Patent Publication Number: US-2006017649-A1

Title: Helical antenna with integrated notch filter

Description:
TECHNICAL FIELD  
      This invention relates in general to an antenna and more particularly to a helical antenna with integrated notch filter used with a portable two-way radio for mitigating out of band interference.  
     BACKGROUND  
      Portable radio equipment is commonly used for many public service applications such as police, fire and other governmental service organizations. These public service organizations often operate in a licensed radio spectrum typically in the 800-900 MHz range. Typically, this spectrum is very crowded not only with public safety applications but also with commercial vendors offering cellular telephone services. This often creates a problem with portable radio equipment used by public safety users since the strong cellular signals from powerful cell sites and the like create strong intermodulation distortion products associated with their own radio frequency signals. These distortion products can interference with the receiver in the portable radio and may at times render the radio useless. This occurs since the radio receiver front end circuitry becomes overloaded from the strong off-channel interference, rendering it unable to receive incoming voice or data.  
      One way to reduce and/or eliminate the incoming distortion products caused by adjacent channel interference is through the use of discrete filters used in connection with incoming signals at the front end of the radio receiver. One application of this technology is to directly filter the incoming interfering signals at the radio antenna. Prior art designs have incorporated filters with the antenna by using discrete components in a circuit board design that is built into the antenna. Although this solution helps to attenuate incoming distortion products at the receiver, this is an expensive solution. The antenna must use a special design which incorporates discrete circuit components such as inductors and capacitors that form the discrete filter. Moreover, the components integrated with these filers often are not capable of handling the high transmitter power requirements for a given portable radio. This causes the filter circuit components to ultimately fail, requiring expensive repair or replacement of the antenna.  
      Consequently, the need exists to provide an inexpensive antenna design which incorporates a filter that can effectively reduce incoming radio frequency distortion products from strong adjacent channel interferers.  
     SUMMARY OF THE INVENTION  
      Briefly, according to the invention, there is provided a two-pitch helical antenna with center feeding where the antenna&#39;s shape effectively forms a notch filter used for filtering distortion products from the front end of a portable communications receiver. The antenna includes a helical element positioned above a feed point that is tuned to a passband frequency. The electrical length of the helical element is a quarter wavelength, half wavelength, or appropriately tuned such that it synthesizes the passband characteristic of a 50 ohm terminated parallel tuned circuit for the desired passband. The helical element below the feed point should provide a short circuit condition as the desired stop band frequencies are either an open ended quarter wave in electrical length or shortened half wave length in electrical length. Both elements form a second order notch filter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings in which like reference numerals identify like elements, and in which:  
       FIG. 1  is a front elevational view of the helical antenna with integrated notch filter according to the preferred embodiment of the invention.  
       FIG. 2  is a close-up elevational view of the helical antenna illustrating the pitch length and wire diameter of the radiating element as seen in  FIG. 1 .  
       FIG. 3  is a top view of the helical antenna illustrating the helix diameter of the radiating element as seen in  FIG. 1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward.  
      Referring now to  FIG. 1 , the helical antenna with integrated notch filter  100  includes a main helical radiating element  101 . The radiating element  101  is a tuned element that offers both a tuned inductance formed by the coil of the radiation and a tuned capacitance formed by the distance between the metallic radiating elements. This distributed inductance and capacitance work to form a tuned notch filter operating at some predetermined frequency. As will be recognized by those skilled in the art, the notch filter works to reduce and/or eliminate a predetermined range of frequencies that present themselves at the antenna  100 .  
      An open ended quarter wavelength helical resonator  103  is attached at the proximal end of the radiating element  101 . The quarter wavelength helix resonator  103  is a coil formed with and connected to the radiating element  101  that works to match the radiating element  103  to a feed point  105 . The feed point  105  connects with a feedline or stripline connection  107  that can be either directly or parasitically coupled to the feed point  105 . Preferably, the feed point  105  should match a 50 ohm non-reactive load impedance.  
      The helical antenna  100  offers a number of distinct advantages of prior art antenna systems since the intrinsic characteristics of the helical radiating element  101  provides a frequency response of a second order band stop filter. This configuration creates a shunt helix resonator in the radiating element  101  with a quarter wave helix  103 . Both the radiating element  101  and the helix resonator  103  operate to attenuate a specific range of interfering frequency spectrum enabling the radio receiver to operate normally without being overloaded from adjacent channel interference in the same frequency spectrum. Moreover, since no discrete components are used in connection with the notch filter, the antenna is capable of accommodating power levels at and above 5 watts without the burden and expense of failing filter components.  
      As seen in  FIGS. 2 and 3 , the radiating element  101  includes a period length  201  and a wire diameter  203 .  FIG. 3  illustrates a top view of the radiating element where the helix diameter  301  of the helix is shown. The resonant frequency of the integrated notch filter  100  uses the period length  201 , diameter  203  and helix diameter  301 .  
      Thus, the invention provides an inexpensive antenna helical design that incorporates an integrated filter using non-discrete components that effectively reduces incoming radio frequency distortion products from strong adjacent channel interferers.  
      While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.