Abstract:
A dual band multi-pitch helical antenna ( 101 ) includes a first section ( 201 ) positioned adjacent to the feed point having a widely spaced pitch. A second section ( 203 ) is attached to the first section ( 201 ) having a narrowly spaced pitch. A third section ( 205 ) is attached to the second section ( 203 ) having a widely spaced pitch, while a fourth section ( 207 ) is attached to the third section ( 205 ) having a narrowly spaced pitch. The antenna further includes a parasitic element ( 213   a/   213   b ) that is positioned adjacent to each of the first section ( 201 ), second section ( 203 ), third section ( 205 ), and fourth section ( 207 ) for enhancing broad-band antenna performance. A matching network ( 216 ) is connected between an antenna feed point and the first section ( 201 ) for matching the dual band multi-pitch helical antenna to a predetermined feed point impedance such that the antenna is resonant in at least two frequency bands.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to helically wound portable antennas and more particularly to a dual-band multi-pitch helical antenna with integrated half-wave radiator that may be used in the very high frequency (VHF) and 800 MHz frequency spectrum. 
       BACKGROUND  
       [0002]    Helically wound antennas have been used for many years on portable, handheld two-way radio equipment offering a convenient and moderately effective radiator. Since the antenna is wound shorter in size, depending on the frequency of use, it may lose some overall efficiency as compared with a full size antenna. However, what is lost in efficiency is gained in convenience in that the antenna is often very small and versatile in view of its flexible outer sheath making it difficult to bend or break. 
         [0003]    The conventional Association of Public Safety Officials (APCO) type public safety radios provide services in both VHF (136-174 MHz) and 800 MHz (760-870 MHz) frequency bands. The challenges in designing a VHF/800 antenna are in obtaining the proper resonances for each band simultaneously in the same antenna structure. Additionally, the broadside radiation pattern of the 800 band for a nominal dual band antenna covering these bands has the tendency to be downward pointing which is detrimental to the performance in achieving optimal signal transmission and reception. This is due to the close proximity of the transmitting radio&#39;s chassis currents to the radiating antenna elements. Finally, the dependency of the radio housing or chassis for radiation in the 800 MHz band also limits an antenna&#39;s radiation performance. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES  
         [0004]    The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention. 
           [0005]      FIG. 1  is perspective view of the MPPH antenna in accordance with an embodiment of the invention. 
           [0006]      FIG. 2  is a magnified view of the MPPH antenna shown in  FIG. 1 . 
           [0007]      FIG. 3  is graphical diagram return loss versus frequency of the MPPH antenna as shown in  FIG. 1 . 
           [0008]      FIG. 4  is a polar plot diagram showing the antenna radiation pattern of the MPPH antenna as shown in  FIG. 1  as compared with other types of antennas. 
       
    
    
       [0009]    Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
       DETAILED DESCRIPTION  
       [0010]    Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to a complementary cumulative distribution driven level convergence system and method. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
         [0011]    In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. 
         [0012]      FIG. 1  is perspective view of the MPPH antenna  100  in accordance with an embodiment of the invention Typically, the antenna  101  is mounted to a radio housing  103  which has upper surface or top  105 . The antenna typically might be mounted along an edge surface  107  such that it is spatially oriented in relation to the upper surface  105  having dimensions X  109 , Y  111 , and Z  113 . 
         [0013]      FIG. 2  is a magnified view  200  of the MPPH antenna shown in  FIG. 1 . The antenna  101  is comprised of a first section  201  having a wide-spaced pitch for operating as a base helix providing a higher band resonance and matching that typically might be in the 800 MHz spectrum. As will be recognized by those skilled in the art, the term “pitch” refers to the distance between turns in helical coil forming section  201  and each subsequent section as described herein. The second section  203  is a narrow-pitch helix and is designed to produce an electrical one-quarter wavelength at a higher radio frequency (RF) frequency band to provide sufficient electrical choking for preventing RF energy from entering the outer portions of the antenna. Thus, the second section  203  provides isolation between the resonances of the two bands, e.g., VHF and 800 MHz. A third section  205  is a wide pitch section and provides discontinuity to the RF antenna current distribution. This occurs by creating the abrupt change in helix pitch where the third section  205  electrical length is an additional one-quarter wave length. A fourth section  207  is also a narrow pitch section which sums or totals up the electrical wave length of the helix  101  to provide resonance in the VHF band. The antenna may include a top or cap that may be frictionally engaged to an end  209 . Finally, a parasitic element  213   a / 213   b  is electrically coupled to the first section  201 , second section  203 , third section  205 , and fourth section  207  which positioning is a predetermined distance from these elements. The parasitic element  213   a / 213   b  is linear in nature extending substantially the length of the four helical sections which form the MPPH antenna  101 . Parasitic element  213   a  is positioned outside the four helical sections, while in an alternative embodiment parasitic element  213   b  is positioned inside the four helical sections. 
         [0014]    In order to properly match the antenna to the radio&#39;s 50 ohm antenna termination impedance, a matching section  216  is used between the first section  101  and radio connector (not shown) mechanically mounted to the top  105  of the radio housing. The matching circuit is positioned a distance “X” below the stub meeting the first section  201  and provides broad band matching to the antenna at its various operating frequencies. The matching circuit may typically be comprised of a inductive-capacitive (LC) matching section (not shown) in order to cancel various electrical reactance which results from antenna mismatch. In that the MPPH antenna  101  of the present invention utilizes an end-fed half wavelength parasitic element as described herein, this may result in high inductive and/or capacitive reactive components at the feed point requiring use of the matching section  216 . 
         [0015]    In operation, both the pitch of the helix in each of the four sections as well as its length may be adjusted to obtain the correct resonant frequency. The parasitic element  213   a / 213   b  is placed in close proximity to each of the MPPH helical elements which also will affect resonance. The length of the parasitic element  213   a / 213   b  is sized to one-half of a wave length at the lowest operating frequency such that it is substantially the same size as the combined length of the first section  201 , second section  203 , third section  205  and fourth section  207 . As seen in  FIG. 2 , the parasitic element  213   a  and parasitic element  213   b  may also be placed on either the outside or inside of the first section  201 , second section  203 , third section  205 , or fourth section  207 . With regard to parasitic element  213   a,  it is positioned a distance “Y” from center of the sections forming the helix. The parasitic element  213   a / 213   b  may also be molded inside a sheath (not shown) or mechanically fastened onto the sheath using a conductive tape, glue, or non-conductive hardware. Moreover, a plastic, rubber, or other material may be used to protectively cover the antenna making it in the form of a durable “rubber ducky” type of antenna used with portable two-way radio transceivers. The parasitic element  213   a / 213   b  can also be realized in the form of a helix with the appropriate electrical length, wrapping around the first section  201 , second section  203 , third section  205 , and fourth section  207 . The lengths of the parasitic element  213  and the number of turns of the fourth section of the helix  207  are to be altered to tune the resonant frequencies of the MPPH antenna  101  to the desired frequencies in both bands. 
         [0016]      FIG. 3  is graphical diagram return loss versus frequency of the MPPH antenna as shown in  FIG. 1 . The range of loss used in this example is between 1 dB and −20 dB over frequency spectrum between 100 MHz to 1 GHz. As seen in the graphical representation, the radiation pattern illustrates a comparison of 800 MHz band antennas mounted on a typical portable radio chassis. These antennas include the antenna QW which is the radiation pattern of a quarter wave antenna, showing downward radiation pattern, antenna P 1  which is the radiation pattern of a multi pitch helix without the parasitic radiator that shows a similar downward radiation to that of QW. The antenna HW is the radiation pattern of a half wave whip antenna which has a typical half wave radiation pattern. Finally, antenna MPPH is the radiation pattern of the MPPH antenna described in an embodiment of the present invention which shows a radiation pattern covering a majority of directions in the horizontal plane and upward. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Freespace 
                 Handheld 
               
               
                 Antenna 
                 Description 
                 Pattern 
                 Eff (%) 
                 Eff (%) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 P1 
                 Mackinaw P1 antenna 
                 Downward 
                 62 
                 41.38 
               
               
                 QW 
                 Short quarter wave 
                 Downward 
                 65.2 
                 46 
               
               
                   
                 monopole antenna 
               
               
                 HW 
                 Standard half wave whip 
                 Half wave 
                 59 
                 56.73 
               
               
                   
                 antenna 
                 Dipole 
               
               
                 MPPH 
                 Multi pitch helix with 
                 Upward 
                 67.8 
                 55.53 
               
               
                   
                 Parasitic Half wave 
               
               
                   
                 radiator 
               
               
                   
               
             
          
         
       
     
         [0017]    Table 1 above shows the measured radiation efficiencies of the antennas in both free space and hand-held positions where radiation efficiency is defined as the ratio of the total power radiated by the antenna to the net power accepted by the antenna by a connected transmitter. This data illustrates the efficiency of the MPPH antenna that is consistently comparable to that of a single band half wavelength whip-type antenna. 
         [0018]      FIG. 4  is a polar plot diagram showing the antenna radiation pattern of the MPPH antenna as shown in  FIG. 1  as compared with other types of antennas shown in Table 1 above. In the plot shown in  FIG. 4 , the MPPH antenna uses a concentric parasitic element  213   b  placed inside the sections of the multi-pitch helix. As seen in the plot, resonances in both the VHF and 800 MHz bands are present in the MPPH antenna which illustrates the utility for an integration between a quarter-wave-based helix and a parasitic half-wave radiator in a VHF/800 MHz helical antenna. Thus, this type of helical antenna is designed with the base helical antenna section such that it provides a wide band matched impendence at the frequency bands of interest namely between 100-200 MHz and 800 MHz. 
         [0019]    Thus, the present invention is a dual band multi-pitch parasitic half-wave antenna which utilizes an additional section of the helix that by manipulating the pitch and the number of turns provides the appropriate isolation of the base section from the remainder of the helix. The subsequent sections of the antenna total up its electrical length of the whole antenna to provide resonance and antenna functionality for the VHF band. A broad-banding matching circuit is used at the base of the antenna to broaden the matched bandwidth of the antenna, if necessary. 
         [0020]    In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.