Abstract:
An arrangement for enhancing electrical isolation between antennas in antenna structures comprising at least two antennas, and a radio device applying the arrangement. The interfering antenna comprises components causing substantial degradation in radiation characteristics in the operating band of another antenna. For example, a PIFA ( 310 ) may comprise, instead of a short-circuit conductor, a conductive structure ( 312, 313, 314 ) having a parallel resonance in the operating band of another antenna ( 320 ). Mutual interference of radio parts using separate antennas can be made relatively small without electrical isolation arrangements between antenna elements. Moreover, the invention makes antenna filter design easier and reduces disadvantages caused by antenna filters.

Description:
FIELD OF THE INVENTION 
     The invention relates to an arrangement for enhancing electrical isolation between antennas in antenna structures comprising at least two antennas. The invention also relates to a radio device employing a dual antenna according to the invention. 
     BACKGROUND OF THE INVENTION 
     Portable communications devices operating in two or more radio systems have become common in recent years. If such a communications device functions only in one system at a time, it is usually equipped with an antenna that has two operating bands or one band which is wide enough to cover both bands used by the two systems, for example. Two separate antennas may be used if the communications device can function simultaneously in two systems, especially if the frequency bands of the systems are relatively close to one another. With separate antennas, the mutual interference of the systems can be made smaller than with a common antenna. However, the mutual interference is not completely removed because there exists a certain electromagnetic coupling between the antennas. This problem can be in principle alleviated by increasing the distance between the antennas, which, however, will in practice make the structure too large. An interfering transmitter may also be equipped with an antenna filter the attenuation of which increases steeply on that side of the pass band where the operating band of the affected receiver is located. The order of such a filter is high, resulting in higher production costs and problems related to the pass-band attenuation of the filter. All increases in losses between the power amplifier and antenna will result in increased current consumption in the power amplifier and potential heating problems in the device. 
     Electromagnetic coupling between antennas can also be reduced by arranging electrical isolation between them.  FIG. 1  illustrates such a known solution.  FIG. 1  shows the antenna end of a transmitter operating according to a first system, and the antenna end of a receiver operating according to a second system. The transmitter includes a series connection of a RF power amplifier PA, transmitting end antenna filter SFI and a transmitting antenna  110 . The filter SFI is relatively simple in that its pass-band attenuation is not harmfully high. The receiver includes a receiving antenna  120  which is connected to a receiving end antenna filter RFI which in turn is connected to a low-noise amplifier LNA. The first system is for example GSM1800 (Global System for Mobile Communications) and the second system e.g. GPS (Global Positioning System) in which the receiving frequency is 1575.42 MHz. In that case GPS reception will be susceptible to interference from GSM transmissions because the gap between the GPS receiving frequency and GSM transmission band is only 135 MHz. In  FIG. 1  there is a line  105  between the antenna symbols, referring to an arrangement which electromagnetically isolates the transmitting and receiving antennas. Such an arrangement may be e.g. a grounded metal strip placed between the antenna elements. A disadvantage of this solution is that it increases the amount of hardware as well as production costs. Moreover, the directional characteristics of the antennas may suffer. 
     An object of the invention is to reduce said disadvantages associated with the prior art. An antenna structure according to the invention is characterized by that which is specified in the independent claim  1 . A radio device according to the invention is characterized by that which is specified in the independent claim  13 . Some advantageous embodiments of the invention are specified in the other claims. 
     SUMMARY OF THE INVENTION 
     The basic idea of the invention is as follows: An antenna structure comprises at least two adjacent but separate antennas with different operating bands. An interfering antenna comprises structural parts which cause substantial degradation of radiation characteristics at the operating band frequencies of the other antenna. This reduces interference level in the receiver to which the other antenna is connected. To realize the invention, a PIFA (planar inverted F antenna), for instance, may have, instead of a short-circuit conductor, a conductor structure which has a parallel resonance in the operating band of the other antenna. 
     An advantage of the invention is that mutual interference of radio parts using separate antennas can be made relatively small without using an arrangement for electrical isolation between the antenna elements. This is based on the fact that the transmission power of the interfering antenna drops in the operating band of the other antenna. Another advantage of the invention is that it makes antenna filter design easier and reduces disadvantages caused by antenna filters. A further advantage of the invention is that an arrangement according to the invention will not affect the directional characteristics of the antennas. A yet further advantage of the invention is that the necessary structural parts can be partly implemented in conjunction with antenna element manufacturing, without extra production stages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is below described in detail. The description refers to the accompanying drawings in which 
         FIG. 1  shows an antenna isolation solution according to the prior art, 
         FIG. 2  schematically shows an antenna isolation solution according to the invention, 
         FIG. 3  shows an example of an antenna structure according to the invention, 
         FIG. 4  shows a second example of an antenna structure according to the invention, 
         FIG. 5  shows a third example of an antenna structure according to the invention, 
         FIG. 6  shows a fourth example of an antenna structure according to the invention, 
         FIGS. 7   a,b  show a fifth example of an antenna structure according to the invention, 
         FIGS. 8   a,b  show a sixth example of an antenna structure according to the invention, 
         FIG. 9  shows an example of the effect of an arrangement according to the invention on antenna isolation, and 
         FIG. 10  shows an example of a radio device equipped with an antenna according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  was already discussed in connection with the description of the prior art. 
       FIG. 2  schematically shows an antenna isolation solution according to the invention. Like in  FIG. 1 , here, too, are shown the antenna end of a transmitter operating according to a first system, and the antenna end of a receiver operating according to a second system. The difference from  FIG. 1  is that the electromagnetic isolation arrangement between the transmitting antenna  210  and receiving antenna  220  is now missing. Instead,  FIG. 2  shows symbol  215  referring to an arrangement included in the transmitting antenna structure to provide electromagnetic isolation of the antennas. Isolation is realized such that the arrangement  215  causes substantial deterioration in the radiation characteristics of the transmitting antenna  210  in the operating band of the receiving antenna  220 . 
       FIG. 3  shows an example of an antenna structure according to the invention. It includes two PIFA-type antennas where a unitary, relatively massive ground plane GND serves as a ground electrode. The first antenna  310  includes a radiating plane  311 . Let us call it a transmitting antenna even though it may also function as a receiving antenna of a bi-directional system. The second antenna  320  includes a radiating plane  321 . Let us call it a receiving antenna even though it may also function as a transmitting antenna of a bi-directional system. The receiving antenna  320  also includes a conventional short-circuit conductor  322  and feed conductor  325 . 
     The feed conductor  315  of the transmitting antenna  310  is conventional, too. The short-circuit conductor, instead, is in accordance with the invention. In this example, the short-circuit conductor or, actually, short-circuit arrangement comprises a conductive wire  314  and an extension  312  to the radiating plane  311 , directed towards the ground plane, which extension has a conductive plate  313  parallel to the ground plane GND. The conductive plate  313  and ground plane are so close to each other that there is a significant capacitance C between them. The shape of the conductive wire  314  is in this example arcuate. It is connected by one end to the ground plane and by the other end to the radiating plane near the beginning of its extension  312 . The conductive wire is so thin that it causes a significant inductance L beside the capacitance C. The resulting parallel resonance circuit is dimensioned so as to have a resonance frequency equal to the center frequency of the reception band of the receiving antenna  320 . The impedance of said resonance circuit in the operating band of the transmitting antenna  310  is small, so the antenna radiates and receives well. In the operating band of the receiving antenna the impedance of said resonance circuit is high, whereby the matching of the transmitting antenna is poor and it radiates weakly. Matching is of course degraded alone by the fact that operation is now off from the operating band proper of the transmitting antenna. However, this does not produce sufficient isolation between the antennas if their bands are relatively close to one another. The arrangement according to the invention decidedly enhances the isolation. 
       FIG. 3  does not show any support structure for the radiating planes. Such a structure may comprise e.g. a dielectric frame along the edges of the plane. 
       FIG. 4  shows a second example of an antenna structure according to the invention. There are two parallel antennas in close proximity to each other, like in FIG.  3 . The radiating elements of the antennas are in this case conductive patterns on the surface of a printed circuit board  401 . The radiating/receiving element of the receiving antenna  420  is a meandering pattern. The transmitting antenna  410  is a PIFA. In this example is has two bands, because the radiating plane  411  is divided by a nonconductive slot  419  into two branches of different lengths. The transmitting antenna comprises a short-circuit arrangement functioning as a parallel resonance circuit, like the structure in FIG.  3 . In this case the short-circuit arrangement includes a first conductive block  412  connected to the radiating plane  411 , a second conductive block  413  connected to the ground plane GND, and a conductive wire  414 . The first and second conductive blocks face each other. Their facing surfaces are planar and so close to each other that there exists a significant capacitance C between the first and second conductive blocks. The first conductive block may form a single entity with the radiating plane  411  and the second conductive block with the ground plane. The conductive wire  414  starts from the ground plane, makes a single loop, goes through a via in the circuit board, and ends at the radiating plane next to the connection point of the first conductive block. The conductive wire  414  has a certain inductance L. 
       FIG. 5  shows a third example of an antenna structure according to the invention. In this example the first, i.e. transmitting, antenna is a PIFA and the second, or receiving, antenna is a monopole the whip element  521  of which can be pushed inside the radio device. The ground plane GND shared by the both antennas is now a conductive plane on a surface of a printed circuit board  505  in the radio device. The short-circuit conductor  512  of the transmitting antenna is in this example conventional. The antenna feed arrangement, instead, is in accordance with the invention. A conventional feed conductor is replaced by a series connection of a discrete capacitor  516  and conductor  515 . The capacitor is located on the opposite side of the printed circuit board  505  as seen from the radiating plane  511  of the transmitting antenna. One electrode of the capacitor is connected to the feeding antenna port AP, and one end of the conductor  515  to the feed point F of the radiating plane  511 . The thickness of the conductor  515  is chosen such that its inductance L is suitable. The series resonance circuit is designed so that its resonance frequency equals the center frequency of the operating band of the transmitting antenna. The impedance of the series resonance circuit in the operating band of the transmitting antenna is small, so the antenna radiates and receives well. In the operating band of the receiving antenna the impedance of the series resonance circuit is high, whereby the matching of the transmitting antenna is poor and it radiates weakly. 
       FIG. 5  shows a short part of the frame  508  supporting the radiating plane  511 . The support structure for the whip element  521  is not shown except for a dielectric block  529  on the printed circuit board  505  next to the lower end of the extended whip element. The feed conductor  525  of the whip antenna comes through said block into a contact surface on said block  529 . 
       FIG. 6  shows a fourth example of an antenna structure according to the invention. Of the two antennas there is shown only the one the transmission of which tends to interfere with the reception of the other. In this example, too, the transmitting antenna  610  is a PIFA; it is fed at a point F of the radiating plane and it has a short-circuit conductor  612 . A conductive layer on the upper surface, or the surface nearest to the radiating plane, of a circuit board  605  in the radio device serves as a ground plane GND. The feeding is capacitive. A “hot” pole of the antenna port AP of the transmitting antenna is galvanically connected to a conductive area  602  on the upper surface of the circuit board  605 , which area is insulated from the ground plane. Above this conductive area there is a parallel conductive plate  617 , galvanically coupled through conductor  615  to the radiating plane at its feed point F. Between the conductive area  602  and conductive plate  617  there is a certain capacitance C. The gap between the conductors in question may contain air or some dielectric material to increase the capacitance and stabilize the structure. The short-circuit conductor  612  is so thin that its inductance L is significant to the operation of the antenna. Instead of the straight conductor shown here it may naturally be a conductor wound in a coil. 
       FIG. 6  further shows a simplified equivalent circuit of the antenna  610 . Starting from the antenna port AP and following the feed conductor, there is first a capacitance C and feed point F. Between the latter and signal ground there is antenna radiation resistance R r . From the feed point there is a certain, mainly reactive, impedance Z to the short-circuit point S of the radiating plane. Between the short-circuit point and signal ground there is an inductance L. The other pole of the antenna port is connected to the signal ground. The values of the capacitance C and inductance L are chosen such that the transmitting antenna is matched in its own operating band, i.e. the impedance that cab be “seen” in the antenna port is nearly resistive and relatively near the internal impedance of the feeding source. When shifting into the operating band of the other antenna, the matching of the transmitting antenna deteriorates as the radiation resistance goes reactive and, according to the invention, because of the inductance L and capacitance C. 
       FIGS. 7   a  and  b  show a fifth example of an antenna structure according to the invention. Of the two antennas there is shown only the one the transmission of which tends to interfere with the reception of the other. A transmitting PIFA  710  is shown in  FIG. 7   a  from the side of the feed and short-circuit conductors, and in  FIG. 7   b  also laterally but 90 degrees horizontally rotated from the position shown in  FIG. 7   a . Between the radiating plane  711  and ground plane GND, extending up to both of these, there is in this example a small circuit board  707 . The circuit board  707  includes a straight microstrip  712  which serves as a short-circuit conductor, and a microstrip  715  which serves as a feed conductor. The latter is so thin that it has a significant inductance. The feed strip  715  is here connected by its lower end to the antenna port AP of the antenna  710 . An intermediate point in the feed strip is capacitively coupled to ground via a chip capacitor  716  on the circuit board  707 . This kind of feed arrangement is designed so that the matching of the transmitting antenna is good in its operating band but relatively poor in the operating band of the receiving antenna. 
       FIGS. 8   a  and  b  illustrate a sixth example of an antenna structure according to the invention. In this case, too, the receiving antenna to be shielded is not shown. A transmitting PIFA  810  is shown in  FIG. 8   a  from the side of the feed and short-circuit conductors. Between the radiating plane  811  and ground plane GND, extending up to both of these, there is a small printed circuit board  807 . This is in  FIG. 8   b  shown from the back, i.e. from inside the antenna  810 . The printed circuit board  807  includes a straight feed microstrip  815  and short-circuit strips  812   a  and  812   b . A first short-circuit strip  812   a  on the front side of the printed circuit board starts from the radiating plane  811  and forms a rectangular “spiral” to increase the inductance. It continues, after a via, on the back side of the printed circuit board in the other short-circuit strip  812   b . The latter is connected to the ground plane by its lower end. On the back side of the printed circuit board there is also a chip capacitor  813  connected in parallel with a coil formed by the short-circuit strips  812   a,b . The resulting resonance circuit is designed like in the cases depicted by FIGS.  3  and  4 : In the operating band of the transmitting antenna  810  the impedance of the resonance circuit is small but in the operating band of the receiving antenna it is high. 
       FIG. 9  shows an example of the improved electrical isolation that can be achieved between antennas in accordance with the invention. A test signal is fed into an antenna of the GSM1800 system, and a level measurement is done in the output of the antenna of a GPS receiver in the same radio device. Curve  91  represents the isolation attenuation of the antennas with no special GPS reception shielding. The isolation attenuation is of course at its smallest when the frequency of the test signal is 1575.42 MHz, or the frequency used in the GPS system; the attenuation is then only 3.8 dB. Curve  92  shows the isolation attenuation of the antennas when the transmitting antenna has been modified in accordance with the invention in order to shield GPS reception. A resonance circuit in the transmitting antenna raises the isolation attenuation by about 17 dB at the GPS frequency, making it 20.8 dB. A prior-art isolation arrangement corresponding to  FIG. 1  will in practice produce an isolation attenuation of about 10 dB, so the improvement from that arrangement, too, is considerable. 
       FIG. 10  shows a radio device MS. It has a first  010  and second  020  antenna. The first antenna includes an arrangement  012  according to the invention. 
     Above we described a few solutions according to the invention. The invention does not limit the shapes of antenna elements and additional parts according to the invention, nor the method of manufacturing of the antenna. Also both of the two antennas may include an arrangement according to the invention. This may be the case e.g. when a device includes separate UMTS (Universal Mobile Communication System) and WLAN (Wireless Local Area Network) antennas. The inventional idea may be applied in various ways within the scope defined by the independent claim  1 .