Abstract:
A speed measuring device that employs a Fresnel zone plate lens antenna. The Fresnel lens antenna is mounted to one end of a low profile collection housing, typically cylindrical in configuration. An opposite end of the collection housing includes a back plate having an opening. A transceiver unit is mounted to the outside surface of the back plate so that a transmitter and a detector within the transceiver are in communication with the opening. A signal is transmitted from the transceiver unit through the opening, and is directed by the lens antenna. A reflected signal is received and focused by the lens antenna, and collected by the housing to be directed through the opening to the transceiver.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a speed measuring device and, more particularly, to a speed measuring device including a low profile collection housing and a Fresnel zone plate lens antenna mounted thereto that focuses a transmit beam and a receive beam. 
         [0003]    2. Discussion of the Related Art 
         [0004]    Speed measuring devices have many applications in the art, such as vehicle speed detection. One type of speed measuring device uses RF signals and the Doppler effect to determine the speed of an object. A speed measuring device that uses the Doppler effect includes a transceiver that transmits a narrow band RF beam towards a target, and receives a reflected beam from the target. The reflected beam will be shifted in frequency from the transmitted beam relative to the speed of the target. Known speed measuring devices of this type typically employ an antenna horn that directs and focuses the transmitted beam from the transceiver, collects the reflected beam and focuses the reflected beam onto a detector in the transceiver. However, horn antennas typically have a long profile that is determined based on the frequency being transmitted, which adds significant size to the device. Further, horn antennas have a small aperture size, which reduces the system gain. 
       SUMMARY OF THE INVENTION 
       [0005]    In accordance with the teachings of the present invention, a speed measuring device is disclosed that employs a Fresnel zone plate lens antenna. The Fresnel lens antenna is mounted to one end of a low profile collection housing, typically cylindrical in configuration. An opposite end of the collection housing includes a back plate having an opening. A transceiver unit is mounted to the outside surface of the back plate so that a transmitter and a detector within the transceiver are in communication with the opening. A signal is transmitted from the transceiver unit through the opening, and is directed by the lens antenna. A reflected signal is received and focused by the lens antenna, and collected by the housing to be directed through the opening to the transceiver. The transceiver uses the Doppler effect to detect a difference in frequency between the transmitted beam and the reflected beam to determine the speed of a target from which the transmitted beam is reflected. 
         [0006]    Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a front view of a speed measuring device employing a Fresnel zone plate lens antenna, according to an embodiment of the present invention; 
           [0008]      FIG. 2  is a back view of the speed measuring device shown in  FIG. 1 ; 
           [0009]      FIG. 3  is an exploded rear perspective view of the speed measuring device shown in  FIG. 1 ; 
           [0010]      FIG. 4  is an exploded front perspective view of the speed measuring device shown in  FIG. 1  without the Fresnel zone plate lens antenna; 
           [0011]      FIG. 5  is a front view of a transceiver that is part of the speed measuring device of the invention; 
           [0012]      FIG. 6  is a plan view of the front of a Fresnel zone plate lens antenna showing the radius of the zones relative to the lens focal point; and 
           [0013]      FIG. 7  is a schematic diagram of the speed measuring device of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0014]    The following discussion of the embodiments of the invention directed to a speed measuring device employing a Fresnel zone plate lens antenna is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
         [0015]      FIG. 1  is a front view,  FIG. 2  is a rear view,  FIG. 3  is an exploded rear perspective view and  FIG. 4  is an exploded front perspective view of a speed measuring device  10 , according to an embodiment of the present invention. The device  10  includes a cylindrical collection housing  12  having a forward facing edge  14 , a side plate  28  and a back plate  16 , where a cavity  30  is defined within the housing  12 . In this non-limiting embodiment, the collection housing is made of aluminum, and is operable to reflect RF waves. Alternately, the collection housing  12  can be made of a low weight and durable material, such as a suitable plastic, and an inside surface or an outside surface of the collection housing  12  can be coated with a metal layer to provide the wave reflection. The diameter of the collection housing  12  determines the gain of the device  10 . In one non-limiting embodiment, the collection housing  12  has a diameter in the 4-5 inch range and a height of about 2 inches. 
         [0016]    A planar Fresnel zone plate lens antenna  20  is mounted, here by bolts, to a shoulder  18  provided in the collection housing  12 .  FIG. 4  shows the device  10  without the Fresnel lens antenna  20  mounted thereto so as to show the interior cavity  30  of the collection housing  12 . The back plate  16  includes a square opening  22  through which the transmitted and reflected RF beams propagate. A transceiver module  24  is mounted, here by bolts, to the back plate  16  so that an opening  26  in the transceiver module  24  is in communication with the opening  22 .  FIG. 5  is a front view of the transceiver module  24  separated from the device  10 . In one non-limiting embodiment, the speed measuring device  10  operates in the K band, i.e., 12-93 GHz, where the K a  band is in the 18-40 GHz range and is mainly used for radar and general communications, and the K u  band is the 12-18 GHz range and is mainly used for satellite communications. 
         [0017]    The Fresnel lens antenna  20  is made of a circular planar dielectric material and includes a plurality of spaced apart metallized rings  32  separated by dielectric rings  38 . In this non-limiting embodiment, the lens antenna  20  includes two metallized rings  34  and  36 , where the interior ring  34  is wider than the exterior ring  36 , but where the area of the rings  34  and  36  is about the same. The width of the rings  32  determines the focal length of the lens antenna  20 . In one embodiment, the material of the lens antenna  20  is a low-loss dielectric material, such as polystyrene, and the rings  32  are deposited thereon by a suitable deposition process. In one non-limiting embodiment, the lens antenna  20  has a dielectric constant of ε r =4.7+0.03j. The thickness of the lens antenna  20  is determined by choosing low reflection coefficients from wave transmitting simulations through multi-layered dielectrics. In one non-limiting embodiment, the Fresnel lens antenna  20  has approximately a 22 dB gain, a diameter of about 4.0 inches and a thickness of about one-eighth of an inch. 
         [0018]    The operation of a Fresnel lens is well understood to those in the art. An RF beam propagates through the dielectric rings  38  between the metallized rings  32 , and is prevented from propagating through the metallized rings  32  of the lens antenna  20 . Therefore, a Fresnel lens can be designed so that the parts of the beam that are at one phase are blocked, and the parts of the beam that are in phase with each other pass through the lens antenna  20  and can be combined. 
         [0019]    For a phase-reversing zone plate, the successive radius of the zones (rings) are chosen so that the distance from a selected focal point, such as the opening  22 , on the central axis increases by one-half the wavelength of the center frequency of the beam going from the inner radius to the outer radius of any ring  32 . This is illustrated in  FIG. 6  where a Fresnel lens  40  is shown including opaque rings  42  separated by dielectric rings  44 . The distance between the focal point  46  and the center of the lens  40  is shown as distance D, the radius of the rings  42  are shown as radius R from the focal point  46  and the wavelength of the RF signal is A. As the RF beam propagates through the lens antenna  20  it is defracted at the edges of the rings  32 , and focuses at the focal point at the opening  22 . 
         [0020]    The following equations are used to define the size of the zones for phase calculation purposes. 
         [0000]        R   i1   =D+ 0.5λ  (1) 
         [0000]        r   i1   =√ {square root over (R i1   2   −D   2 )}  (2) 
         [0000]        R   o1   =D+λ   (3) 
         [0000]        r   o1   =√ {square root over (R o1   2   −D   2 )}  (4) 
         [0000]        R   i2   =D+ 1.5λ  (5) 
         [0000]        r   i2   =√ {square root over (R i2   2   −D   2 )}  (6) 
         [0000]        R   o2   =D+ 2λ  (7) 
         [0000]        r   o2   =√ {square root over (R o2   2   −D   2 )}  (8) 
         [0021]    In this non-limiting embodiment, the transceiver module  24  includes a Gunn diode transceiver  48 . The transceiver module  24  is a commercially available integrated module with a Gunn diode mounted in a cavity for the transmitter and one or two Shottkey barrier diode in the receiver. In one non-limiting embodiment, the transceiver module  24  is one of several modules available from MDT depending on the transmit frequency. An IF output is generated whose frequency is proportional to the targets velocity. With the two-mixer design, the direction-of-motion is obtained as a phase difference between the two intermediate frequency (IF) outlets. The Doppler sensor within the transceiver module  24  has about 5 mWs of output power and supports dual IFs, which are capable of detecting the direction of the moving object. 
         [0022]      FIG. 7  is a schematic block diagram of a speed measuring system  50  including an antenna  52  representing the Fresnel lens antenna  20 , and a K-band Doppler transceiver  54 , representing the transceiver module  24 . The system  50  includes an IF processing circuit  56  including an IF amplifier  58 , a receive signal strength indicator (RSSI) processor  60  and an attenuator/amplifier  62 . The RSSI processor  60  measures the amplified IF signal. Based on the measured RSSI value and a predetermined threshold, the attenuator/amplifier  62  operates as either an attenuator to reduce the strength of the signal or as an amplifier to increase the strength of the signal so that the signal is substantially constant for signal processing. The strength of the reflected signal will depend on how close the target is to the system  50 . The conditioned signal from the attenuator/amplifier  62  is passed to a digital controller  64 , such as a DSPIC  30 . The digital controller  64  includes an analog-to-digital converter and a digital signal processor functioning with a 16-bit microcontroller architecture. The controller  64  converts the received analog signal to a digital signal and converts the signal from the time domain to the frequency domain using a fast Fourier transform (FFT) with implemented software. The power spectral density of the processed signal is then analyzed for frequency content, which indicates the Doppler shifted frequency of the target. 
         [0023]    The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.