Abstract:
An adjustable monopole antenna especially intended for the mobile terminals. The adjusting circuit ( 930 ) of the antenna is located between the radiator ( 920 ) and the antenna port of a radio device and forms, together with the antenna feed conductor ( 901 ), a feed circuit. This circuit comprises an adjustable reactance between the feed conductor and the ground in series with the feed conductor or in both of those places. For example, the feed conductor can be connected by a multi-way switch to one of alternative transmission lines, which are typically short-circuited or open at their tail end and shorter than the quarter wave, each line acting for a certain reactance. The antenna operating band covers at a time only a part of the frequency range used by one or two radio systems, in which case the antenna matching is easier to arrange than of a real broadband antenna. The space required for both the radiator and the adjusting circuit is relatively small. There is no need to arrange a coupling to the radiator for the antenna adjusting, which means a simpler antenna structure and thus savings in production costs.

Description:
PRIORITY AND RELATED APPLICATIONS 
       [0001]    This application claims priority to International PCT Application No. PCT/FI2006/050418 entitled “Adjustable antenna” having an international filing date of Sep. 28, 2006, which claims priority to Finland Patent Application No. 20065116 of the same title filed Feb. 15, 2006, as well as Finland Patent Application No. 20055554 filed Oct. 14, 2005, each of the foregoing incorporated herein by reference in its entirety. This application is related to co-owned and co-pending U.S. patent application Ser. No. 12/083,129 filed Apr. 3, 2008 entitled “Multiband Antenna System And Methods” (Attorney docket No. LKP.014A/OP101722), Ser. No. 12/080,741 filed Apr. 3, 2008 entitled “Multiband Antenna System and Methods” (Attorney docket No. LKP.015A/OP101819), Serial No. 12/______ filed Apr. 10, 2008 entitled “Internal Antenna and Methods” (Attorney docket No. LKP.016A/OP101815), Ser. No. 12/009,009 filed Jan. 15, 2008 and entitled “Dual Antenna Apparatus And Methods”, Ser. No. 11/544,173 filed Oct. 5, 2006 and entitled “Multi-Band Antenna With a Common Resonant Feed Structure and Methods”, and co-owned and co-pending U.S. patent application Ser. No. 11/603,511 filed Nov. 22, 2006 and entitled “Multiband Antenna Apparatus and Methods”, each also incorporated herein by reference in its entirety. This application is also related to co-owned and co-pending U.S. patent application Ser. Nos. 11/648,429 filed Dec. 28, 2006 and entitled “Antenna, Component And Methods”, and 11/648,431 also filed Dec. 28, 2006 and entitled “Chip Antenna Apparatus and Methods”, both of which are incorporated herein by reference in their entirety. This application is further related to U.S. patent application Ser. Nos. 11/901,611 filed Sep. 17, 2007 entitled “Antenna Component and Methods”, 11/883,945 filed Aug. 6, 2007 entitled “Internal Monopole Antenna”, 11/801,894 filed May 10, 2007 entitled “Antenna Component”, and 11/______ entitled “Internal multiband antenna and methods” filed Dec. 28, 2007, each of the foregoing incorporated by reference herein in its entirety. 
     
    
     COPYRIGHT 
       [0002]    A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
         [0003]    The invention relates to an adjustable antenna especially intended for mobile terminals. 
       BACKGROUND OF THE INVENTION 
       [0004]    The adjustability of an antenna means in this description that a resonance frequency of the antenna can be changed electrically. The aim is that the operating band of the antenna around the resonance frequency always covers the frequency range, which the function presumes at each time. There are different causes for the need for adjustability. As portable radio devices, like mobile terminals, are becoming smaller thickness-wise, too, the distance between the radiating plane and the ground plane of an internal planar antenna unavoidably becomes shorter. This results in e.g. that the antenna bandwidths will decrease. Then, as a mobile terminal is intended for operating in a plurality of radio systems having frequency ranges relatively close to each other, it becomes more difficult or impossible to cover frequency ranges used by more than one radio system. Such a system pair is for instance GSM1800 and GSM1900 (Global System for Mobile telecommunications). Correspondingly, securing the function that conforms to specifications in both transmitting and receiving bands of a single system can become more difficult. If the system uses sub-band division, it is advantageous, from the point of view of the radio connection quality, if the resonance frequency of the antenna can be tuned in a sub-band being used at each time. 
         [0005]    In the invention described here the antenna adjusting is implemented by a switch. The use of switches for the purpose in question is well known as such. For example the publication EP1 113 524 discloses an antenna, where a planar radiator can at a certain point be connected to the ground by a switch. When the switch is closed, the electric length of the radiator is decreased, in which case the antenna resonance frequency becomes higher and the operating band corresponding to the resonance frequency is displaced upwards. A capacitor can be in series with the switch to set the band displacement as large as desired. In this solution the adjusting possibilities are very limited. 
         [0006]    In  FIG. 1  there is a solution including a switch, known from the publication EP 1 544 943. Of the antenna base structure, only the radiator  120  is drawn in the figure, which radiator can be a part of a larger radiating plane. The antenna comprises, in addition to the base structure, an adjusting circuit with a parasitic element  131 , a transmission line  132 , a two-way switch  133 , a first reactive circuit X 1  and a second reactive circuit X 2 . The head end of the first conductor of the transmission line is connected to the parasitic element, and the head end of the second conductor is connected to the ground. In practice, the second conductor can belong to the ground plane, which as such has no head and tail end. Each reactive circuit includes for example two or three reactive components. The transmission line  132  will be terminated, depending on the switch state, by one of the reactive circuits. When the switch is controlled so that its state changes, the electric length and resonance frequency of a certain part of the antenna change. This means that the corresponding operating band is displaced. 
         [0007]    The solution according to  FIG. 1  is intended for a multi-band antenna. In it the influence of the adjusting can be directed, when needed, only on one operating band of the antenna, and a good impedance matching can be arranged for the antenna in the band to be displaced. These matters are due to that there are several variables when designing the adjusting circuit. However, the solution is suitable only for the antennas of PIFA type, and the parasitic element used in it increases the structure costs. 
       SUMMARY OF THE INVENTION 
       [0008]    In a first aspect of the invention, an antenna of monopole type is disclosed. In one embodiment, the antenna comprises an adjusting circuit to change its resonance frequency and thus the place of its operating band. In this case the operating band covers at a time only a part of a frequency range used by one or two radio systems. The adjusting circuit is located between the radiator and the antenna port/switch of a radio device and forms, together with the antenna feed conductor, a feed circuit. This circuit comprises an adjustable reactance between the feed conductor and the ground or in series with the feed conductor or in both of those places. For example, the feed conductor can be connected by a multiple-way switch to one of alternative transmission lines, which are typically short-circuited or open at their tail end and shorter than the quarter wave, each line acting for a certain reactance. The lengths of the transmission lines and the values of the possible discrete components then are variables from the point of view of the antenna adjusting. 
         [0009]    An advantage of this exemplary embodiment of the invention is that the space required for an antenna according to it is very small due to the monopole structure. Despite its small size, a basic antenna having a relatively narrow band functions in practice as a broad band antenna, because only a part of this broad band is needed at a time. In addition, a good matching and efficiency are achieved over the whole width of the band, because the matching of a relatively narrowband antenna can be arranged more comfortably than of a real broadband antenna. A further advantage of this exemplary embodiment of the invention is that the space required for the adjusting circuit of the antenna is relatively small. This is due in part to physically short transmission lines in the adjusting circuit. A still further advantage of the invention is that the adjusting according to it does not require arrangement of a coupling to the antenna radiator, which means a simpler antenna structure and thus savings in production costs. 
         [0010]    In another aspect of the invention, an adjustable antenna is disclosed. In one embodiment, the antenna comprises: a radiator electrically coupled to an adjusting circuit, said adjusting circuit comprising a plurality of reactive circuits disposed between a feed conductor and a signal ground. Each of said plurality of reactive circuits generates a unique resonance frequency for said antenna. 
         [0011]    In one variant, the antenna further comprises an antenna switch, said antenna switch implementing time divisional sharing between a plurality of transmit/receive components. The plurality of transmit/receive components comprise for example a first transmitter and receiver for a first system, and a second transmitter and receiver for a second, different system. 
         [0012]    In another variant, said adjusting circuit further comprises at least one switch coupled to said plurality of reactive circuits, said at least one switch electrically coupled to a control feed. Signals received via said control feed trigger said at least one switch to change states thereby selecting one of said plurality reactive circuits. 
         [0013]    In yet another variant, said at least one switch comprises two switches disposed in electrical series with one another, said two switches enabling at least four reactive circuits between said feed conductor and said signal ground. For example, the at least four reactive circuits comprise a plurality of inductive and a plurality of capacitive electronic components. 
         [0014]    In a further variant, said at least one switch comprises a first and a second state, said first and second states characterized by a first and a second electronic component, respectively, said first and second electronic components disposed in electrical parallel with one another. 
         [0015]    In still another variant, said adjusting circuit comprises a phase shifter and a capacitance diode, said plurality of reactive circuits generated via adjustments generated by a control signal to said capacitance diode. 
         [0016]    In another aspect of the invention, a method of operating an adjustable antenna is disclosed. In one embodiment, the adjustable antenna comprises an adjusting circuit, a radiator and a feed conductor electrically coupling said adjusting circuit to said radiator, and said method comprises: operating said adjusting circuit in a first mode of operation, said first mode of operation associated with a first resonance frequency; receiving a control signal at said adjusting circuit to change states; and operating said adjusting circuit in a second mode of operation, said second mode of operation associated with a second resonance frequency. 
         [0017]    In one variant, the adjustable antenna further comprises an antenna switch coupled between a plurality of transmit/receive nodes, and said method further comprises: operating said antenna switch such that it time-shares between said plurality of transmit/receive nodes. 
         [0018]    In another aspect of the invention, an adjusting circuit useful in an antenna system is disclosed. In one embodiment, said adjusting circuit comprises a plurality of reactive circuits disposed between a feed conductor and a signal ground. Each of said plurality of reactive circuits generates a unique resonance frequency for said antenna. 
         [0019]    In one variant, said adjusting circuit further comprises at least one switch coupled to said plurality of reactive circuits, said at least one switch electrically coupled to a control feed. The signals received via said control feed trigger said at least one switch to change states, thereby controllably selecting one of said plurality reactive circuits. 
         [0020]    In another variant, said at least one switch comprises two switches disposed in electrical series with one another, said two switches enabling at least four distinct reactive circuits between said feed conductor and said signal ground. 
         [0021]    In a further variant, said at least four reactive circuits comprise a plurality of inductive and a plurality of capacitive electronic components. 
         [0022]    In yet another variant, said at least one switch comprises a first and a second state, said first and second states characterized by a first and a second electronic component, respectively, said first and second electronic components disposed in electrical parallel with one another. 
         [0023]    In a further variant, said adjusting circuit comprises a phase shifter and a capacitance diode, said plurality of reactive circuits generated via adjustments generated by a control signal to said capacitance diode. 
         [0024]    In yet another aspect of the invention, an adjustable antenna comprising a signal ground, monopole radiator having a feed conductor and an adjusting circuit to displace an operating band of the antenna, wherein the adjusting circuit and feed conductor together form a feed circuit of the antenna, the feed circuit comprising a reactive circuit between the feed conductor and the signal ground and at least one node pair, the reactance of a circuit between the nodes of which pair can be altered to change a resonance frequency of the antenna. 
         [0025]    In one variant, the number of said node pairs is one, one node of said pair being located at the feed conductor, and the other node of said pair being located in the signal ground, the circuit between the nodes of said pair comprises at least two inductive elements and a multiple-way switch to comprise a connection between the feed conductor and signal ground through one inductive element at a time. The inductive elements comprise for example short transmission lines. In one variant, the number of said transmission lines is three, and the operating bands corresponding thereto collectively substantially cover a frequency range at least 100 MHz wide. 
         [0026]    In another variant, the frequency range comprises a range of approximately 470-702 MHz associated with a DVB-H system. 
         [0027]    In yet another variant, said inductive elements comprise discrete coils. 
         [0028]    In still a further variant, the number of said node pairs is one, each node of said pair being located at the feed conductor, the circuit between the nodes of said pair being disposed in series with the feed conductor and comprising at least two capacitive elements and a multiple-way switch to constitute a connection between the nodes through one capacitive element at a time, said reactive circuit comprising a fixedly connected coil. 
         [0029]    In yet another variant, said at least one node pair comprises two node pairs, one node of a first of said node pairs being disposed at the feed conductor and the other node of said first pair being disposed at least partly in the signal ground, a circuit between the nodes of said first pair comprising at least two inductive elements and a multiple-way switch to form a connection between the feed conductor and signal ground through one inductive element at a time, and each node of a second pair of said two node pairs being disposed substantially at the feed conductor, the circuit between said second pair of nodes being disposed in electrical series with the feed conductor and comprising at least two capacitive elements and a multiple-way switch to form a connection between the nodes of said second pair through one of said capacitive elements at a time. 
         [0030]    In a further variant, wherein the number of said node pairs is one, one node of said pair being located substantially at the feed conductor and the other node of said pair being associated with the signal ground, and said circuit between the nodes comprising (i) a capacitance diode to change the reactance of the circuit, and (ii) a phase shifter to shift the adjustment range of the reactance of the circuit. 
         [0031]    In yet another variant, the adjusting circuit further comprises an LC circuit disposed electrically between the feed conductor and said switch to at least protect the switch against electrostatic discharge. 
         [0032]    In still another variant, said switch is selected from the group consisting of FET, PHEMT or MEMS devices. 
         [0033]    In a further variant, said antenna comprises an inverted L antenna (ILA). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    The invention is below described in detail. Reference will be made to the accompanying drawings where 
           [0035]      FIG. 1  presents an example of an adjustable antenna according to the prior art, 
           [0036]      FIG. 2  presents a principled structure of an antenna according to the invention, 
           [0037]      FIG. 3  presents as a block diagram an example of an adjusting circuit of an antenna according to the invention, 
           [0038]      FIG. 4  presents another example of an adjusting circuit of an antenna according to the invention, 
           [0039]      FIG. 5  presents a third example of an adjusting circuit of an antenna according to the invention, 
           [0040]      FIG. 6  presents a fourth example of an adjusting circuit of an antenna according to the invention, 
           [0041]      FIG. 7  presents as a circuit diagram an example of the implementation of an adjusting circuit according to  FIG. 3 , 
           [0042]      FIG. 8  presents an example of the implementation of the adjusting circuit according to  FIG. 7  by a circuit board, 
           [0043]      FIG. 9  presents an example of the wholeness of an antenna according to the invention, 
           [0044]      FIG. 10  presents an example of the displacement of an operating band of an antenna according to the invention, when the adjusting circuit is controlled, 
           [0045]      FIG. 11  presents as a Smith diagram an example of the impedance of an adjusting circuit of an antenna according to the invention, and 
           [0046]      FIG. 12  presents an example of the gain of an antenna according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0047]    Reference is now made to the drawings wherein like numerals refer to like parts throughout. 
         [0048]      FIG. 1  was already described in conjunction with the description of the prior art. 
         [0049]      FIG. 2  shows the principled structure of an antenna according to the invention as a simple block diagram. The radiator  220  of an antenna  200  is of monopole type. Also the feed conductor  201  and the adjusting circuit  230  of the antenna are here included in the antenna. Naturally also the common signal ground GND, necessary in the function of the structure, belongs to it. The feed conductor has been connected to the radiator at its one end and to the rest of the radio device in question at its other end. In the example of  FIG. 2  the radio device has the transmitters TX 1 , TX 2  and receivers RX 1 , RX 2  in compliance with two different systems, and its function is time divisional. For this reason the feed conductor is connected the transmitters and receivers through the antenna switch ASW. The adjusting circuit  230  engages the feed conductor  201  and forms together with it a feed circuit. The adjusting circuit is reactive by nature to avoid losses, and it receives a control CO from the radio device. A reactance value influencing in the circuit is altered by that control so that a resonance frequency of the antenna and along with it the place of an operating band change as desired. 
         [0050]    There is at least one node pair in the feed circuit, the reactance between which nodes can be altered by the control CO. One node of the pair is located along the feed conductor, and the other node can be located in the signal ground or at another point of the feed conductor. In the latter case the reactance to be altered is in series with the feed conductor. In all cases there is a reactive circuit, adjustable or constant, between the feed conductor and signal ground. Examples of the feed circuit are in  FIGS. 3-6 . 
         [0051]    In  FIG. 3  there is as a block diagram an adjusting circuit according to the invention, where the adjusting circuit  330  has been connected between the antenna feed conductor  301  and the signal ground GND. The adjusting circuit comprises an LC circuit  332 , a multiple-way switch  333  and three alternative reactive structure parts X 1 , X 2 , X 3 . The LC circuit has been connected to the feed conductor at its one end and to the switch input at its other end. Its aim is to attenuate the harmonic frequency components being generated in the switch and to function as an electrostatic discharge (ESD) protector of the switch. The switch  333  has three outputs, to one of which the switch input can be connected at a time by the control CO. Each output of the switch has been fixedly connected to one of said reactive structure parts, the reactances of which exist against the signal ground. The interchanging of the reactance by controlling the switch changes the resonance frequency of the antenna and thus the place of its operating band. The operating band of the antenna then has three alternative places in this example. 
         [0052]    In  FIG. 4  there is a feed circuit according to the invention, the adjusting circuit  430  of which comprises a part between the feed conductor  401  and the signal ground and another part in series with the feed conductor. The former part is located before the latter part, as seen from the antenna port/switch. Both parts are adjustable in this example. The part between the feed conductor and signal ground comprises a two-way switch SW 1  and two inductive structure parts L 41  and L 42 . Depending on the state of the switch SW 1 , one of the inductive structure parts L 41 , L 42  is connected from the feed conductor to the signal ground. The part in series comprises another two-way switch SW 2  and two capacitive structure parts C 41  and C 42 . Depending on the state of the switch SW 2 , one of the capacitive structure parts C 41 , C 42  is connected in series with the feed conductor  401 . The two-way switches SW 1  and SW 2  together form a switching unit  433 , which is controlled by the control signals CO. If the controls of the two-way switches are distinct, four alternative places are in principle obtained for the antenna operating band. 
         [0053]    In  FIG. 5  there is a feed circuit according to the invention, the adjusting circuit  530  of which comprises a part between the feed conductor  501  and the signal ground and another part in series with the feed conductor. The former part is located after the latter part, as seen from the antenna port/switch, and only the part in series is adjustable. The part between the feed conductor and signal ground consists of an inductive structure part L 51 . The part in series comprises a two-way switch  533  and two capacitive structure parts C 51  and C 52 . Depending on the state of the switch  533 , one of the capacitive structure parts C 51 , C 52  is connected in series with the feed conductor  501 . The switch is controlled by the control signal CO. In this case the antenna operating band has two alternative places. 
         [0054]    The inductive structure part can be located at antenna port&#39;s side of the part in series with the feed conductor instead of the radiator&#39;s side of the part in series as presented in  FIG. 5 . Inside the part in series the order of the two-way switch and capacitive structure parts can be any, in other words the two-way switch can be located also at radiator&#39;s side of the capacitive structure parts. 
         [0055]    In  FIG. 6  there is a feed circuit according to the invention, the adjusting circuit  630  of which comprises only a part between the feed conductor  601  and the signal ground. That part consists of a phase shifter  632  and a capacitance diode CDI, which are in series. The adjustment takes place by controlling the capacitance diode by the control signal CO, which can be continuous in this example. The antenna operating band can then be displaced continuously in a defined total range. By designing the phase shifter suitably, the adjustment range of the reactance of the adjusting circuit can be shifted as desired. For example, it can be shifted wholly to the inductive side. 
         [0056]      FIG. 7  shows as a circuit diagram an example of the implementing of an adjusting circuit according to  FIG. 3 . Said LC circuit comprises a coil L 7  connected between the input conductor of the adjusting circuit  730  and the signal ground and a capacitor CB 1  in series with the input conductor of the adjusting circuit, which input conductor is connected to the antenna feed conductor  701 . The capacitor CB 1  functions also as a blocking capacitor preventing the forming of a direct current circuit through the antenna feed conductor as seen from the control circuit of the switch of the adjusting circuit. One terminal of the capacitor CB 1  has been connected to the input of the switch  733 . The reactive structure parts connected to the three outputs of the switch are implemented by short transmission lines, each of which comprising a ground conductor and another conductor insulated from the ground, which conductor is here called a separate conductor. An open transmission line shorter than the quarter wave represents a certain capacitance, and the short-circuited line represents a certain inductance. These transmission lines, which implement the alternative reactances, are called tuning lines. In this example the first tuning line  734  is short-circuited at its tail end, the second tuning line  735  is short-circuited as well at its tail end and the third tuning line  736  is terminated by a discrete tuning capacitor CT at its tail end. A blocking capacitor CB 2  is at the head end of the separate conductor of the short-circuited first tuning line to prevent the forming of a direct current circuit through the tuning line and the control circuit of the switch. For same reason there is a blocking capacitor CB 3  at the head end of the separate conductor of the second tuning line. 
         [0057]      FIG. 8  shows an example of the implementation of the adjusting circuit according to  FIG. 7  by a circuit board. The upper surface of the circuit board PCB is mostly conductive ground plane GND functioning as the signal ground. The feed conductor  701  of the antenna is a conductor strip on the surface of the circuit board continuing to a monopole radiator from an edge of the circuit board. The input conductor of the adjusting circuit is a conductor strip, which branches from the feed conductor. Said coil L 7  and capacitor CB 1  are discrete components. The switch  733  is an integrated component. The switching parts are type of FET (Field Effect Transistor), PHEMT (Pseudomorphic High Electron Mobility Transistor) or MEMS (Micro Electro Mechanical System), for example. The switch is controlled from the opposite side of the circuit board through a via. The tuning lines  734 ,  735 ,  736  are planar transmission lines on the surface of the circuit board. A short-circuited line is produced, when the tail end of the separate conductor of the line joins the surrounding ground plane. 
         [0058]      FIG. 9  shows an example of the wholeness of an antenna according to the invention. A portion of the circuit board PCB of a radio device is seen in the figure. The monopole radiator  920  is a plate-like and rigid sheet metal strip. It has been connected to the antenna feed conductor  901  at the feed point FP being located near a corner of the circuit board. The radiator is directed first from that point over the edge of the end of the circuit board outside the board and turns after that, onwards level with the upper surface of the circuit board, in the direction of the end. On the circuit board there is the signal ground GND at a certain distance from the radiator  920 . The antenna of the example is then an ILA (Inverted L-antenna), which is a version of the monopole antenna. The radiator has a perpendicular fold part at the outer edge of the portion along the end of the circuit board to increase its electric length. On the circuit board, in the end on the radiator side, there is the adjusting circuit  930  of the antenna. It has been presented only as an area confined by a broken line in  FIG. 9 . 
         [0059]      FIG. 10  shows an example of the displacement of an operating band of an antenna according to the invention, when the adjusting circuit is controlled. The example relates to the antenna comprising an adjusting circuit according to  FIG. 8 . The first tuning line  734  of the antenna is 17 mm long, the second tuning line  735  is 1.5 mm long and the third tuning line  736  is 3.5 mm long. The capacitance of the tuning capacitor CT is 10 pF. The circuit board material is FR-4, the dielectric constant of which is about 4.5. The antenna has been designed for the DVB-H system (Digital Video Broadcasting), which uses the frequency range 470-702 MHz. Curve A 01  shows fluctuation of the reflection coefficient as a function of frequency, when the feed conductor is connected to the first tuning line, curve A 02  shows fluctuation of the reflection coefficient, when the feed conductor is connected to the second tuning line and curve A 03  shows fluctuation of the reflection coefficient, when the feed conductor is connected to the third tuning line. From the curves can be seen that the above-mentioned frequency range will be covered so that the reflection coefficient is −3 dB or better apart from just the upper end of the range. The use of the first tuning line is most advantageous in the lower band BL, 470-540 MHz, the use of the second tuning line in the middle band BM, 540-635 MHz and the use of the third tuning line in the upper band BU, 635-702 MHz. The measured antenna with its adjusting circuit is a prototype and can be improved by a more accurate design. 
         [0060]      FIG. 11  shows as a Smith diagram an example of the impedance of the adjusting circuit of an antenna according to the invention. The example relates to the same structure as the matching curves in  FIG. 10 . Curve B 01  shows fluctuation of the impedance as a function of frequency, when the radiator is connected to the first tuning line, curve B 02  shows fluctuation of the impedance, when the radiator is connected to the second tuning line and curve B 03  shows fluctuation of the impedance, when the radiator is connected to the third tuning line. The ends of the curves correspond to the boundary frequencies of the above-mentioned bands BL, BM and BU. In an ideal case the curves would be situated on the outer circle of the diagram, which case would correspond to a lossless case. In practice the adjusting circuit is not lossless, of course. However, the resistive proportion of the impedances is small, order of 5O, when the characteristic impedance of the lines is 50O. It can be seen from the diagram that the impedance of all tuning lines is inductive. The third tuning line  736  would be capacitive as open, but terminating the line by the 10 pF capacitance converts it to slightly inductive. A corresponding short-circuited line would be so short that it would not function correctly in practice. 
         [0061]      FIG. 12  shows an example of the gain of an antenna according to the invention. It relates to the maximum gain G max  or the gain in the most advantageous direction. The example concerns the same structure as the matching curves in  FIG. 10 . Curve C 01  shows the fluctuation of the maximum gain as a function of frequency, when the radiator is connected to the first tuning line, curve C 02  shows fluctuation of the maximum gain, when the radiator is connected to the second tuning line and curve C 03  shows fluctuation of the maximum gain, when the radiator is connected to the third tuning line. It can be seen from the curves that the maximum gain fluctuates from −5 to −10 dB in most of the using range of each tuning line. 
         [0062]    The adjustable monopole antenna according to the invention has been described above. Its structure can naturally differ in details from that presented. For example the number of the switch operating states and of the tuning lines or circuits corresponding those states can be also greater than three to implement more alternative places for the operating band. The reactive circuit from the feed conductor to the ground is advantageously inductive, but can also be capacitive. Correspondingly the possible series circuit is advantageously capacitive, but also can be inductive. The invention does not limit the manufacturing manner of the antenna radiator. The inventive idea can be applied in different ways within the scope defined by the independent claim  1 .