Abstract:
In a radar system, harmonic excitation of an antenna is carried out in different frequency ranges. The antenna characteristic which varies as a function thereof is used to analyze different solid angle ranges around an object.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to a radar system, in particular for measuring distance and/or speed in motor vehicles, in which harmonics of a fundamental frequency are used. 
   BACKGROUND INFORMATION 
   A pulse-Doppler radar system for measuring distance or speed in a motor vehicle is described in U.S. Pat. No. 6,362,777. Therein, a multiplier or a mixer is provided in the transmit path for supplying a signal having double the frequency of a reference oscillator to the transmitting antenna. Frequency doubling is carried out there in order to be able to use a reference oscillator having a lower frequency and thus more stable behavior. 
   SUMMARY OF THE INVENTION 
   Using principles of the present invention, i.e., the varying antenna characteristic resulting from harmonic excitation of the same antenna in different frequency ranges is used to analyze various solid angle ranges around an object, different radar analyses are possible without changing the hardware. 
   Instead of using a plurality of radar sensors for different applications, e.g., long-range radar, short-range radar, park assist systems, stop and go, etc., different applications using just one radar sensor may be implemented via the measures according to the present invention. 
   Frequency generation and modulation are achievable at lower frequencies in a cost-effective and stable manner. Just one frequency multiplier is required, which may have active or passive circuits. The amplifiers and mixers required may either be switched over within the frequency, or advantageously have a broadband design without switch-over. For the mixer, simple and thus cost-effective sub-harmonic mixer designs may be used. In the case of speed measurement, the Doppler effect is multiplied. The dynamic range may therefore be reduced or expanded. 
   Harmonic excitation of the same antenna constitutes a particular advantage. Usually, resonant antennas are one half of a wavelength long. They may nonetheless be excited on all harmonics. The radiation angle then varies with higher order (from perpendicular in the direction of wire/patch). Thus in the case of vehicle systems used for all-round view, the area in front of the vehicle may be covered at the lower frequency, and additionally the area to the side at the higher frequency. By optimizing the design and phase position it is possible to cover any desired direction. Furthermore, a plurality of basic elements (dipole/patch) may be used for the antenna system to achieve the desired power and antenna characteristic. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram for a multiple-frequency radar system. 
       FIG. 2  shows a broadband layered antenna for use in the radar system shown in  FIG. 1 . 
       FIG. 3  shows a section of the antenna system shown in  FIG. 2 . 
       FIG. 4  shows a view of the antenna system shown in  FIG. 2  with the patch removed. 
       FIG. 5  shows an antenna characteristic when excited using the fundamental wave. 
       FIG. 6  shows an antenna characteristic when excited using the third harmonic. 
       FIGS. 7 ,  8  and  9  show embodiments of broadband biconical antennas. 
   

   DETAILED DESCRIPTION 
   For the radar system according to the present invention, a system as shown in  FIG. 1  is advantageously used. A reference oscillator  1 , which is modulated by a baseband modulator  2 , is connected to transmitting antenna  5  via a switchable frequency multiplier  3  and downstream amplifier  4 . The received radar signal, which is reflected off at least one object, passes from receiving antenna  6  to a down mixer  8  via a low noise amplifier  7  (LNA=low noise amplifier). Using the output signal of frequency multiplier  3 , the down mixer converts the received antenna signal into a low-frequency analysis signal, which is subjected to further processing in unit  9 , in particular via analog-digital conversion. Like amateur radio practices, the frequency ranges in which the radar system according to the present invention operates utilize the harmonics, the bands 3.5, 7, 14, 21, and 28 MHz as well as 144, 432, and 1296 MHz being used. Frequency generation and modulation advantageously are carried out at low frequencies (fundamental wave). Only frequency multiplier  3  and mixer  8  and the amplifiers need to be designed to handle high-frequency signals. They may either be switchable within the frequency, or advantageously have a broadband design without switch-over. For mixer  8  cost-effective sub-harmonics mixer designs may also be used. The radar system may operate via pulse, continuous wave (CW), frequency modulated CW (FMCW), or via mixed types of operation. For pulse operation, a controlled switch is required in the transmit path, and in the receive path a switch of the same type is also required, this being operable relative to the switch in the transmit path subject to a delay equal to the propagation time of the radar pulse for a predefined distance zone. 
   A broadband layered antenna having a transmitting and/or receiving dipole, in front of which an electrically coupled patch element is located at a predefined distance from the dipole, is suitable as the transmitting and/or receiving antenna.  FIG. 2  shows the basic design of such an antenna system, which is described in detail in German Patent No. DE 103 53 686.3. Patch  10 , a small rectangular metal plate, is located parallel to the layering of antenna system  11  at a distance of approximately 0.1 times the fundamental wavelength of the transmitted beam at 26 GHz, above flat dipole  12  on the layer configuration. The distance is not limited to the aforementioned dimensioning, but rather may vary. A range of 0.01 to 0.2 times the wavelength is suitable. Patch  10  is for example attached to the device housing (not shown) above and clear of dipole  12 , or is attached to dipole  12  via a foam layer (see  FIGS. 3 and 4 ). Dipole  12  includes two separate, symmetrical, rectangular metal surfaces, which are situated on a dielectric substrate  13 , e.g., a circuit board, a ceramic material, or a softboard material. The halves of the dipole each have a length of about one quarter of the fundamental wavelength. The wavelength is evaluated not in air but rather as effectively loaded by the dielectric. 
   Each individual dipole half is fed via a signal supply conductor  14  (open two-wire line, known as “chicken-ladder”). The two signal supply conductors  14  are situated parallel to one another and thus form a differential input. They extend across the surface of substrate layer  13  and are for example printed or etched. A metallic ground layer  15 , which screens off the radiation, is applied on substrate layer  13 , the ground layer having recesses only in the area of signal supply conductors  14  and dipole  12 . In addition, screening metallic ground layer  16  covers the entire area of the rear side (not shown) of the antenna system. Dipole  12  and patch  10  are situated parallel to one another, and the two signal supply conductors  14  extend perpendicular thereto. Thus the field vectors of the electrical field of dipole  12 , patch  10 , and supply conductors  14  are situated parallel to one another and point in the same direction. As shown in  FIG. 3 , the inner edges of the separate halves of the dipole are in contact with signal supply conductors  14 . Metallic chamber strips  17  (indicated by broken lines) are located in the layers beneath ground layer  15 , and extend to rear-side ground layer  16 . These chamber strips  17  conductively connect the two outer ground layers  18  and surround dipole  12  except for a through-opening for signal supply conductors  14 . This ground screening largely suppresses lateral radiation. The surrounding chamber strips  17  are at a distance from dipole  12  of one quarter of the wavelength of the transmitted radiation. Radiation beamed into substrate  13  is reflected off chamber strips  17  and fed back in the correct phase sequence. 
   The resonant length of patch  10  is from left to right. In contrast to ordinary patch antennas the patch is longer than it is wide here. On the fundamental wave the resonant length is one half of a wavelength. In the case of excitation on harmonics of this fundamental wave, at approximately 26 GHz, the radiation angle varies with higher order. The resonant length(s) of the dipole and/or the patch element(s) is/are then greater than one half of the operating wavelength. Thus for vehicle systems used for all-round view, excitation in a low frequency range provides coverage in front of the vehicle, i.e., distances to objects in a direction perpendicular to the antenna exciter surface may be measured, and at higher harmonics of the fundamental frequency solid angle ranges to the side of the vehicle may also be analyzed. This is advantageous in particular for park assist systems or for determining the distance to the edge of the lane. By optimizing the design and phase position it is possible to cover any desired direction. Furthermore, a plurality of basic elements (dipole/patch) may be used for the antenna system to achieve the desired power and antenna characteristic. Moreover, antenna characteristics having differing harmonic frequency ranges may also be used to provide a joint analysis profile. In this case, for example, the analysis profile is stored during excitation on the fundamental frequency and correlated with a current analysis profile on a higher harmonic frequency. Further harmonics may be used for harmonic excitation of the antenna, e.g., N=1, 2, 3 or N=1, 3, 5 or N=1, 2, 4, 8 etc., or alternatively it is possible to use only harmonics and not the fundamental wave, e.g., N=2, 3 or N=3, 5. 
   To set the desired degree of multiplication N, frequency multiplier  3  and if necessary down mixer  8  are controlled accordingly by unit  9 . The antenna characteristic of the above-described antenna with regard to the fundamental wave, i.e., at about 26 GHz, is shown in  FIG. 5 . In the case of the fundamental wave, the patch beams forward (z direction) perpendicular to the patch surface. The gain relative to an omnidirectional radiator in the z direction is 8.18 dBi. No lateral minor lobes are created. In the case of higher direct harmonic excitation, this direction occupies a zero position, and the radiation is shifted based on an angle determined by harmonic number N.  FIG. 6  shows the antenna characteristic in the case of excitation on the third harmonic at approximately 78 GHz. Four main lobes are created, which are rotated by a fixed angle symmetrically relative to the z direction, as well as smaller minor lobes. 
   The antenna&#39;s bandwidth may be increased by designing the dipole and/or patch element as biconical, which is advantageous in particular in the case of excitation using higher harmonics, since the modulation signal is also multiplied. Exemplary embodiments of this kind are shown in  FIGS. 7 through 9 . In  FIG. 7 , patch  10  and dipole  12  are both biconical. In  FIG. 8 , dipole  12  is biconical and the patch is rectangular. In  FIG. 9 , patch  10  is biconical and dipole  12  is rectangular. 
   The above-described patch configurations are merely examples, and many other types are possible, e.g., individual patch, coupled patches, waveguide radiators, printed wires or surfaces etc. Asymmetrical excitations are also possible.