Patent Publication Number: US-9847574-B2

Title: Multiband helical antenna

Description:
REFERENCE TO RELATED APPLICATIONS 
     Reference is hereby made to U.S. Provisional Patent Application 61/817,909, entitled SIMPLIFIED STRUCTURE FOR MULTIBAND ANTENNA, filed May 1, 2013, the disclosure of which is hereby incorporated by reference and priority of which is hereby claimed pursuant to 37 CFR 1.78(a)(4) and (5)(i). 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to antennas and more particularly to multiband antennas. 
     BACKGROUND OF THE INVENTION 
     Various types of multiband antennas are known in the art. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved multiband helical antenna having a highly compact structure. 
     There is thus provided in accordance with a preferred embodiment of the present invention a multiband antenna including a feed point, a helical radiating element galvanically connected to and fed by the feed point, the helical radiating element resonating in a Very High Frequency range and an elongate radiating element arranged coaxially within the helical radiating element and galvanically connected to and fed by the feed point, the elongate radiating element extending along only a portion of the helical radiating element, the elongate radiating element having a first resonant frequency and a second resonant frequency, the elongate radiating element operating as a quarter-wavelength monopole at the first resonant frequency and as an eighth-wavelength monopole at the second resonant frequency. 
     Preferably, the helical radiating element operates in a frequency range of 136-174 MHz. 
     Preferably, the first resonant frequency is generally equal to 800 MHz and the second resonant frequency is generally equal to 400 MHz. 
     Alternatively, the first resonant frequency is generally equal to 1600 MHz and the second resonant frequency is generally equal to 800 MHz. 
     Preferably, the frequency range of operation of the helical radiating element is offset from the first resonant frequency by at least 250 MHz. 
     In accordance with a preferred embodiment of the present invention, the elongate radiating element extends along less than 35% of the helical radiating element. 
     Preferably, the elongate radiating element extends along between 3-7 cm of the helical radiating element. 
     Preferably, the elongate radiating element extends along between 4-6 cm of the helical radiating element. 
     In accordance with another preferred embodiment of the present invention, the helical radiating element has a dual-pitch. 
     Preferably, the helical radiating element includes a first portion proximal to the feed point and having a first pitch and a second portion distal from the feed point and having a second pitch. 
     Preferably, the second pitch is smaller than the first pitch. 
     Preferably, the first portion is shorter than the second portion. 
     Preferably, the multiband antenna also includes a threaded insert extending along the first portion, for maintaining the first pitch. 
     Preferably, the multiband antenna also includes a matching circuit connected to the helical radiating element and the elongate radiating element. 
     There is further provided in accordance with another preferred embodiment of the present invention a multiband antenna including a feed point, a dual-pitch helical radiating element galvanically connected to and fed by the feed point, the dual-pitch helical radiating element resonating in a Very High Frequency range and an elongate radiating element arranged coaxially within the dual-pitch helical radiating element and galvanically connected to and fed by the feed point, the elongate radiating element extending along only a portion of the dual-pitch helical radiating element, the elongate radiating element having a first resonant frequency and a second resonant frequency, the elongate radiating element operating as a quarter-wavelength monopole at the first resonant frequency and as an eighth-wavelength monopole at the second resonant frequency. 
     Preferably, the dual-pitch helical radiating element includes a first portion having a first pitch and a second portion having a second pitch, the second pitch being smaller than the first pitch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
         FIGS. 1A, 1B, 1C and 1D  are simplified respective side, cross-sectional, exploded and perspective view illustrations of a multiband antenna constructed and operative in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to  FIGS. 1A, 1B, 1C and 1D , which are simplified respective side, cross-sectional, exploded and perspective view illustrations of a multiband antenna constructed and operative in accordance with a preferred embodiment of the present invention. 
     As seen in  FIGS. 1A-1D , there is provided an antenna  100 , including a feed point  102  and a helical radiating element  104  galvanically connected to and fed by feed point  102 . Helical radiating element  104  is preferably embodied as a cylindrical helical radiating element. It is appreciated, however, that helical radiating element  104  may alternatively be embodied in a variety of configurations, including hexagonal or square-helical. 
     Helical radiating element  104  is preferably embodied as a dual-pitch helical radiating element, preferably including a first lower portion  106 , proximal to feed point  102  and having a first pitch and a second upper portion  108 , distal from feed point  102  and having a second pitch. As seen most clearly in  FIG. 1C , the second pitch of second upper portion  108  is preferably smaller than the first pitch of first lower portion  106 . 
     First portion  106  may, by way of example, be shorter and comprise a fewer number of turns than second portion  108 . By way of example, first portion  106  may comprise 16 helical turns, each spaced apart by approximately 3.5 mm and second portion  108  may comprise 65 helical turns, each spaced apart by approximately 2.7 mm. First and second portions  106  and  108  may have a diameter of approximately 5.8 mm and be formed by a coiled wire having a thickness of approximately 0.9 mm. It is appreciated, however, that these particular described configurations of first and second portions  106  and  108  are exemplary only and may be modified according to the desired operating characteristics of antenna  100 , as will be described henceforth. Helical radiating element  104  preferably has an electrical length for resonating in the Very High Frequency (VHF) range, preferably spanning approximately 136-174 MHz. 
     An elongate radiating element  110  is preferably arrange coaxially within helical radiating element  104  and is galvanically connected to and fed by feed point  102 . It is appreciated that feed point  102  thus serves as a common galvanic feed point for both helical radiating element  104  and elongate radiating element  110 . Elongate radiating element  110  is preferably embodied as a straight insulated wire formed by a suitable conductive material such as copper. 
     It is a particular feature of a preferred embodiment of the present invention that elongate radiating element  110  does not extend fully along a length of helical radiating element  104  but rather extends only partially along and within helical radiating element  104 . By way of example, helical radiating element  104  may have a physical length of approximately 18 cm and elongate radiating element  110  may have a physical length of approximately 5.1 cm, such that elongate radiating element  110  extends along only a small portion of the physical length of helical radiating element  104 . Preferably, elongate radiating element  110  extends along less than approximately 35% of helical radiating element  104 . Particularly preferably, elongate radiating element  110  extends along between approximately 3-7 cm and even more particularly preferably along between approximately 4-6 cm of helical radiating element  104 . 
     Elongate radiating element  110  preferably operates as a monopole radiating element having a first resonant frequency, wherein the first resonant frequency has a corresponding associated first wavelength and elongate radiating element  110  has an electrical length generally equal to a quarter of that first wavelength. It is appreciated that elongate radiating element  110  thus operates as a quarter-wavelength monopole at its first resonant frequency. The operation of an elongate radiating element as a quarter-wavelength monopole will be readily understood by one skilled in the art as a typical mode of operation of a whip monopole element. The first resonant frequency of elongate radiating element  110  may be in the 800 MHz range. 
     In addition to the first resonant frequency of elongate radiating element  110 , however, it has been found that when elongate radiating element  110  is positioned as described above within helical radiating element  104 , elongate radiating element  110  operates as a monopole radiating element exhibiting an additional second resonant frequency. The second resonant frequency of elongate radiating element  110  has a corresponding associated second wavelength and the electrical length of elongate radiating element  110  is preferably generally equal to an eighth of that second wavelength. It is appreciated that elongate radiating element  110  thus operates as an eighth-wavelength monopole at its second resonant frequency. The second resonant frequency of elongate radiating element  110  may be in the 400 MHz range. 
     As will be readily appreciated by one skilled in the art, the operation of elongate radiating element  110  as an eighth-wavelength monopole radiating element is surprising and atypical of whip monopole elements. The operation of elongate radiating element  110  as an eighth-wavelength monopole radiating element seems to arise due to the particular location thereof within VHF helical radiating element  104  and due to the disparity in the preferable respective operating frequencies of helical radiating element  104  and elongate radiating element  110 . The VHF operating frequency of helical radiating element  104  is preferably offset from the first resonant frequency of elongate radiating element  110  by at least 250 MHz. 
     The embodiment of helical radiating element  104  as a dual-pitch helical radiating element, as shown in  FIGS. 1B and 1C , has been found to provide particularly advantageous performance of antenna  100 , as it allows tuning of the first, second and VHF resonant frequencies of antenna  100  by way of adjustment of the parameters of the helices respectively forming first and second portions  106  and  108  of helical radiating element  104 . However, the above-described surprising operation of elongate radiating element  110  as an eighth-wavelength monopole when so disposed within helical radiating element  104  is not limited to the case wherein helical radiating element  104  is a dual-pitch helical radiating element. Helical radiating element  104  thus may alternatively be embodied as a single-pitch helical radiating element, depending on the required operating characteristics of antenna  100 . 
     It is understood that as a result of the VHF resonant frequency arising due to the operation of helical radiating element  104  and the first and second resonant frequencies arising due to the operation of elongate radiating element  110 , antenna  100  is preferably operative as a tri-band antenna. In contrast to conventional somewhat comparable multiband antennas, which conventional multiband antennas typically have complex structures, antenna  100  has an advantageously simple structure including only a few parts and is thus compact, highly flexible, cost-efficient, light and easy to assemble. 
     It is appreciated that the operation of elongate radiating element  110  is not limited to the 400/800 MHz range. Elongate radiating element  110  may alternatively have an electrical length such that elongate radiating element  110  radiates in the 800/1600 MHz range. In this case, the radiation pattern of elongate radiating element  110  in the 1600 MHz range is predominantly directed upwards, this being particularly advantageous for GPS applications. 
     As seen most clearly at enlargement  111  in  FIG. 1B , helical radiating element  104  and elongate radiating element  110  are preferably connected to a radio-frequency connector  112  by way of a matching circuit  114 , which matching circuit  114  is preferably formed on a surface of a printed circuit board  116 . It is appreciated, however, that the inclusion of matching circuit  114  in antenna  100  is optional and that matching circuit  114  may be obviated should helical and elongate radiating elements  104  and  110  be sufficiently well matched to an input impedance of radio-frequency connector  112 . 
     Antenna  100  may further include a threaded insert  116 , seen most clearly in  FIG. 1C . Threaded insert  116  preferable functions to maintain the first pitch of lower portion  106  of helical radiating element  104  as well as to hold elongate element  110  concentrically in place within the bore of helical radiating element  104 . It is appreciated, however, that threaded insert  116  may be obviated or replaced by other holding means as are well known in the art. 
     Antenna  100  may be installed as an external whip-type antenna attached to a portable electronic device such as a Land Mobile Radio (LMR). In this case, antenna  100  may be housed by an outer protective insulative cover, such as a cover  120  seen most clearly in  FIG. 1D . It is appreciated that cover  120  is omitted from  FIG. 1C  for the sake of clarity of presentation only. It is further understood that antenna  100  is not limited to installation on LMR devices, and may alternatively be employed as an internal or external antenna in a variety of appropriate portable or non-portable electronic devices. 
     It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly claimed hereinbelow. Rather, the scope of the invention includes various combinations and subcombinations of the features described hereinabove as well as modifications and variations thereof as would occur to persons skilled in the art upon reading the forgoing description with reference to the drawings and which are not in the prior art.