Patent Publication Number: US-2023139294-A1

Title: Cloaked low band elements for multiband radiating arrays

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of and claims priority from U.S. patent application Ser. No. 17/038,070, filed Sep. 30, 2020, which is a continuation application of U.S. patent application Ser. No. 16/711,536, filed Dec. 12, 2019, which is a continuation application of U.S. patent application Ser. No. 16/655,479 filed Oct. 17, 2019, which is a continuation application of U.S. patent application Ser. No. 16/277,044, filed Feb. 15, 2019, which is a continuation of U.S. patent application Ser. No. 15/517,906, filed Apr. 7, 2017, which is a 35 U.S.C. § 371 national stage application of PCT International Application No. PCT/US2015/044020, filed Aug. 6, 2015, which itself claims priority to U.S. Provisional Patent Application No. 62/081,358, filed Nov. 18, 2014, the disclosure and content of each of the above applications is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to wide-band multi-band antennas with interspersed radiating elements intended for cellular base station use. In particular, the invention relates to radiating elements intended for a low frequency band when interspersed with radiating elements intended for a high frequency band. This invention is aimed at minimizing the effect of the low-band dipole arms, and/or parasitic elements if used, on the radio frequency radiation from the high-band elements. 
     BACKGROUND 
     Undesirable interactions may occur between radiating elements of different frequency bands in multi band interspersed antennas. For example, in some cellular antenna applications, the low band is 694-960 MHz and the high band is 1695-2690 MHz. Undesirable interaction between these bands may occur when a portion of the lower frequency band radiating structure resonates at the wavelength of the higher frequency band. For instance, in multiband antennas where a higher frequency band is a multiple of a frequency of a lower frequency band, there is a probability that the low band radiating element, or some component or part of it, will be resonant in some part of the high band frequency range. This type of interaction may cause a scattering of high band signals by the low band elements. As a result, perturbations in radiation patterns, variation in azimuth beam width, beam squint, high cross polar radiation and skirts in radiation patterns are observed in the high band. 
     SUMMARY 
     In one aspect of the present invention, a low band radiating element for use in a multiband antenna having at least a high band operational frequency and a low band operational frequency is provided. The low band element comprises a first dipole element having a first polarization and comprising a first pair of dipole arms and a second dipole element having a second polarization and comprising a second pair of dipole arms oriented at approximately 90 degrees to the first pair of dipole arms. Each dipole arm includes a plurality of conductive segments, each having a length less than one-half wavelength at the high band operational frequency, coupled in series by a plurality of inductive elements, having an impedance selected to attenuate high band currents while passing low band currents in the dipole arms. The inductive elements are selected to appear as high impedance elements at the high band operational frequency and as lower impedance elements at the low band operational frequency. 
     In another aspect of the present invention, a multiband antenna is provided. The multiband antenna includes a reflector, a first array of first radiating elements and a second array of second radiating elements. The first radiating elements have a first operational frequency band and the second radiating elements have a second operational frequency band. The first radiating elements include two or more dipole arms. Each dipole arm includes a plurality of conductive segments coupled in series by a plurality of inductive elements. The conductive segments each have a length less than one-half wavelength at the second operational frequency band. The first radiating elements may comprise single dipole elements or cross dipole elements. 
     The inductive elements are typically selected to appear as high impedance elements at the second operational frequency band and as lower impedance elements at the first operational frequency band. The first operational frequency band typically comprises a low band of the multiband antenna and the second operational frequency band typically comprises a high band of the multiband antenna. 
     In another aspect of the present invention, parasitic elements may be included on the multiband antenna to shape low band beam characteristics. For example, the parasitic elements may have an overall length selected to shape beam patterns in the first operational frequency band, and comprise conductive segments coupled in series with inductive elements selected to reduce interaction between the parasitic elements and radiation at the second operational frequency band. The conductive segments of the parasitic elements may also have a length of less than one half wave length at the second operational frequency band. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an antenna according to one aspect of the present invention. 
         FIG.  2    is a plan view of a portion of an antenna array according to another aspect of the present invention. 
         FIG.  3    is an isometric view of a low band radiating element and parasitic elements according to another aspect of the present invention. 
         FIG.  4    is a more detailed view of the low band radiating element of  FIG.  3   . 
         FIG.  5    is a first example of a parasitic element according to another aspect of the present invention. 
         FIG.  6    is a second example of a parasitic element accordingly to another aspect of the present invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG.  1    schematically diagrams a dual band antenna  10 . The dual band antenna  10  includes a reflector  12 , an array of high band radiating elements  14  and an array of low band radiating elements  16 . Optionally, parasitic elements  30  may be included to shape azimuth beam width of the low band elements. Multiband radiating arrays of this type commonly include vertical columns of high band and low band elements spaced at pre-determined intervals See, for example, U.S. patent application Ser. No. 13/827,190, now U.S. Pat. No. 9,276,329 to Jones et al., which is incorporated by reference. 
       FIG.  2    schematically illustrates a portion of a wide band dual band antenna  10  including features of a low band radiating element  16  according to one aspect of the present invention. High band radiating elements  14  may comprise any conventional crossed dipole element, and may include first and second dipole arms  18 . Other known high band elements may be used. The low band radiating element  16  also comprises a crossed dipole element, and includes first and second dipole arms  20 . In this example, each dipole arm  20  includes a plurality of conductive segments  22  coupled in series by inductors  24 . 
     The low band radiating element  16  may be advantageously used in multi-band dual-polarization cellular base-station antenna. At least two bands comprise low and high bands suitable for cellular communications. As used herein, “low band” refers to a lower frequency band, such as 694-960 MHz, and “high band” refers to a higher frequency band, such as 1695 MHz-2690 MHz. The present invention is not limited to these particular bands, and may be used in other multi-band configurations. A “low band radiator” refers to a radiator for such a lower frequency band, and a “high band radiator” refers to a radiator for such a higher frequency band. A “dual band” antenna is a multi-band antenna that comprises the low and high bands referred to throughout this disclosure. 
     Referring to  FIG.  3   , a low band radiating element  16  and a pair of parasitic elements  30  are illustrated mounted on reflector  12 . In one aspect of the present invention, parasitic elements  30  are aligned to be approximately parallel to a longitudinal dimension of reflector  12  to help shape the beam width of the pattern. In another aspect of the invention, the parasitic elements may be aligned perpendicular to a longitudinal axis of the reflector  12  to help reduce coupling between the elements. The low band radiating element  16  is illustrated in more detail in  FIG.  4   . Low band radiating element  16  includes a plurality of dipole arms  20 . The dipole arms  20  may be one half wave length long. The low band dipole arms  20  include a plurality of conductive segments  22 . The conductive segments  22  have a length of less than one-half wavelength at the high band frequencies. For example, the wavelength of a radio wave at 2690 MHz is about 11 cm, and one-half wavelength at 2690 MHz would be about 5.6 cm. In the illustrated example, four segments  22  are included, which results in a segment length of less than 5 cm, which is shorter than one-half wavelength at the upper end of the high band frequency range. The conductive segments  22  are connected in series with inductors  24 . The inductors  24  are configured to have relatively low impedance at low band frequencies and relatively higher impedance at high band frequencies. 
     In the examples of  FIGS.  2  and  3   , the dipole arms  20 , including conductive segments  22  and inductors  24 , may be fabricated as copper metallization on a non-conductive substrate using, for example, conventional printed circuit board fabrication techniques. In this example, the narrow metallization tracks connecting the conductive segments  22  comprise the inductors  24 . In other aspect of the invention, the inductors  24  may be implemented as discrete components. 
     At low band frequencies, the impedance of the inductors  24  connecting the conductive segments  22  is sufficiently low to enable the low band currents continue to flow between conductive segments  22 . At high band frequencies, however, the impedance is much higher due to the series inductors  24 , which reduces high band frequency current flow between the conductive segments  22 . Also, keeping each of the conductive segments  22  to less than one half wavelength at high band frequencies reduces undesired interaction between the conductive segments  22  and the high band radio frequency (RF) signals. Therefore, the low band radiating elements  16  of the present invention reduce and/or attenuate any induced current from high band RF radiation from high band radiating elements  14 , and any undesirable scattering of the high band signals by the low band dipole arms  20  is minimized. The low band dipole is effectively electrically invisible, or “cloaked,” at high band frequencies. 
     As illustrated in  FIG.  3   , the low band radiating elements  16  having cloaked dipole arms  20  may be used in combination with cloaked parasitic elements  30 . However, either cloaked structure may also be used independently of the other. Referring to  FIGS.  1  and  3   , parasitic elements  30  may be located on either side of the driven low band radiating element  16  to control the azimuth beam width. To make the overall low band radiation pattern narrower, the current in the parasitic element  30  should be more or less in phase with the current in the driven low band radiating element  16 . However, as with driven radiating elements, inadvertent resonance at high band frequencies by low band parasitic elements may distort high band radiation patterns. 
     A first example of a cloaked low band parasitic element  30   a  is illustrated in  FIG.  5   . The segmentation of the parasitic elements may be accomplished in the same way as the segmentation of the dipole arms in  FIG.  4   . For example, parasitic element  30   a  includes four conductive segments  22   a  coupled by three inductors  24   a . A second example of a cloaked low band parasitic element  30   b  is illustrated in  FIG.  6   . Parasitic element  30   b  includes six conductive segments  22   b  coupled by five inductors  24   b . Relative to parasitic element  30   a , the conductive segments  22   b  are shorter than the conductive segments  22   a , and the inductor traces  24   b  are longer than the inductor traces  24   a.    
     At high band frequencies, the inductors  24   a ,  24   b  appear to be high impedance elements which reduce current flow between the conductive segments  22   a ,  22   b , respectively. Therefore the effect of the low band parasitic elements  30  scattering of the high band signals is minimized. However, at low band, the distributed inductive loading along the parasitic element  30  tunes the phase of the low band current, thereby giving some control over the low band azimuth beam width. 
     In a multiband antenna according to one aspect of the present invention described above, the dipole radiating element  16  and parasitic elements  30  are configured for low band operation. However, the invention is not limited to low band operation, the invention is contemplated to be employed in additional embodiments where driven and/or passive elements are intended to operate at one frequency band, and be unaffected by RF radiation from active radiating elements in other frequency bands. The exemplary low band radiating element  16  also comprises a cross-dipole radiating element. Other aspects of the invention may utilize a single dipole radiating element if only one polarization is required.