Abstract:
A PCB antenna, comprising: a substrate; a radiator, patterned on the substrate, having a branch point; a ground on the substrate; a short path, patterned on the substrate, having two ends where one end is connected to the ground and the other end is connected to the branch point of the radiator; and at least one passive element coupled between the radiator and the short path, is disclosed. The resonant frequency and/or the input impedance of the PCB antenna can be adjusted according to a distance between the passive element and the branch point of the radiator.

Description:
FIELD OF INVENTION 
       [0001]    The invention is related to a printed circuit board (PCB) antenna used in various applications, such as telecommunication systems, and more particularly related to a method for adjusting a resonant frequency and an input impedance of the antenna and a structure thereof. 
       BACKGROUND OF THE INVENTION 
       [0002]    PCB antennas are sensitive to surroundings including PCB material, layout, nearby components, metal materials, housings and so on. For example, two PCB antennas with the same size patterned on different PCBs may demonstrate different performances. Even two identical antennas may have two distinct resonant frequency values and input impedance values when used in different products. If the resonant frequency shifts out of band, the input impedance increases/decreases beyond tolerance or other performances beyond tolerance, the designer will encounter a big problem in designing and verifying procedures of the antenna. 
         [0003]    Generally, a larger-sized PCB antenna may have a wider band in comparison with a small-sized PCB antenna. Therefore, if there is enough space for a larger-sized PCB antenna, a larger-sized PCB antenna is preferred to overcome the shift of frequency and the input impedance increase/decrease. Nevertheless, a larger-sized PCB antenna is obviously unsuitable to be implanted in portable electronic communication devices, because such applications are getting much smaller. The condition becomes worse when the portable electronic communication devices require multiple antennas for multiple applications, e.g. cellular, GPS, Bluetooth and so on. 
         [0004]    When a PCB antenna designed for a specific product is tested and found that its resonant frequency is out of band, input impedance is beyond tolerance or other performances are beyond tolerance, the layout of the PCB antenna will be redesigned to form a modified PCB antenna accordingly. The design and test procedures will be continuously performed until the modified PCB antenna passes the verification test. Besides, if the housing or the PCB material of the product is changed by manufacturers due to some reasons, it usually needs a PCB antenna of new version to fit the change of the surroundings, which is time consuming and cost effective. 
         [0005]    For a designer, adding a matching circuit to a feed pin of the PCB antenna without adjusting the layout of the PCB antenna is another practicable manner. However, there are only several specific matching circuits available in the markets and the properties of matching circuits are different based on different suppliers, such that the performances of the PCB antennas having different matching circuits are discrete. That is, the PCB antenna resonates at M frequency when a M matching circuit is added to the PCB antenna, and the PCB antenna resonates at N frequency when a N matching circuit is added to the PCB antenna. And the designer cannot make the PCB antenna operate at an arbitrarily frequency between M and N because a suitable matching circuit is unavailable. 
         [0006]    Thus, there is a need for a method for adjusts the resonant frequency, the input impedance and other performances of a PCB antenna effectively and economically, and a structure thereof. 
       SUMMARY OF THE INVENTION 
       [0007]    In the present invention, a PCB antenna, comprising: a substrate; a radiator, patterned on the substrate, having a branch point; a ground on the substrate; a short path, patterned on the substrate, having two ends where one end is connected to the ground and the other end is connected to the branch point of the radiator; and at least one passive element coupled between the radiator and the short path, is disclosed. The resonant frequency and/or the input impedance of the PCB antenna can be adjusted according to a distance between the passive element and the branch point of the radiator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1A  shows a perspective view of a PCB antenna; 
           [0009]      FIG. 1B  shows a top view of the PCB antenna illustrated in  FIG. 1A ; 
           [0010]      FIG. 1C  shows a curve of the return loss measurement of the PCB antenna illustrated in  FIG. 1A ; 
           [0011]      FIG. 1D  shows a Smith Chart of the impedance of the PCB antenna illustrated in  FIG. 1A ; 
           [0012]      FIG. 2A  shows a top view of a PCB antenna; 
           [0013]      FIG. 2B  shows a curve of the return loss measurement of the PCB antenna illustrated in  FIG. 2A , 
           [0014]      FIG. 2C  shows a Smith Chart of the impedance of the PCB antenna illustrated in  FIG. 2A , 
           [0015]      FIG. 3A  shows a bottom view of a PCB antenna, 
           [0016]      FIG. 3B  shows a curve of the return loss measurement of the PCB antenna illustrated in  FIG. 3A , 
           [0017]      FIG. 3C  shows a Smith Chart of the impedance of the PCB antenna illustrated in  FIG. 3A , 
           [0018]      FIG. 4A  shows a top view of a PCB antenna, 
           [0019]      FIG. 4B  shows a curve of the return loss measurement of the PCB antenna illustrated in  FIG. 4A , 
           [0020]      FIG. 4C  shows a Smith Chart of the impedance of the PCB antenna illustrated in  FIG. 4A , 
           [0021]      FIG. 5A  shows a bottom view of a PCB antenna, 
           [0022]      FIG. 5B  shows a curve of the return loss measurement of the PCB antenna illustrated in  FIG. 5A , 
           [0023]      FIG. 5C  shows a Smith Chart of the impedance of the PCB antenna illustrated in  FIG. 5A , 
           [0024]      FIG. 6A  shows a curve of the return loss measurement of a PCB antenna illustrated in  FIG. 4A , 
           [0025]      FIG. 6B  shows a Smith Chart of the impedance of the PCB antenna illustrated in  FIG.4A , 
           [0026]      FIG. 7A  shows a curve of the return loss measurement of a PCB antenna illustrated in  FIG. 5A , 
           [0027]      FIG. 7B  shows a Smith Chart of the impedance of the PCB antenna illustrated in  FIG. 5A , 
           [0028]      FIG. 8A  shows a top view of a PCB antenna, 
           [0029]      FIG. 8B  shows a curve of the return loss measurement of the PCB antenna illustrated in  FIG. 8A , 
           [0030]      FIG. 8C  shows a Smith Chart of the impedance of the PCB antenna illustrated in  FIG. 8A , 
           [0031]      FIG. 9A  shows a bottom view of a PCB antenna, 
           [0032]      FIG. 9B  shows a curve of the return loss measurement of the PCB antenna illustrated in  FIG. 9A , 
           [0033]      FIG. 9C  shows a Smith Chart of the impedance of the PCB antenna illustrated in  FIG. 9A , 
           [0034]      FIG. 10A  shows a curve of the return loss measurement of a PCB antenna illustrated in  FIG. 8A , 
           [0035]      FIG. 10B  shows a Smith Chart of the impedance of the PCB antenna illustrated in  FIG. 8A , 
           [0036]      FIG. 11A  shows a curve of the return loss measurement of a PCB antenna illustrated in  FIG. 9A , and 
           [0037]      FIG. 11B  shows a Smith Chart of the impedance of the PCB antenna illustrated in  FIG. 9A . 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    A compact PCB antenna is disclosed. In the following, the present invention can be further understood by referring to the exemplary, but not limiting, descriptions accompanied with the drawings in  FIG. 1  to  FIG. 11 . 
         [0039]    Now referring to  FIG. 1A , a perspective view of a PCB antenna  100  is shown. The PCB antenna  100  includes a substrate  102 , having a top surface and a bottom surface, a radiator  110  patterned on the substrate  102 , a ground  104  on the substrate  102 , and a shorting path  106  patterned on the substrate  102 . Specifically, the substrate  102  is a printed circuit board, such as FR4, FR408, or Rogers 4003 as known to those skilled in the art. One end of the shorting path  106  is connected to the ground  104  at point A and the other end of the shorting path  106  is connected to the radiator  110  at a branch point, namely point B. One end of the radiator  110  has a feed pin, namely point C, and the other end of the radiator  110  further extends to the bottom surface of the substrate  102  through a via hole. In addition, a top view of the PCB antenna  100  is shown in  FIG. 1B , and the experimental results of the PCB antenna  100  are shown in  FIG. 1C  and  FIG. 1D . Through  FIG. 1B , the top surface of the substrate  102  can be seen. From  FIG. 1C , the return loss measurement (S 11 ) shows that the resonant frequency of the PCB antenna  100  is 2.39 GHz. In  FIG. 1D , the Smith Chart plots the reflection coefficient in the complex plane, and it shows that the impendence of the PCB antenna  100  is near 50Ω. Notably, the return loss is defined as the absolute value of the reflection coefficient in dB, and the return loss measurement and the reflection coefficient can be measured by a Vector Signal Analyzer or other instruments as known to those in the art. 
         [0040]    In one embodiment, referring to  FIG. 2A , the present invention adds a 0402 resistor  220  on the top surface of the substrate  102  to form a PCB antenna  200 , wherein one end of the 0402 resistor  220  is connected to the radiator  110  and the other end is connected to the short path  106 . The experimental results of the PCB antenna  200  are shown in  FIG. 2B  and  FIG. 2C . From  FIG. 2B , the return loss measurement (S 11 ) shows that the resonant frequency of the PCB antenna  200  is shifted from 2.39 GHz (shown in  FIG. 1C ) to 2.49 GHz. From  FIG. 2C , the Smith Chart shows that the impendence of the PCB antenna  200  is shifted from about 50Ω (shown in  FIG. 1D ) to a higher value about 70Ω. 
         [0041]    In another embodiment, the present invention adds a 0402 resistor  320  on the bottom surface of the substrate  102  to form a PCB antenna  300 , wherein the 0402 resistor  320  bypasses the radiator  110 , as shown in  FIG. 3A . The experimental results of the PCB antenna  300  are shown in  FIG. 3B  and  FIG. 3C .  FIG. 3A  is a bottom view of the PCB antenna  300 , and the radiator  110  shown in  FIG. 3A  is extended from the top surface of the substrate  102  through the via hole as described above. That is,  FIG. 2A  and  FIG. 3A  are the top view and the bottom view of the PCB antenna shown in  FIG. 1A  respectively, except the 0402 resistors  220  and  320  are added on different surfaces. From  FIG. 3B , the return loss measurement (S 11 ) shows that the resonant frequency of the PCB antenna  300  is shifted from 2.39 GHz (shown in  FIG. 1C ) to 2.54 GHz. From  FIG. 3C , the Smith Chart shows that the impendence of the PCB antenna  300  is shifted from about 50Ω (shown in  FIG. 1D ) to a lower value about 40Ω. 
         [0042]    It should be noted that “the 0402 resistor 320 bypasses the radiator 110” described above means that one end of the 0402 resistor  320  is connected to the radiator  110  at one point and the other end of the 0402 resistor  320  is connected to the radiator  110  at another point. Accordingly, the term “bypass” means joining two points in the radiator  110 . Furthermore, not only 0402 resistors can be added in the present invention, another passive element, such as different resistors, such as 0201, 0402, 0603, 0805, 1206, 1210, 2010, 1812, 2512, capacitors or inductors, may also be utilized for different purposes. 
         [0043]    It should also be noted that though only one resistor is mounted on the PCB antenna  200  and  300  respectively, the present invention might mount more than one resistor on the top surface or bottom surface of a PCB antenna. Besides/more than one resistor may be mounted both on the top and bottom surface of a PCB antenna. Furthermore, not only resistors can be utilized in present invention, other passive element, such as capacitors, inductors or combination, thereof may also be utilized. 
         [0044]    Although a resistor is mounted on the PCB antenna  200  or  300 , an inductor may be used in the present invention. Specifically, if the 0402 resistor  220  in  FIG. 2A  is replaced by a 0402 inductor  420  with inductance of 1.5 nH to form a PCB antenna  400  as shown in  FIG. 4A . The experimental results of the PCB antenna  400  are shown in  FIG. 4B  and  FIG. 4C . Five curves  450 ,  452 ,  454 ,  456  and  458  are depicted in  FIG. 4B  based on increasing/decreasing the distance “d 1 ” labeled in  FIG. 4A . The curve  452  is depicted while d 1 =1 mm. The curve  454  is depicted while d 1 =3 mm. The curve  456  is depicted while d 1 =5 mm. The curve  458  is depicted while d 1 =7 mm. The curve  450  is depicted while no element added. Five curves  460 ,  462 ,  464 ,  466  and  468  are also depicted in  FIG. 4C  corresponding to the curves  450 ,  452 ,  454 ,  456  and  458 . As a result, the curves depicted in  FIG. 4B  show that the longer the d 1  the higher the resonant frequency of the PCB antenna  400 , and the curves depicted in  FIG. 4C  show that the longer the d 1  the higher the impedance of the PCB antenna  400 . In other words, the PCB antenna  400  of the present invention can achieve the desired resonant frequency, impedance or other performances by adjusting the distance d 1 . 
         [0045]    Moreover, if the 0402 resistor  320  in  FIG. 3A  is replaced by a 0402 inductor  520  with inductance of 1.5 nH to form a PCB antenna  500  as shown in  FIG. 5A , the experimental results of the PCB antenna  500  are shown in  FIG. 5B  and  FIG. 5C . There are four curves  550 ,  552 ,  554  and  556  depicted in  FIG. 5B  based on increasing/decreasing the distance “d 2 ” labeled in  FIG. 5A . The curve  552  is depicted while d 2 =1 mm. The curve  554  is depicted while d 2 =3 mm. The curve  556  is depicted while d 2 =5 mm. And, the curve  550  is depicted while no element been mounted. There are also four curves  560 ,  562 ,  564  and  566  depicted in  FIG. 5C  corresponding to the curves  550 ,  552 ,  554  and  556 . As a result, the curves depicted in  FIG. 5B  show that the longer the d 2  the higher the resonant frequency of the PCB antenna  500 , and the curves depicted in  FIG. 5C  show that the longer the d 2  the lower the impedance of the PCB antenna  500 . In other words, the PCB antenna  500  of the present invention can achieve the desired resonant frequency, impedance or other performances by adjusting the distance d 2 . However, the variations in impedance are slight. 
         [0046]    Back to  FIG. 4A , if d 1  is fixed at  5 mm and the inductance of the 0402 inductor  420  is varied, the experimental results of the PCB antenna  400  are shown in  FIG. 6A  and  FIG. 6B . There are four curves  650 ,  652 ,  654  and  656  depicted in  FIG. 6A . The curve  652  is depicted when the inductance of the 0402 inductor  420  has a value of 1 nH. The curve  654  is depicted when the inductance of the 0402 inductor  420  has a value of 2 nH. The curve  656  is depicted when the inductance of the 0402 inductor  420  has a value of 4 nH. And, the curve  650  is depicted when a 0 nH element mounted. There are also four curves  660 ,  662 ,  664  and  666  depicted in  FIG. 6B  corresponding to the curves  650 ,  652 ,  654  and  656 . As a result, the curves depicted in  FIG. 6A  show that the larger the inductance added, the lower the resonant frequency of the PCB antenna  400 , and the curves depicted in  FIG. 6B  show that the larger the inductance added, the lower the impedance of the PCB antenna  400 . 
         [0047]    Back to  FIG. 5A , if d 2  is fixed at 5 mm and the inductance of the 0402 inductor  520  is varied, the experimental results of the PCB antenna  500  are shown in  FIG. 7A  and FIG.  7 B. There are four curves  750 ,  752 ,  754  and  756  depicted in  FIG. 7A . The curve  752  is depicted when the inductance of the 0402 inductor  420  has a value of 1 nH. The curve  754  is depicted when the inductance of the 0402 inductor  420  has a value of 2 nH. The curve  756  is depicted when the inductance of the 0402 inductor  420  has a value of 4 nH. The curve  750  is depicted when a 0 nH element added. There are also four curves  760 ,  762 ,  764  and  766  depicted in  FIG. 7B  corresponding to the curves  750 ,  752 ,  754  and  756 . As a result, the curves depicted in  FIG. 7A  show that the larger the inductance the lower the resonant frequency of the PCB antenna  500 , and the curves depicted in  FIG. 7B  show that the larger the inductance the higher the impedance of the PCB antenna  500 . 
         [0048]    It should be noted that, from  FIG. 6A ,  FIG. 6B ,  FIG. 7A  and  FIG. 7B , the variations in resonant frequency and impedance are enlarged with the inductance of the 0402 inductor decreasing. 
         [0049]    Although a resistor or an inductor is mounted on the PCB antenna  200 ,  300 ,  400  or  500 , a capacitor may be used in the present invention. Specifically, if the 0402 resistor  220  in  FIG. 2A  is replaced by a 0402 capacitor  820  with capacitance of 1.5 pF to form a PCB antenna  800  as shown in  FIG. 8A . The experimental results of the PCB antenna  800  are shown in  FIG. 8B  and  FIG. 8C . There are four curves  850 ,  852 ,  854  and  856  depicted in  FIG. 8B  based on increasing/decreasing the distance “d 3 ” labeled in  FIG. 8A . The curve  852 .is depicted while d 3 =1 mm. The curve  854  is depicted while d 3 =2 mm. The curve  856  is depicted while d 3 =3 mm. And, the curve  850  is depicted while no element been mounted. There are also four curves  860 ,  862 ,  864  and  866  depicted in  FIG. 8C  corresponding to the curves  850 ,  852 ,  854  and  856 . As a result, the curves depicted in  FIG. 8B  show that the longer the d 3  the lower the resonant frequency of the PCB antenna  800 , and the curves depicted in  FIG. 8C  show that the longer the d 3  the lower the impedance of the PCB antenna  800 . 
         [0050]    Moreover, if the 0402 resistor  320  in  FIG. 3A  is replaced by a 0402 capacitor  920  with capacitance of 1.5 pF to form a PCB antenna  900  as shown in  FIG. 9A , the experimental results of the PCB antenna  900  are shown in  FIG. 9B  and  FIG. 9C . There are four curves  950 ,  952 ,  954  and  956  depicted in  FIG. 9B  based on increasing/decreasing the distance “d 4 ” labeled in  FIG. 9A . The curve  952  is depicted while d 4 =1 mm. The curve  954  is depicted while d 4 =1.5 mm. The curve  956  is depicted while d 4 =2 mm. And, the curve  950  is depicted while no element been mounted. There are also four curves  960 ,  962 ,  964  and  966  depicted in  FIG. 9C  corresponding to the curves  950 ,  952 ,  954  and  956 . As a result, the curves depicted in  FIG. 9B  show that the longer the d 4  the lower the resonant frequency of the PCB antenna  900 , and the curves depicted in  FIG. 9C  show that the longer the d 4  the higher the impedance of the PCB antenna  900 . 
         [0051]    Back to  FIG. 8A , if d 3  is fixed at 1 mm and the capacitance of the 0402 capacitor  820  is varied, the experimental results of the PCB antenna  800  are shown in  FIG. 10A  and  FIG. 10B . There are four curves  1052 ,  1054 ,  1056  and  1058  depicted in  FIG. 10A . The curve  1052  is depicted when the capacitance of the 0402 capacitor  820  has a value of 1 pF. The curve  1054  is depicted when the capacitance of the 0402 capacitor  820  has a value of 2 pF. The curve  1056  is depicted when the capacitance of the 0402 capacitor  820  has a value of 3 pF. The curve  1058  is depicted when the capacitance of the 0402 capacitor  820  having a value of 4 pF. There are also four curves  1062 ,  1064 ,  1066  and  1068  depicted in  FIG. 10B  corresponding to the curves  1052 ,  1054 ,  1056  and  1058 . As a result, the curves depicted in  FIG. 10A  show that the larger the capacitance the lower the resonant frequency of the PCB antenna  800 , and the curves depicted in  FIG. 10B  show that the larger the capacitance the lower the impedance of the PCB antenna  800 . 
         [0052]    Back to  FIG. 9A , if d 4  is fixed at 1 mm and the capacitance of the 0402 capacitor  920  is varied, the experimental results of the PCB antenna  900  are shown in  FIG. 11A  and  FIG. 11B . There are four curves  1152 ,  1154 ,  1156  and  1158  depicted in  FIG. 11A . The curve  1152  is depicted when the capacitance of the 0402 capacitor  920  has a value of 1 pF. The curve  1154  is depicted when the capacitance of the 0402 capacitor  920  has a value of 2 pF. The curve  1156  is depicted when the capacitance of the 0402 capacitor  920  has a value of 3 pF. The curve  1156  is depicted when the capacitance of the 0402 capacitor  920  having a value of 4 pF. There are also four curves  1162 ,  1164 ,  1166  and  1168  depicted in  FIG. 11B  corresponding to the curves  1152 ,  1154 ,  1156  and  1158 . As a result, the curves depicted in  FIG. 11A  show that the larger the capacitance the lower the resonant frequency of the PCB antenna  900 , and the curves depicted in  FIG. 11B  show that the larger the capacitance the higher the impedance of the PCB antenna  900 . 
         [0053]    It should be noted that, from  FIG. 10A ,  FIG. 10B ,  FIG. 11A  and  FIG. 11B , the variations in resonant frequency and impedance are enlarged with the capacitance of the 0402 capacitor increasing. 
         [0054]    Consequently, if an inductor is mounted on the top surface as shown in  FIG. 4A , the resonant frequency is increased and the input impedance is increased while the distance (i.e. d 1  as described above) increases. If an inductor is mounted on the bottom surface as shown in  FIG. 5A , the resonant frequency is increased and the input impedance is decreased while the distance (i.e. d 2  as described above) increases. In addition, if a capacitor is mounted on the top surface as shown in  FIG. 8A , the resonant frequency is decreased and the input impedance is decreased while the distance (i.e. d 3  as described above) increases. If a capacitor is mounted on the bottom surface as shown in  FIG. 9A , the resonant frequency is decreased and the input impedance is increased while the distance (i.e. d 4  as described above) increases. In other words, if increasing the resonant frequency with decreasing the input impedance of a PCB antenna is desired, mounting a capacitor on the bottom surface with suitable distance may be chose. And if the scale-up of variation is desired, a capacitor with higher capacitance may be chose. 
         [0055]    Throughout the present invention, a method for adjusting the resonant frequency and input impedance of a PCB antenna and a structure thereof are provided for a designer to tune the resonant frequency and input impedance of the PCB antenna easily and economically without any other matching circuit needed. Moreover, it is advantageous that the resonant frequency and input impedance of the PCB antenna may be tuned to desired value. That is, the performances of the PCB antenna of the present invention having different passive elements at different distances or locations are continuous. 
         [0056]    The present invention has been described above with reference to preferred embodiments. However, those skilled in the art will understand that the scope of the present invention need not be limited to the disclosed preferred embodiments. On the contrary, it is intended to cover various modifications and equivalent arrangements within the scope defined in the following appended claims. The scope of the claims should be accorded the broadest interpretation so as to encompass all such modifications and equivalent arrangements.