Patent Publication Number: US-11050146-B2

Title: Wideband antenna balun

Description:
The present teachings generally relate to antennas. More specifically, the present teachings relate to antennas for receiving and transmitting an electromagnetic signal. 
     Bandwidth of dipole antenna elements can be increased by arranging them in a biconical or a bow-tie type shape, or their likes. A dipole antenna should preferably be fed using a balanced transmission line, however feeding lines such as coaxial type are unbalanced transmission lines wherein, one terminal is typically at a ground potential. Such unbalanced transmission lines can also be called single-ended transmission lines, or simply single-ended lines. Since a dipole antenna presents a balanced input, i.e., both terminals typically having an equal but opposite voltage with respect to ground, when a balanced antenna is fed with an unbalanced or a single-ended line, common mode currents can cause the coax line to radiate in addition to the antenna itself. Unwanted effects arising when a dipole antenna is fed with an unbalanced line may include, distortion in the radiation pattern, and alteration in the impedance seen by the line. 
     One way to properly feed the dipole whilst retaining its expected characteristics is by using a balun in between the coaxial feedline and the antenna terminals. 
     Transformer type baluns are commonly used with HF-antennas, but become lossy and bulky at higher frequencies as VHF and UHF, where delay line type baluns are more common. Delay line baluns may however suffer from limited bandwidth, as the phase-shift of a delay line is frequency dependent. In addition Pawsey stub type balun configurations are also known that require a quarter wavelength stub to balance the antenna feed. 
     The above-mentioned and other problems inherent to the prior art will be shown solved by the features of the accompanying independent claims. 
     According to an object of the present teachings it can be provided a balun that reduces radiation from an unbalanced feeder of a wideband dipole, whilst extending the low frequency range. Said unbalanced feeder is for example a coaxial cable. Accordingly, the frequency range of the wideband dipole can be enhanced, or the dipole can be made more wideband. 
     According to another aspect a use of a connection point between the conductors of the feeding network to increase the wideband performance of an antenna can be provided. 
     In a further object of the present teachings it can be provided an antenna or antenna arrangement to simultaneously provide a second feed for an auxiliary antenna. 
     A typical application of the present teachings can be a combined multi-band mobile and GPS antenna, however the person skilled in the art will understand that the present teachings can be applied in other wireless and transmission-line applications as well. All embodiments and aspects of the present teachings in this disclose are meant to be examples demonstrating the present invention in a simple form for ease of understanding. The examples are not meant as limitations, or as affecting the scope of the present teachings. The skilled person will further appreciate that the teachings can also be applied to other kinds of symmetrical or balanced antennas, and is not limited to dipole antennas only. For keeping the discussions concise and simple the case the term dipole antenna is used in the rest of the disclosure without limitation and without affected the scope. 
    
    
     
       The invention will now be discussed more in detail using the following drawings illustrating certain aspects of present teachings by way of examples. The figures are not necessarily drawn to scale. 
         FIG. 1  illustrates a simple bow-tie type dipole antenna connected to a balanced feeder 
         FIG. 2  illustrates a simple bow-tie type dipole antenna connected to a coaxial feeder 
         FIG. 3  illustrates an embodiment of the present teachings when applied to a bow-tie dipole antenna 
     
    
    
       FIG. 1  shows a simple bow-tie type dipole antenna  100 . The antenna  100  comprises two identical elements  101  and  102 . The first antenna element  101 , on the left hand side, in the figure is electrically connected to a first feeding line  103 . The second antenna element  102 , on the right hand side, in the figure is electrically connected to a second feeding line  104 . Each of the antenna elements  101  and  102  have identical width  107 . Furthermore, the length  110  of the first antenna element  101  is identical to the length  120  of the second antenna element. In terms of physical dimensions, the dipole antenna  100  has a total length  105  which is usually half the wavelength of frequency of interest. Due to the width  107  the antenna appears longer and also provides more bandwidth. 
     The antenna elements  101  and  102 , also called poles of the dipole  100 , are spaced apart by a relatively small spacing width  108 . This spacing width is typically substantially small such that the sum of the lengths  110  and  120  of the antenna elements  101  and  102  is approximately equal to the length  105  of the antenna  100 . In  FIG. 1 , the feeder line consisting of the first feeding line  103  and the second feeding line  104  is shown symmetrical and balanced. 
       FIG. 2  shows a case where a dipole antenna  200  is connected to an unbalanced feeder. In this case, the unbalanced feeder is shown in the form of a coaxial cable  201 . The shown coaxial cable  201  comprises two conducting paths; first is a braided shield or mesh  206  that is connected to the first antenna element  101  through a first conductor  203 . The first conductor  203 , for example can be a solder patch or such. The second is an inner conductor  204  of the coaxial cable  201 . The inner conductor  204  is connected to the second antenna element  102 . This conductor  204  can also be soldered, or be connected by any other suitable means to the second antenna element  102 . The inner conductor  204  is electrically insulated from the braided shield by a dielectric or insulator  205 . The braided mesh  206  is further shown isolated by black isolation except for the portion of the cable  201  that is shown exposed in the figure, i.e., the portion where the braided mesh  206  is visible. A person skilled in the art will note that coaxial cable is shown as an example only, in practice an unbalanced feed even as a coaxial conductor may also be applied as a PCB trace or similar. In other words, the feed need not be using an actual cable per se. 
     An unbalanced feed may also be implied by non-symmetrical signals applied to a symmetrical antenna. 
     As discussed previously, such an unbalanced arrangement as shown in  FIG. 2  is not desirable due to problems mentioned previously in this disclosure. 
     Now referring to  FIG. 3 , which shows an antenna arrangement  300  that shows embodiments according to an aspect of the present teachings when applied to more of an hourglass shaped dipole antenna. In this example, an hourglass shape is demonstrated for example to achieve a more wideband response. The antenna shape is not to be considered limiting to the scope or generality of the present teachings; it is rather selected dependent upon what kind of characteristics are desired. The dipole antenna shown in  FIG. 3  comprises two electrically conducting elements or poles  301  and  302  respectively. The poles are aligned symmetrically along an axis  380  that runs along the  105  length of the antenna  300 . The antenna arrangement  300  also comprises a first feed line  303 . The first feed line  303  comprises two conducting lines  313  and  306 . The first feed line  303  has a first length spanning between a first device end  320  and a first antenna end  321 . The first conducting line or first electrical conductor  313  is shown as an outer conductor of a coaxial type arrangement. For the case of a coaxial cable, the first conducting line  313  would correspond to the braided shield, for example as  206  shown in  FIG. 2 . For other types of coaxial arrangements, such as delay line type PCB traces, the first conducting line  313  will correspond to the outer conductor. The second conducting line or second electrical conductor  306  is shown as an inner conductor of a coaxial type arrangement. The first electrical conductor  313  and the second electrical conductor  306  are electrically isolated from each other along the first length. For a coaxial cable, the first conductor  313  and the second conductor  306  are typically isolated by a dielectric. For the case of a coaxial cable, the second conducting line  306  would correspond to the inner conductor, for example as  204  shown in  FIG. 2 . For other types of coaxial arrangements, such as delay line type PCB traces, the second conducting line  306  will correspond to the inner conductor. The first device end  320  is arranged to be connected to a device (not shown in the figure) using the antenna arrangement  300  (the device is not shown in the figure). At the antenna end  321  the first conducting line  313  is electrically connected to the first antenna element  301 . The second conducting line  306  is electrically connected to the second antenna element  302 . A person skilled in the art will appreciate that an arrangement consisting of the first antenna element  301 , the second antenna element  302 , and the first feed line  303  arranged as explained above corresponds to the antenna arrangement of  FIG. 2 . 
     The antenna arrangement  300  further comprises a second feed line  304 . The second feed line  304  comprises two conducting lines  323  and  308 , namely third conducting line or third electrical conductor  323  and fourth conducting line or fourth electrical conductor  308  respectively. The second feed line  304  is shown spilt in two parts  304   a  and  304   b  respectively in the figure. The first part  304   a  has a second length spanning between a second device end  330  and a second antenna end  331 . The second part  304   b  has a third length spanning between a third antenna end  341  and a remote end  343 . The functionality associated with the second part  304   b  of the second feed line  304  is to be covered under another aspect of the present teachings, and should be considered as a preferable embodiment of the teachings rather than being essential to the most general embodiment of the present teachings. Furthermore, a corner or knee between ends  321  and  331  is shown primarily to highlight that the third conducting line  323  is connected to the second antenna element  302  at or close to the second antenna end  321 . The fourth conducting line  308  is not conductively connected to either of the antenna elements  301  or  302 , instead the fourth conducting line  308  is connected to or forms an auxiliary antenna  350 . The third conducting line  323  is also further conductively connected to the second antenna element  302  at or close to the remote end  343 . In case of a coaxial arrangement the first conducting line  313  and the third conducting  323  line can also be called screening conductors. A conductive connection  333 , preferably in the form of a short circuit, is made between the first conducting line  313  and the second conducting line  323  at a predetermined length  390  from the antenna elements. The predetermined length  390  is less than the first length and the second length. According to another embodiment, the predetermined length is less than the width  107  of the elements. 
     According to one aspect of the present teachings, by electrically connecting the first conducting line  313  and the third conducting  323  line, at the connection point  333  located at a predetermined distance  390  from the antenna elements, the connection point  333  becomes an artificial neutral ground level, as the opposite polarized voltages at the feeds of the dipole add to a zero at this point. The predetermined distance can be defined with respect to a reference point related to the antenna elements. The predetermined distance or predetermined length  390  is in this case has been defined as the distance between the axis  380  and the location of the electrical connection  333 . 
     A skilled person will appreciate that usually for a given lowest frequency of interest, the antenna length, for example  105 , should be essentially half wavelength corresponding to the lowest frequency of interest. In conventional antennas, for frequencies below the lowest frequency of interest, the antenna impedance becomes capacitive such impedance mismatch will occur and the antenna will become ineffective. The applicant has realized that this can be compensated by choosing the distance  390  of the connection point  333  such that reactive current flowing through the connection point  333  via conductors  313  and  323  becomes essentially inductive and thus essentially compensates the capacitive nature of the antenna such that the antenna may be used at frequencies lower than given by the physical length of the antenna according to the half wavelength principle explained above. It can be appreciated the location of the connection point  333 , or the distance  390  of it, will alter the effective inductive impedance offered by the connection point  333 . The distance  390  of the connection point  333  can thus be chosen such that desired compensation is achieved. 
     From the above, it can thus be appreciated that such an antenna, according to the present teachings, can save physical space and costs. In simpler terms, now in context of  FIG. 3 , it can be said that the distance  390  to the connection point  333  can be selected such that the impedance presented by the antenna arrangement is predominantly resistive even at frequencies below half wavelength physical length where the dipole itself becomes predominantly capacitive. According to the present teachings, the impedance presented by the antenna arrangement with the connection point is made predominantly resistive by compensating the capacitive behavior of the dipole at low frequencies by the essentially inductive behavior introduced by the connection point  333 . 
     According to one aspect, the distance  390  of connection point  333  is at or below λ/4, where λ/2 is the half wavelength of the antenna elements defined by the physical dimensions of the antenna. Accordingly, due to the connection point  333 , the antenna is able to operate at frequencies substantially lower than the λ/2 defined due to physical dimensions of the antenna. According to another aspect, the distance  390  is at or below λ/6. According to yet another aspect, the distance  390  is at or below λ/8. 
     As an example, with a bow-tie antenna having a length of around 10 cm, a distance  390  of around 5 cm the applicant has been able to operate the same antenna for lowest frequencies close to 700 MHz, which otherwise would have required an antenna length of around 21.5 cm. An appreciable space and cost benefit is thus achieved. A skilled person will realize that limits to lower frequencies will depend on the antenna design, and that presented example does not specify a limit of operation. 
     The present teachings can therefore improve the impedance of the dipole antenna arrangement at low frequency without degenerating the impedance at the upper frequency range. 
     Person skilled in the art will notice that the second feed line  304  adapted to carry the signal for an auxiliary antenna  350  is optional. Instead of the auxiliary antenna  350 , the fourth conducting line may also be connected to a sensor located on one of the poles,  301  or  302 , of the main antenna. The second feed line can thus offer additional space and cost advantages according to another aspect of the present teachings. 
     According to another embodiment, the second part  304   b  of the second feed line  304  lies at least partially over or partially overlaps the the second element  302 . 
     As another example, the antenna has the length  105  is around 100 mm, the width  107  is around 60 mm, and the predetermined distance  390  is around 22 mm. According to this embodiment, the antenna is operable typically from around 800 MHz and two octaves up. A person skilled in the art will understand that these dimensions are provided just as an example, the scope of this embodiment also covers the proportions between the mentioned dimensions, as well as dimensioning of various elements to achieve a given response or performance. 
     By adapting the antenna design, other frequency ranges can be achieved. For example, by appropriately shaping the antenna elements to increase the area, and by reducing the opening angle to adapt more of a Vivaldi antenna shape, the upper frequency may be extended appreciably. Shaping of the antenna elements can also be done without reducing the opening angle, and vice versa, as per the requirements. 
     The antenna formed by elements  301  and  302  can be called a main antenna, whereas the other antenna  350  can be used for auxiliary functions. For example, such an arrangement can be used to build a combined multi-band antenna, which can save costs and space. 
     In an another embodiment, at least one of the, antenna elements, and/or the feed lines are realized as PCB traces. In yet another embodiment, at least one of the, antenna elements, and/or the feed lines are realized as traces using any thin-film or thick film process. In yet another embodiment, at least one of the, antenna elements, and/or the feed lines are realized in a semi-conductor manufacturing process. 
     Present teachings also relate to the use of a connection point according to the teachings of this disclosure for enhancing the frequency range or wideband performance of an antenna. More specifically, the teachings relate to the use of a conductive connection  333  in the feeding network for feeding an antenna. The antenna comprises two antenna elements or poles,  301  and  302  respectively. preferably in the form of a short circuit, made between the first conducting line  313  and the second conducting line  323  at a predetermined length  390  from the antenna elements. The feeding network comprises the first conducting line  313  and the second conducting line  323 . The details of the antenna and the feeding network are already discussed in this disclosure, e.g., in context of  FIG. 3 . Accordingly, the present teachings also relate to the use of an antenna balun comprising the connection point for enhancing the frequency range or wideband performance of an antenna. 
     The examples in this disclosure are shown in their simplest sense for ease of explanation, and without limiting the scope or generality of the present teachings. A person skilled in the art will understand that the present teachings can be applied to different types of antennas. The teachings can be applied to any wireless application where a balun type functionality is required. The skilled person will also appreciate that the embodiments explained in this disclosure can be combined with each other to realize a wireless device according to specific requirements. Discussion of an embodiment separately does not mean that the embodiment cannot be used with the rest of the examples or embodiments presented herein. 
     To summarize, the present teachings relate to an antenna comprising a first element and a second element. The first element and the second element are arranged to be poles of the antenna. The antenna is adapted to be fed by a feeding network, wherein the feeding network comprises a first feed line having a first antenna end and a first device end. The first feed line has a first length spanning between the first antenna end and the first device end. The first feed line comprises a first electrical conductor and a second electrical conductor. The first electrical conductor is electrically continuous or has electrical conductivity along the first length. This means that two connections made at the first device end and the first antenna end respectively with the first electrical conductor will be electrically connected through the first electrical conductor spanning along the first length. Similarly, the second electrical conductor also has electrical continuity along said first length. The first electrical conductor and the second electrical conductor however are electrically isolated from each other along the first length. The antenna also comprises a second feed line having second antenna end and a second device end. The second feed line has a second length spanning between the second antenna end and the second device end. The second feed line comprises a third electrical conductor. The third electrical conductor has electrical continuity along said second length. In other words the third electrical conductor is electrically continuous along the first length. This means that two connections made at the second device end and the second antenna end respectively with the third electrical conductor will be electrically connected through the third electrical conductor spanning along the second length. The first electrical conductor is adapted to be electrically connected to the first antenna element at or close to the first antenna end. The second electrical conductor is adapted to be electrically connected to the second antenna element at or close to the first antenna end. The third electrical conductor is adapted to be electrically connected to the second antenna element at or close to the second antenna end. The feeding network also comprises an electrical connection between the first electrical conductor and the third electrical conductor, the electrical connection is done at a connection point located at a predetermined distance from a reference point related to the antenna elements. The predetermined distance is shorter than at least one of the first length and the second length. This is especially the case when predetermined distance is measured from the axis of symmetry running along the length of the antenna—when the antenna is dipole type. Preferably, the location of the electrical connection is chosen such that the impedance presented by it is inductive at low frequency range where the impedance of the first element and the second element is capacitive. The location is chosen such that the inductive impedance due to the connection point at least partially compensates for the capacitive impedance of the antenna elements below the half wavelength frequency. The connection point lies preferably at a distance at or below λ/4 from the antenna defined from the physical dimensions of the antenna. 
     In another embodiment, the second feed line also comprises a fourth electrical conductor, the fourth electrical conductor having electrical continuity along said certain length of the second feed line. The third electrical conductor and the fourth electrical conductor are electrically isolated from each other along the length of the second feed line. 
     According to another embodiment, the fourth electrical conductor is connected to an auxiliary antenna or a sensor. 
     According to yet another embodiment, the second feed line also comprises a third length. Said third length is spanning between a third antenna end and a remote end. The third antenna end is close to the second antenna end. The third antenna end and the second antenna end are preferably the same. Each of the third electrical conductor and the fourth electrical conductor are electrically continuous along their respective lengths between the second device end and the remote end. In other words, the third and fourth conductors have each continuity between the second device end and remote end. The third electrical conductor and the fourth electrical conductor however are electrically isolated from each other between the remote end and the device end. 
     In another embodiment, said auxiliary antenna is connected to the fourth electrical conductor at or close to the remote end. 
     In yet another embodiment, said sensor is connected to the fourth electrical conductor at or close to the remote end. 
     In another embodiment, at least one of the first feed line and the second feed line is a co-axial cable. In yet another embodiment, at least one of the first feed line and the second feed line is a microstrip. In yet another embodiment, at least one of the first feed line and the second feed line is a stripline. In yet another embodiment, at least one of the first feed line and the second feed line is a coplanar waveguide. 
     In further embodiments, at least one of the first device end and the second device end is operatively connected to a transmitter, receiver, or a transponder. 
     In another embodiment, at least the first element and the second element are a part of a GPS antenna. 
     In yet another embodiment, the auxiliary antenna is a mobile communications antenna. By mobile communications it is meant, GSM, CDMA, or such. 
     In another embodiment, the antenna is a combined multi-band antenna. 
     In another embodiment, the first element and the second element form at least partially a bow-tie or an hourglass type dipole having an antenna length and an element width. Preferably, the antenna length is around 100 mm; the element width is around 60 mm; and the predetermined distance is around 22 mm when measured from axis of symmetry running along the antenna length. 
     The present teachings also relate to an antenna balun comprising a first feed line and a second feed line. The first feed line has a first length spanning between a first antenna end and a first device end. The first feed line comprises a first electrical conductor and a second electrical conductor. The first electrical conductor and the second electrical conductor are electrically isolated from each other along the first length. The second feed line has a second length spanning between a second antenna end and a second device end. The second feed line comprises a third electrical conductor and a fourth electrical conductor. The first electrical conductor and the second electrical conductor are electrically isolated from each other along the second length. The balun comprises an electrical connection between the first electrical conductor and the third electrical conductor, the electrical connection being at a connection point located at a predetermined distance from the first antenna end or the second antenna end. The first antenna end is configured to be connected to a first antenna element, and the second antenna end is configured to be connected to a second antenna element. Preferably, the location of the electrical connection is chosen such that the impedance presented by it is inductive at low frequency range where the impedance of the first element and the second element is capacitive. The location is chosen such that the inductive impedance due to the connection point at least partially compensates for the capacitive impedance of the antenna elements below the half wavelength frequency. 
     The present teachings also relate to the use of the antenna balun for enhancing the frequency range of operation of an antenna. More specifically, the frequency range below the half wavelength frequency, defined by the physical dimensions of the antenna, is enhanced.