Abstract:
A higher band radiating element for use in a multiband antenna includes first and second dipole arms supported by a feedboard. The feedboard includes first and second matching circuits, each comprising a capacitor-inductor-capacitor (CLC) matching circuit. The matching circuit further includes a CM tuning circuit connecting a portion of the matching circuit to ground via a microstrip trace selected to pass lower band currents while blocking higher band currents. The CM tuning circuit moves the common mode resonance of the higher band support PCB down below the operating frequency of additional, lower band radiating elements present in the multiband antenna, which is preferable to moving the common mode resonance above the lower band frequencies.

Description:
[0001]    This application claims priority to and incorporates by reference U.S. Provisional Patent Application No. 62/103,799, filed Jan. 15, 2015 and titled “Low Common Mode Resonance Multiband Radiating Array” 
     
    
     BACKGROUND 
       [0002]    Multiband antennas for wireless voice and data communications are known. For example, common frequency bands for GSM services include GSM900 and GSM1800. A low band of frequencies in a multiband antenna may comprise a GSM900 band, which operates at 880-960 MHz. The low band may also include Digital Dividend spectrum, which operates at 790-862 MHz. Further, it may also cover the 700 MHz spectrum at 698-793 MHz. Ultra wide band antennas may cover all of these bands. 
         [0003]    A high band of a multiband antenna may comprise a GSM1800 band, which operates in the frequency range of 1710-1880 MHZ. A high band may also include, for example, the UMTS band, which operates at 1920-2170 MHz. Additional bands may comprise LTE2.6, which operates at 2.5-2.7 GHz and WiMax, which operates at 3.4-3.8 GHz. Ultra wide band antennas may cover combinations of these bands. 
         [0004]    When a dipole element is employed as a radiating element, it is common to design the dipole so that its first resonant frequency is in the desired frequency band. To achieve this, the dipole arms are about one quarter wavelength, and the two dipole arms together are about one half the wavelength of the desired band. These are commonly known as “half-wave” dipoles. 
         [0005]    However, in multiband antennas, the radiation patterns for a lower frequency band can be distorted by resonances that develop in radiating elements that are designed to radiate at a higher frequency band, typically 2 to 3 times higher in frequency. For example, the GSM1800 band is approximately twice the frequency of the GSM900 band. 
         [0006]    There are two modes of distortion that are typically seen, Common Mode resonance and Differential Mode resonance Common Mode (CM) resonance occurs when a portion of the higher band radiating element structure resonates as if it were a one quarter wave monopole at low band frequencies. For example, when the higher band radiating element comprises a dipole element coupled to a feed network with an associated matching circuit, the combination of a high band dipole arm and associated matching circuit may resonate at the low band frequency. This may cause undesirable distortion of low band radiating patterns. 
         [0007]    For example, low band elements, in the absence of high band elements, may have a half power beam width (HPBW) of approximately 65 degrees. However, when high band elements are combined with low band elements on the same multi-band antenna, Common Mode resonance of the low band signal onto the high band elements may cause an undesirable broadening of the HPBW to 75-80 degrees. 
         [0008]    Approaches for reducing CM resonance include adjusting the dimensions of a high band element to move the CM resonance up or down to move it out of band of the low band element. In one example, the high band radiators are effectively shortened in length at low band frequencies by including capacitive elements in the feed, thereby tuning the CM resonance to a higher frequency and out of band. See, for example, U.S. Provisional Application Ser. No. 61/987,791, the disclosure of which is incorporated by reference. While this approach is cost-effective, tuning the CM resonance above the low band often results in an undesirable broadening of the azimuth beamwidth of the low band pattern. 
         [0009]    Another approach for reducing CM resonance is to increase the length of the stalk of a high band element by locating it in a “moat”. A hole is cut into the reflector around the vertical stalks of the radiating element. A conductive well is inserted into the hole and the stalk is extended to the bottom of the well. This lengthens the stalk, which lowers the resonance of the CM, allowing it to be moved out of band, while at the same time keeping the dipole arms approximately ¼ wavelength above the reflector. See, U.S. patent application Ser. No. 14/479,102, the disclosure of which is incorporated by reference. While this approach desirably tunes the CM resonance down and below the low band, it requires more space and entails extra complexity and manufacturing cost. 
       SUMMARY 
       [0010]    According to one aspect of the present invention, a higher band radiating element for use in a multiband antenna includes first and second dipole arms supported by a feedboard. Each dipole arm has a capacitive coupling area. The feedboard includes a balun and first and second matching circuits coupled to the balun. The first matching circuit is capacitively coupled to the first dipole arm and the second matching circuit is capacitively coupled to the second dipole arm. The first and second matching circuits each comprise a capacitor-inductor-capacitor (CLC) matching circuit having, in series, a stalk, coupled to the balun, a first capacitive element, an inductor, and a second capacitive element, the second capacitive element being coupled to a dipole arm. The feed circuit further includes a CM tuning circuit connecting the first capacitive element and the inductor to the stalk. The CM tuning circuit may comprise a microstrip line providing a DC connection to the stalk and having a length selected to appear as a high impedance at an operating frequency of the radiating element. The CM tuning circuit moves the common mode resonance of the support PCB down below the operating frequency of additional, lower band radiating elements present in the multiband antenna, which is preferable to moving the common mode resonance above the lower band frequencies. The capacitive elements may be selected to block out-of-band induced currents while passing in-band currents. 
         [0011]    The capacitors of the CLC matching circuits may be shared across different components. For example, the first capacitive element and an area of the stalk may provide the parallel plates of a capacitor, and the feedboard PCB substrate may provide the dielectric of the capacitor. The second capacitive element may combine with the capacitive coupling area of the dipole arm to provide the second capacitor. 
         [0012]    The radiating element may comprise a cross dipole radiating element. In one example, the multiband antenna comprises a dual band antenna having high band radiating elements and low band radiating elements. The high band radiating elements have a first operational frequency band within a range of about 1710 MHz-2700 MHz, and the low band radiating elements have a second operational frequency band within a range of about 698 MHz-960 MHz. In such an example, the common mode tuning circuit is dimensioned to pass low band current and block high band current. 
         [0013]    In another example, a multiband antenna, may include a first array of first radiating elements having a first operational frequency band and a second array of second radiating elements having a second operational frequency band. The second operational frequency band is higher than the first operational frequency band, and often a multiple of the first operational frequency band. The second radiating elements further comprising first and second dipole arms, each dipole arm having a capacitive coupling area, and a feedboard having a balun and first and second matching circuits coupled to the balun. The first matching circuit is coupled to the first dipole arm and the second matching circuit is coupled to the second dipole arm. The first and second matching circuits each include, in series, a stalk, coupled to the balun, a first capacitive element, an inductor, and a second capacitive element, the second capacitive element being associated with one of the first and second dipole arms. Each matching circuit further includes a common mode tuning circuit connecting the first capacitive element and the inductor to the stalk, the common mode tuning circuit comprising a microstrip line dimensional to short any induced low band currents to the stalk without substantially affecting high band currents. 
         [0014]    The first operational frequency band comprises a mobile communications low band and the second operational frequency band comprises a mobile communications high band. For example, the first operational frequency band may located within an approximate range of 698 MHz to 960 MHz, and the second operational frequency band may located within an approximate range of 1710 MHz to 2170 MHz. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a plan view of a panel antenna having arrays of high band radiating elements and low band radiating elements. 
           [0016]      FIG. 2  is a diagram of a low band radiating element and a plurality of high band radiating elements. 
           [0017]      FIG. 3  is an isometric view of a sub-array of high band radiating element feedboards according to one aspect of the present invention. 
           [0018]      FIGS. 4 a  and 4 b    illustrate one example of layers of metallization according to another aspect of the present invention. 
           [0019]      FIGS. 5 a -5 c    illustrate another example of layers of metallization according to another aspect of the present invention. 
           [0020]      FIG. 6  is a schematic diagram of a radiating element dipole and feed circuit of the elements illustrated in  FIGS. 3, 4   a - 4   b , and  5   a - 5   c.    
           [0021]      FIG. 7  is a graph showing improved azimuth beamwidth performance due to the present invention. 
           [0022]      FIG. 8  is a graph illustrating typical common mode and differential mode performance. 
           [0023]      FIG. 9  is a graph illustrating improved common mode and differential mode performance due to the present invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0024]      FIG. 1  schematically diagrams a dual band antenna  10 . The dual band antenna  10  includes a reflector  12 , arrays of high band radiating elements  14 , and an array of low band radiating elements  16  interspersed with the high band elements. The high band radiating element  14  and low band element  16  may each comprise a cross dipole. Other radiating elements may be used, such as dipole squares, patch elements, single dipoles, etc. The present invention is not limited to dual band antennas, and may be used in any multiband application where higher band radiating elements and lower band radiating elements are present. 
         [0025]      FIG. 2  illustrated the dual band antenna of  FIG. 1  in more detail. The low band element  16  may optionally include a chokes on the dipole arms  17  to reduce undesirable interference from the low band elements on the high band radiation pattern. See, e.g., PCT/CN2012/087300, which is incorporated by reference. The dipole arms  15  of the high band element  14  may be supported over the reflector  12  by feed boards  18 . 
         [0026]    The high band radiating elements  14  may be arranged in a sub-array. For example, referring to  FIG. 3 , feed boards  18  are arranged on a backplane with a portion of a feed network to create a sub array. 
         [0027]    Referring to  FIGS. 4 a  and 4 b   , a first example of a feed board  18   a  for a high band radiating element  14  according to one aspect of the present invention is illustrated. In this example, the stalk traces capacitively couple signals from the feed network to the dipole arms of radiating elements  14 . 
         [0028]    In the example of  FIGS. 4 a  and 4 b   , two metallization layers of a feed board  18   a  are illustrated. These metallization layers are on opposite sides of a printed circuit board substrate. A first layer is illustrated in  FIG. 4 a    and a second layer is illustrated in  FIG. 4 b   . The first layers implements CM tuning circuits  20 , hook balun  22 , first capacitor sections  34 , inductive elements  32 , and second capacitor sections  30 . The second layer implements stalks  24 . 
         [0029]    Another example of a feed board including CM tuning circuits  20  is illustrated in  FIGS. 5 a -5 c   . In this example, similar CLC and CM tuning circuits are employed, but are implemented on three layers of metallization. A first outer layer is illustrated in  FIG. 5 a   , an inner layer is illustrated in  FIG. 5 b   , and a second outer layer is illustrated in  FIG. 5 c   . The middle layer implements the stalks  24 . The first and second outer layers implement the CM tuning circuits  20 , first capacitor sections  34 , inductive elements  32 , and second capacitor sections  30 . Additionally, the first outer layer implements hook balun  22 . 
         [0030]    A schematic diagram of a high band radiating element  14  according to either of the examples of  FIGS. 4 a -4 b    and  FIGS. 5 a -5 c    is illustrated in  FIG. 6 . Hook balun  22  couples with stalks  24  through the substrate of feed board  18  to transform a Radio Frequency (RF) signal in transmit direction from single-ended to balanced. (In the receive direction, the balun couples from balanced to unbalanced signals.) Stalks  24  propagate the balanced signals toward the dipole arms  15 . First capacitor sections  34  capacitively couple to the stalks  24  through the substrate of feed board  18 . Inductive traces  32  connect first capacitor sections  34  to second capacitor sections  30 . Second capacitor sections  30  capacitively couple the RF signals to the dipole arms  15 . The first capacitor section  34  is introduced to couple capacitively from the stalks  24  to the inductive sections  32  at high band frequencies where the dipole is desired to operate and acts to help block some of the low band currents from getting to the inductor sections  32 . 
         [0031]    CM tuning circuits  20  provide a direct current (DC) path from first capacitor sections  34  to stalks  24  though a microstrip line and plated through-hole. Because stalks  24  are connected to ground at their lower-most edge, CM tuning circuits  20  provide a DC path to ground. The CM tuning circuits  20 , in combination with capacitor sections  34 , are preferably configured to act differently at low band and high band frequencies, and to suppress CM resonance at low band frequencies. The impedance of the CM tuning circuits  20  may be adjusted by varying a length and width of the metallic trace, and/or locating the CM tuning circuits over or to the side of a ground plane (e.g., stalk) on an opposite side of a layer of PCB substrate. 
         [0032]    For example, CM tuning circuit  20  may comprise a narrow, high impedance microstrip line having length lw. The CM tuning circuit  20  may be dimensioned with a length to appear as a high impedance element at high band RF frequencies where it connects to capacitor section  34  near inductive section  32 . However, the electrical length of 20 inversely proportional to frequency, and appears electrically shorter and lower in impedance at low band frequencies where it connects to capacitor section  34 . With the addition of CM tuning circuit  20 , the main path for any induced low band current is through the CM tuning circuit  20 , because the first capacitor section  34  acts as a high impedance at low band frequencies. The narrow, high impedance microstrip may affect the high band CLC match and radiation pattern only at high band wavelengths close to lw=nλ/2, where n may be any integer. The length lw may therefore be selected such that CM tuning circuit  20  does not adversely affect high band signals. 
         [0033]    Referring to  FIG. 8 , a plot of CM resonance versus frequency is illustrated. In the case of  FIG. 8 , the high band radiating element is a dipole with a CLC feed circuit, but no CM tuning circuit  20 . There is considerable CM resonance in the band between 790 MHz and 960 MHz.  FIG. 9  shows a similar plot of CM resonance, but in this case the high band radiating element is a dipole with a CLC feed circuit and CM tuning circuit  20 . CM resonance is considerably reduced at low band frequencies, with a deep notch between 700 MHz and 800 MHz and a CM resonance below 700 MHz. 
         [0034]    The CM tuning circuit  20  may be configured to move the CM resonance down below the low band frequency range. The CM resonance of the high band radiating element structure may be shifted by adjusting the length of the CM tuning circuit  20 . In particular, the CM resonance may be shifted lower by increasing length lw. 
         [0035]    For example, referring to  FIG. 7 , three plots of low band beamwidth versus frequency are shown. In a first case, the low band radiating element, in the absence of any high band radiating element, has a beamwidth of 58-65 degrees in at low band frequencies. In a second case, a high band element with a CM tuning circuit  20  having a length lw=22 mm is included. The beamwidth undesirably widens to more than 74 degrees at about 840 MHz, which is within the low band. The widening of the beamwidth is due to the CM resonance in the high band radiating element. This in-band CM resonance may also cause additional beam pattern distortions, such as large azimuth beam squint and poor Front/Back ratios. Also, in this second case, the beamwidth is much better above the CM resonance frequency (less than 60 degrees) than below the CM resonance frequency (more than 70 degrees), illustrating the benefit of tuning the CM resonance frequency to down below the low band. 
         [0036]    In a third case, a high band element with a CM tuning circuit  20  having a length lw=34 mm is included. In this case, the CM resonance is indicated where the beamwidth widens to almost 80 degrees, which is at about 720 MHz. This is well below 760 MHz, which is outside the lower end of the low band frequency range. Advantageously, the beamwidth of the low band radiating elements is about 62 degrees, which is an improvement over techniques that tune the CM resonance frequency to be above the low band range, and the HB radiators of the present invention do not require expensive and bulky moats. A length lw=34 mm also has very little effect on the high band pattern and impedance matching. Other lengths for lw may also be utilized. For example, a length lw=65 mm moves the CM resonance down to 640 MHz. 
         [0037]    In another example of the present invention, the place where the CM tuning circuit  20  connects to the feed stalk may be varied to move CM resonance lower and out of band without detuning the high band radiating element. This solution is advantageous when a desired length lw of the CM tuning circuit  20  degrades or detunes the high band dipole. For example, applying the equation lw=nλ/2, a length lw=65 mm (as in the above example) may affect high band CLC match and radiation pattern at 2300 MHz. If 2300 MHz is within the operational band of the high band element, a different length  1   w  may be selected to achieve good higher band performance. Significantly, the high band impedance of CM tuning circuit  20  depends solely on length lw, whereas the common mode responds is dependent on the total length of the signal path from second capacitor section  30  to stalk  24 . Accordingly, the CM tuning circuit  20  attachment point may be adjusted closer to or further away from the second capacitor section  30  to adjust overall length of the CM tuning circuit  20  and to move the CM resonance back to the desired frequency. 
         [0038]    In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope of these claims.