least one peripheral dielectric strip  113  having a dielectric constant ∈ r1  and a central dielectric strip  115  having a dielectric constant ∈ r2  and feeding means in the form of a feed network having a microstrip feed line  117  and a feed slot  116 . Though the term suspended in air is used above, the patch  109  is typically supported by a thin dielectric substrate. This allows for accurate patch spacing and size without substantially affecting efficiency of the antenna. The operating frequency of the antenna is determined by the dielectric permittivity, dimensions, and location of the peripheral dielectric strips  113 . The maximum effect will occur at the location where the electric field is maximum. The central dielectric strip  115  is used to perform the function of matching the impedance of the antenna to that of the feed network. In FIGS. 3 a ,  3   b ,  3   c , and  3   d , the feed network is represented by a feed slot  116  in the ground plane  103 , however, this approach is equally valid for other forms of feeding means to provide radio signal energy to the patch radiator  109 , including a probe and proximity coupled microstrip lines (not shown). 
     Preferred embodiments are described to illustrate the performance of dielectric loaded microstrip patches. In a first embodiment, dielectric loading is used to reduce a resonant frequency. A slot-fed square patch  109  having an unloaded operating frequency of 9.69 GHz, is loaded with a dielectric strip  115  (with a dielectric constant ∈ r2 =20) causing the resonant frequency of the loaded patch  109  to drop to 5.86 GHz, a reduction to 60% of the original frequency. These results are seen in the measured return loss plots of FIG.  4 . 
     In a second embodiment, dielectric loading is used for increasing the bandwidth of the patch. Here, the unloaded square patch  109  has a 10 dB return loss bandwidth of approximately 4%. When loaded with a dielectric strip  115  with a dielectric constant ∈ r =40, the bandwidth increases to approximately 21%. These results are seen in the measured return loss plots of FIG.  5 . 
     Referring to FIGS. 6 a  and  6   b , an embodiment of the invention is shown wherein the antenna is for radiating circularly polarised radiation. Loading of the slot  116   a  is achieved using dielectric means in the form of dielectric strip  115   a  and loading of the slot  116   b  is achieved using dielectric means in the form of dielectric strip  115   b , which are strategically placed adjacent the respective slots  116   a  and  116   b  between the microstrip patch radiator  103  and the ground plane  103 . The antenna geometry shown comprises a conducting microstrip patch  109  on a very thin substrate spaced above a ground plane  103  by an air dielectric. As shown in the cross sectional view, the thin dielectric layer  107  and patch  109  thereon are 6 mm away from the ground plane  103 . It will be evident to those of skill in the art that spacing of this magnitude with an air dielectric results in poor coupling efficiency between the feed slots  116   a  and  116   b  and the patch  109 . That said, increased spacing also results in higher bandwidth, which is often desirable. 
     A first peripheral dielectric strip  115   a  acts to modify the radiation field from the feed  116   a  and a second peripheral dielectric strip  115   b  acts to modify the radiation field from the feed  116   b . The first and second peripheral dielectric strips  115   a  and  115   b  have a dielectric constant ∈ r1 . Optionally each of the peripheral dielectric strips  115   a  and  115   b  has a different dielectric constant. The other dielectric strips  113   a  and  113   b  have a dielectric constant ∈ r2 . Optionally each of the dielectric strips  113   a  and  113   b  has a different dielectric constant. The other dielectric strips  113   a  and  113   b  act to reduce the overall size of the patch  109  for radiating at a predetermined frequency. The other dielectric strips  113   a  and  113   b  also act to reduce the overall bandwidth. Therefore, there is a design trade-off between operational bandwidth and size of the antenna. The feed slots  116   a  and  116   b  are coupled to microstrip feed lines  117   a  and  117   b , respectively, for providing energy to the feed slots  116   a  and  116   b.    
     The operating frequency of the antenna is determined by the dielectric permittivity, dimensions, and location of the peripheral dielectric strips  113   a  and  113   b . The maximum effect will occur at the location where the electric field is maximum. The slot loading dielectric strips  115   a  and  115   b  are used to perform the function of matching the impedance of the antenna to that of the feed network. In FIGS. 6 a  and  6   b , the feed network is again represented by feed slots  116   a  and  116   b  in the ground plane  103 , however, the invention is equally applicable for other forms of feeding means to provide radio signal energy to the patch radiator  109 . Examples of other feeds include a probe and proximity coupled microstrip lines (not shown). 
     Of course, where bandwidth is the only major concern, the dielectric strips  113   a  and  113   b  are omitted providing for a larger patch  109  and a larger bandwidth than when the dielectric strips  113   a  and  113   b  are present. 
     Theoretically, it is believed that the piece of dielectric material  115  loads the slot and thereby improves overall coupling of the feed  106  to the patch  109 . The dielectric material  115  is almost invisible to the patch  109  since it is loading the slot  106 . Two slots are shown in FIGS. 6 a  and  6   b  to achieve circular polarisation. Of course, a single slot could also be used if it were designed to excite circularly polarised radiation in the patch. One such embodiment involves a slot feed angled at approximately 45 degrees to each patch edge and positioned near a corner of the patch  109  (when viewed from above) to excite the patch  109  along each of its orthogonal axes. Generation of other forms of polarised radiation are also possible such as dual polarised radiation. 
     Conversely, the pieces of dielectric material  113  load the patch and act to reduce the resonant frequency of the patch  109 . This results in a smaller patch size for radiating at a same frequency. Placement of the pieces of dielectric material  113  is shown at the outer edges of the patch  109  (when viewed from above) in order to provide maximum E field loads. The pieces of dielectric material  113  could also be located outside the patch boundaries (when viewed from above) if design requirements are still met. Preferably the pieces of dielectric material  113  are tall and thin blocks or strips of dielectric material. Fatter blocks reduce bandwidth further and are therefore undesirable. Of course, optionally two pieces of dielectric material  113  are located at opposing ends of a same axis of the patch  109  as shown in FIGS. 3 a  and  3   b . Similarly, optionally, four pieces of dielectric material  113  are located, one along each edge of the patch  109 . 
     Referring to FIG. 7, two graphs are presented for measured return loss and impedance locus of a linear-polarised dielectric loaded patch according to the invention. 
     Referring to FIGS. 8 a ,  8   b  and  8   c , graphs are presented for measured radiation patterns of a circularly polarised dielectric loaded patch according to the invention. 
     Referring to FIG. 9, a graph is presented for measured radiation patterns of a linear-polarised dielectric loaded patch according to the invention. 
     A final design to determine the number of dielectric strips, their location, their dimensions, and dielectric permittivity depends on the intended operation of the antenna for the specific application. 
     Though according to the embodiment of FIG. 2, the radiator layer  106  is supported by a ground plane  103  made of electrically conductive material having a plurality of integral support posts or dimples  105  extending substantially perpendicularly from one face of the ground plane  103 , this need not be so. In an alternative embodiment, the support posts  105  are integral with the radiator layer  106  and extend substantially perpendicularly from one face thereof to contact the ground plane  103 . In yet another alternative embodiment, a portion of the support posts  105  are integral with the ground plane  103 , while the remainder are integral with the radiator layer  106 . In yet another embodiment, the support posts  105  are formed such that one or more of the posts are comprised of a first portion that is integral with the ground plane  103  and a mating second portion is integral with the radiator layer  106 . In any case, the support posts  105  support the radiator layer  106  to maintain a substantially uniform air gap  110  of a predetermined thickness between the radiator patch  109  and the ground plane  103 . In yet another embodiment, standard spacers in the form of posts not integral to either the ground plane or the patch substrate are used to position the patch relative to the ground plane. Optionally a further single support post with, for example, an annular shape, is utilised. 
     Even though in each embodiment of the invention described and illustrated the piece of dielectric material is a block or strip, this need not be so. The use of a block or strip is often simpler to model and therefore renders the design process less complicated. That said, it is also possible to use dielectric pieces of arbitrary shape or discontinuous pieces of dielectric material or pieces of dielectric material having other than constant dielectric values. 
     Also, though in each embodiment of the invention described and illustrated the piece of dielectric material for loading the slot feed is positioned directly on the feed slot, this need not be so. The dielectric material for loading the feed slot is positioned according to desired design parameters including loading properties and desired Q factor or Q factor changes. 
     The advantages to an air spaced patch are numerous. That said, many very low dielectric constant materials are known such as foams which are also useful in accordance with the invention. When a foam is used for filling the space between the patch and the ground plane, support posts are obviated. In a simple embodiment, the pieces of dielectric material are positioned on the ground plane according to design parameters and then the foam is injected to fill a space above the ground plane where the pieces of dielectric material are not present. The patch is either placed on top of the foam or, during injection, forms an upper layer to bound the foam dielectric material. 
     Numerous other embodiments can be envisaged without departing from the spirit or scope of the invention.