Abstract:
An antenna unit formed in the shape of a hollow box comprising (a) a substrate forming the front side of the antenna unit, (b) a first microstrip antenna array formed on the substrate, (c) a second microstrip antenna array formed on the substrate, (d) a ground plane forming the rear side of the antenna unit, and (e) a plurality of periodic filters formed on the ground plane. The periodic filters are formed by etching a series of circular patterns, or holes, through the ground plane. The periodic stop band filters provide for improved isolation between the microstrip antenna arrays, without the need for adding additional costly or space consuming components.

Description:
FIELD OF THE INVENTION 
   The present invention relates to antennas, and more specifically to microstrip antenna arrays enhanced with periodic filters. 
   BACKGROUND OF THE INVENTION 
   The use of complex electronic systems in automobiles has increased dramatically over the past several years. Radar systems have been used in advanced cruise control systems, collision avoidance systems, and hazard locating systems. For example, systems are available today that inform the driver if an object (e.g. child&#39;s bicycle, fire hydrant) is in the vehicle&#39;s path even if the object is hidden from the driver&#39;s view. 
   Systems such as these utilize small radar sensor modules that are mounted somewhere on the automobile (e.g., behind the front grill, in the rear bumper). The module contains one or more antennas for transmitting and receiving radar signals. These devices work by transmitting radio frequency (RF) energy at a given frequency. The signal is reflected back from any objects in its path. If any objects are present, the reflected signal is processed and an audible signal is sounded to alert the driver. One example of this type of radar system is the 24 GHz High Resolution Radar (HRR) developed by M/A-Com Inc. (Lowell, Mass.). 
   The radar sensor units used in these systems typically utilize two independent antenna arrays. A first array is used to transmit the outbound signals, and a second antenna array is used to receive the reflected return signals. The two antenna arrays are formed on a single substrate and are generally separated by a space of three to four inches. 
   Microstrip antenna arrays are often used in this type of application because they have a low profile and are easily manufactured at a low cost. In addition, microstrip antenna arrays are versatile and can be used in applications requiring either directional or omni-directional coverage. Microstrip antenna arrays operate using an unbalanced conducting strip suspended above a ground plane. The conductive strip resides on a dielectric substrate. Radiation occurs along the strip at the points where the line is unbalanced (e.g., corners, bends, notches, etc.). This occurs because the electric fields associated with the microstrip along the balanced portion of the strip (i.e., along the straight portions) cancel one another, thus removing any radiated field. However, where there is no balance of electric fields, radiation exists. By controlling the shape of the microstrip, the radiation properties of the antenna can be controlled. 
   Slot-coupled microstrip antennas arrays comprise a series of microstrip patch antennas that are parasitically coupled to a feed microstrip. The feed microstrip resides below the ground plane and is coupled to each of the patch microstrips through a slot in the ground plane. Various numbers of patch antennas can be coupled to a single microstrip input feed to form the array. Six-element arrays and eight-element arrays are commonly used in High Resolution Radar (HRR) sensors, although any number of patch elements can be coupled to the feed microstrip. 
   One problem that arises using this type of antenna design is that the transmit and receive antenna arrays are not perfectly isolated from each other. There is some level of RF signal leakage between the two antenna arrays, either through the air or through the substrate material. The leakage through the substrate is caused by undesired surface wave propagation. This coupling effect between the two antenna arrays lowers antenna gain and reduces performance of the radar sensor. 
   Presently, several techniques are used to improve isolation between microstrip array antennas. Two techniques are shown in FIG.  1 . The first technique, shown in  FIG. 1   a , involves placing a metal wall  11  in the antenna unit  10  between the transmitting antenna array  13  and receiving antenna array  15 . The metal wall  11  improves the isolation between the two microstrip array antennas by blocking or reflecting back signals passing through the air within the cavity  17  formed within the antenna unit  10 . While using a wall  11  such as this will improve isolation between the two antennas, it has several drawbacks. First, the addition of a metal wall  11  in the antenna unit  10  consumes additional space and is cumbersome. As antenna units are becoming increasingly smaller, it is undesirable to introduce an additional space consuming component. Secondly, the isolation achieved by inserting the metal wall  11  is not as high as desired (only about 4 dB improvement in the isolation is obtained). Much of the signal leakage occurs through the substrate rather than by radiated signals traveling through the air within the antenna unit  10 . The metal wall  11  does not sufficiently block any signal coupling which occurs via the substrate layer. 
   A second technique used to provide isolation is illustrated in  FIG. 1   b . This technique involves placing a section  12  of a signal absorbing material in the cavity  18  formed between the transmitting antenna  14  and the receiving antenna  16  within the antenna unit  20 . For example, a section  12  of Eccosorb GDS sheet (Emerson &amp; Cuming Microwave Products, Inc., Randolph, Mass.) can be placed between the antennas to absorb radiation within the unit  20  and thus improve isolation between the antennas. However, this technique also has limitations. While the absorbing materials such as Eccosorb GDS provide an improvement in isolation over the metal wall (about 8 dB improvement in the isolation is obtained), the isolation is not as complete as desired. In addition, the absorbing materials are high in cost. 
   Despite attempts to improve isolation between antennas within an antenna unit using these techniques, often the level of isolation achieved proves to be insufficient. Accordingly, there is a need for an antenna unit that provides a high level of isolation between the antennas, while at the same time is compact, cost efficient, and achieves a high level of gain. The present invention fulfills these needs among others. 
   SUMMARY OF THE INVENTION 
   The present invention provides an antenna unit that improves isolation between a plurality of microstrip antenna arrays while also increasing the radiation gain of each antenna array. This is accomplished by etching a series of openings into the ground plane of an antenna unit comprising at least one slot coupled microstrip antenna array. The openings are configured in such a manner as to act as periodic stop band filters between the antennas. The filters suppress the surface waves propagating from each antenna array, thus increasing the gain of each respective slot coupled microstrip antenna array and the isolation (between two antenna arrays). 
   The openings are arranged in a series of rows and columns. The configuration and positioning of the openings in the ground plane determines the characteristics of the filter. The consistent spacing between the openings results in the periodic nature of the filters with the frequency of the stop band depending upon the spacing chosen. The width of the stop band is determined by the area of the openings. 
   One aspect of the present invention is an automotive sensor unit comprising two microstrip antenna arrays wherein the microstrip antenna arrays have a measured isolation with respect to each other of at least −30 dB in the frequency bandwidth of operation for an HRR sensor (22 to 26 GHz). More preferably, a measured isolation of the antenna arrays with respect to each other of at least −40 dB, or even more preferably of at least −50 dB, can be obtained. In a preferred embodiment, the antenna unit is formed in the shape of a hollow box, and comprises (a) a substrate forming the front side of the antenna unit, (b) a first microstrip antenna array formed on the substrate, (c) a second microstrip antenna array formed on the substrate, (d) a ground plane forming the rear side of the antenna unit, and (e) a plurality of periodic filters formed on the ground plane. The periodic filters are formed by most easily formed etching a series of circular patterns, or holes, through the ground plane. Openings of various other shapes can also be used to produce the filters. The periodic stop band filters provide for improved isolation between the microstrip antenna arrays, without the need for adding additional costly or space consuming components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  is perspective view of an antenna unit using a metal wall for isolation between two microstrip array antennas, in accordance with the prior art. 
       FIG. 1   b  is perspective view of an antenna unit using a section of Eccosorb GDS material for isolation between two microstrip array antennas, in accordance with the prior art. 
       FIG. 2  is a top view of an antenna unit in accordance with the present invention. 
       FIG. 3  is a cross-section of the antenna unit shown in  FIG. 2  in accordance with the present invention. 
       FIG. 4  is a perspective view of an antenna unit in accordance with the present invention. 
       FIG. 5   a  illustrates an antenna unit comprising a slot coupled microstrip antenna array in combination with a series of periodic filters in accordance with an additional embodiment of the present invention. 
       FIG. 5   b  is a graph of the gain pattern achieved using the antenna illustrated in  FIG. 5   a.    
       FIG. 6   a  illustrates a comparative antenna unit comprising a slot coupled microstrip antenna array without the addition of periodic filters, in accordance with the prior art. 
       FIG. 6   b  is a graph of the antenna gain pattern achieved using the antenna illustrated in  FIG. 6   a.   
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 2 , a top view of a preferred embodiment of an antenna unit  30  in accordance with the present invention is shown. The antenna unit  30  contains a transmit slot-coupled microstrip antenna array (TX antenna)  21  and a receive slot-coupled mictrostrip antenna array (RX antenna)  23 . The embodiment illustrated in  FIG. 2  contains two slot coupled microstrip antenna arrays, although the invention is not limited to units having two slot-coupled microstrip antenna arrays. The invention may be practiced with antenna units comprising any number of slot coupled microstrip antenna arrays, or comprising any number of other types of microstrip antenna arrays, or units comprising a combination of both. 
     FIG. 3  shows a cross-section of the antenna unit layers shown in  FIG. 2 , as viewed along cut-line  3 — 3 . The elements within the antenna unit  30  are formed on a multi-layer substrate  32 . Each slot coupled microstrip antenna array comprises a feed microstrip  45  and at least one microstrip patch  39 . The feed microstrip  45  is formed on the inside of a first layer  31  of the multilayer substrate  32 . In the illustrated embodiment, the first layer  31  comprises a layer of 254 micrometer thick Duriod, although the invention may be practiced with other material types. 
   A ground plane  41  resides between the first substrate layer  31  and a second substrate layer  33 . The ground plane  41  comprises an electrically conductive layer of copper. The second substrate layer  33  of 787.4 micrometer thick FR4 resides on top of the ground plane  41 . The FR4 layer  33  acts as a support layer for the Duroid first substrate layer  31 . FR4 material is an inexpensive substrate, thus, it is a favored choice as a carrier layer for support, although various other materials could also be used. 
   A third layer  35  comprising a one millimeter thick radome is formed on the outer surface of the multilayer substrate  30 . The radome can be made of any low loss plastic material. Microstrip patches  39  are etched on a very thin dielectric film (e.g., Kapton) affixed either to the top surface of the second substrate (FR4) layer  33  or the bottom surface of the third (radome) layer  35 . The second substrate (FR4) layer has openings directly underneath the patches  39  which lowers dielectric loss and thus increases the gain of the antenna. 
   The multilayer substrate  32  is positioned within the casing of antenna unit such that an air gap  37  exists between the substrate  32  and the rear or floor  47  of the casing that forms the antenna unit  30 . The overall shape of the antenna unit is shown in FIG.  4 . Referring to  FIG. 4 , the casing  49  of the antenna unit  30  is formed in the shape of an open-faced box. Preferably, the casing comprises a metal material, which prevents radiation from the slots from traveling backward by acting as a reflector. The multilayer substrate  32  serves to close the box by acting as the front face of the unit  30 , creating the air gap  37  between the substrate  32  and the floor  47  of the casing which acts as the rear of the unit  30 . 
   Referring again to  FIG. 2 , a series of openings are shown situated between the RX antenna  23  and the TX antenna  21 . These openings comprise holes  43  etched in the ground plane ( 41  as shown in  FIG. 3 ) of the antenna unit  30 . The holes  43  form periodic stop band filters by suppressing surface waves from the microstrip antenna arrays  21 ,  23 . The period of the filters is determined by the relative spacing of the holes  43  with respect to each other. The stopband center frequency is a function of the period of the structure (i.e., the distance between the rows of holes in the ground plane). The center frequency is approximately velocity divided by twice the period as measured by the distance between the holes. For example, the embodiment illustrated in  FIG. 2  comprises a grid pattern of 8 rows each containing 14 holes. The distance between each row is 3.5 millimeters. This results in a center frequency of approximately 24 GHz, which is desired for HRR applications. 
   The width of the stop band and the attenuation in the stop band are dependent upon the radii of the etched holes  43 . For smaller circle radii, the width of the stop band and attenuation are very small. This follows under the theory that, as the radii of the holes  43  approach zero, the stop band width approaches zero. In other words, the stop band disappears when the holes disappear. The preferred range of radii of the holes for 24 GHz applications is between 1 mm and 1.5 mm. In the embodiment shown in  FIG. 2 , a hole diameter of 1.4 millimeters has been chosen. This provides a stop band sufficiently wide around the critical frequency (24 GHz in a preferred embodiment) to suppress the surface waves and improve the isolation and gain of the antenna. The stop band extends a minimum of 6 GHz on either size of 24 GHz (12 GHz width). 
   In some applications, RF circuits can be located on the rear side of the first substrate layer  31 . Some of these circuits can require a solid ground plane to work properly. This can prevent the openings from being etched on the ground plane  41 . In such instances, the openings can be etched on a metalized plane located on the top surface of the second substrate layer  33  on the bottom surface of the third (radome) layer  35 . While moving the openings off of the ground plane  41  will cause the performance of the antenna to be reduced, it allows the invention to be practiced in units that contain RF circuitry on the rear side of the first substrate layer  31 . 
   A second embodiment of the present invention is shown in  FIG. 5   a .  FIG. 5   a  illustrates an antenna unit  50  comprising a single eight-element slot-coupled microstrip antenna array  51 . The slot coupled microstrip antenna array  51  is constructed according to the configuration described for the two array embodiment (as shown in FIG.  3 ). Periodic filters in the form of holes  53  etched in the ground plane reside on both sides of the array  53 . Isolation from a second antenna array is not a concern in this embodiment, as the antenna unit  50  contains only a single antenna array  51 . However, the periodic filter serve an additional purpose. By suppressing the surface waves generated by the antenna array  51 , the gain of the antenna was increased.  FIG. 5   b  shows the gain pattern simulated at 24 GHz for the antenna in accordance with the embodiment shown in  FIG. 5   a . In contrast,  FIG. 6   a  shows a slot coupled microstrip array antenna  61  without periodic filters etched into the ground plane, with the corresponding gain pattern simulated at 24 GHz shown in  FIG. 6   b . By comparing the two gain patterns, it can be observed that the periodic filters increase the gain of the antenna array. At zero degrees, a computed gain  55  of 15.8 dBi for an antenna unit  50  in accordance with the present invention is compared to a computed gain  65  of 13.8 dBi for an antenna unit  60  that does not have the periodic filters etched in the ground plane. Thus, an increase of about 2 dBi is obtained using holes etched in the ground plane in accordance with the present invention. 
   The antenna unit in accordance with the present invention suppresses undesired surface waves associated with the uses of slot coupled microstrip antenna arrays by using periodic filters etched into the ground plan. By doing so, an increase in isolation between slot coupled microstrip antenna arrays. In the preferred embodiment illustrated in  FIG. 2 , two slot coupled microstrip antenna arrays are separated by a distance of 40 millimeters and have a series of rows of filters etched between them, with each row containing 8 filters. Isolation between the antenna arrays (measured between 22 GHz and 26 GHz) was greater than −30 dB for all frequencies within the measured range. It was measured at greater than −40 dB for some frequencies within this range, and greater than −50 dB for other frequencies within this range. In addition, increased gain of the slot coupled antenna arrays occurs over the same frequency range. 
   It should be understood that the foregoing is illustrative and not limiting and that obvious modifications may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, the specification is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined in the following claims.