Adjustable multiband antenna

Description of equivalent WO 2007012697 A1

Adjustable multiband antenna

The invention relates to an adjustable multiband antenna especially applicable in mobile terminals. The invention further relates to a radio device equipped with such an antenna.

The adjustability of an antenna means in this description, that a resonance frequency or frequencies of the antenna can be changed electrically. The aim is that the operating band of the antenna around a 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 GSM 1800 and GSM 1900 (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 if the resonance frequency of the antenna can be tuned in a sub-band being used at each time, from the point of view of the radio connection quality.

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 EP1113 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. The solution is suit- able for single-band antennas. The controlled displacement of the operating bands of a multi-band antenna is impossible.

In Fig. 1 there is a solution including a switch, known from the publication EP

04008490.7. Of the antenna base structure, only a part of the radiating plane 120 is drawn in the figure. The antenna has two separate operating bands. The an- tenna comprises, in addition to the base structure, an adjusting circuit having a parasitic element 131 , a filter 132, a two-way switch 133, a terminating element 138 and transmission lines. The parasitic element has a significant electromagnetic coupling to the radiating plane and is connected through a short transmission line to the input port of the filter 132. Each transmission line comprises a ground con- ductor and a separate conductor. The output port of the filter is connected through the second short transmission line to the switch 133, the "hot" pole of the output port to the common pole of the switch by the separate conductor of the second transmission line. The common pole of the switch can be connected either to the second or the third pole of the switch by controlling the switch. The second pole of the switch is connected fixedly to the separate conductor 134 of the third short transmission line, which line is open at its opposite end. The third pole of the switch is connected fixedly to the separate conductor 135 of the fourth short transmission line. At the opposite end of the fourth transmission line there is a reactive terminating element 138. Its reactance X can be just a short-circuit (zero in- ductance). The impedance, which the adjusting circuit presents seen from the radiator, depends on the lengths of the transmission lines and the reactance X. The circuit can be designed so that the impedance of the adjusting circuit is very high when the common pole of the switch is connected to the third pole, and the impedance is suitable when the common pole is connected to the second pole. "Suitable" means a value, which causes the operating band to displace as much as desired when the state of the switch is changed.

The object of the filter 132 is to strict the effect of the switching only to one operating band. If it is desired that the effect is stricted e.g. to the upper operating band, the filter is made to be of high-pass type, and its cut-off frequency is arranged be- tween the antenna operating bands. In this case the lower operating band is located in the stop band of the filter, and the impedance of the adjusting circuit at the frequencies of the lower operating band is high in both states of the switch. Changing the switch state then causes neither a change in the electric length of the antenna nor a displacement of the lower operating band.

In the solution according to Fig. 1 it is possible to affect a single operating band of a multi-band antenna without changing the place of the parasitic element used as a coupling element. However, the control of simultaneous displacements of two bands is impossible. In addition, it is difficult to keep the tolerances of the couplings between the paratisitic element and the radiators small enough in the pro- duction. In Fig. 2 there is a solution including switches, known from the publication US 6,650,295. The radiating plane 220 of a planar antenna is seen in the drawing. The radiating plane is located above the circuit board of a radio device, the conductive upper surface of the circuit board functioning as a ground plane 210 of the anrenna and as a ground conductor of the transmission lines, which belong to the structure. The short-circuit conductor 211 and the feed conductor 212 of the antenna join to the radiating plane. Thus the antenna is of the PIFA type (Planar Inverted F-Antenna). In the radiating plane there is a non-conductive slot 225 starting from its edge, which slot divides the plane, as viewed from its short-circuit point, to two branches having different lengths. The PIFA is then a dual-band antenna. The lower operating band is based on the longer branch 221 and the upper operating band on the shorter branch 222.

Both the lower and upper operation band can be displaced in the structure according to Fig. 2. For the displacement of the lower operation band there is the first ad- justing circuit 230 and for the displacement of the upper operation band the second adjusting circuit 240. The first adjusting circuit 230 comprises a first transmission line, a first switch 232 and two extension lines. The first transmission line is longer than the extension lines. The separate conductor 231 of the first transmission line joins the edge of the radiating plane at a point of its longer branch 221. The second end of the separate conductor 231 is connected to the common pole of the first switch 232. This switch has three states. In its first state the second end of the separate conductor 231 is switched to nothing, in the second state it is switched to the separate conductor 233 of the first extension line, and in the third state it is switched to the separate conductor 234 of the second extension line. Each extension line is shorted at its opposite end. They have different lengths, the longer branch of the radiating plane thus having three alternative electric lengths depending on the state of the switch 232, and correspondingly the lower operating band of the antenna having three alternative places. The second adjusting circuit 240 is similar to the first adjusting circuit. The separate conductor 241 of the fourth extension line, corresponding to the separate conductor 231 of the first transmission line, joins the edge of the radiating plane at such a point that the second adjusting circuit mainly affects solely the upper operating band. The place of the upper operating band can be selected from three alternatives by means of the second switch 242.

The lengths of the first and fourth transmission line are in the order of the quarter wave. If that length is shorter than the quarter wave, connecting a short extension line to its end results in that the band is displaced upwards, and if the length is longer than the quarter wave, connecting a short extension line to its end results in that the band is displaced downwards. The losses caused by the switch and thus the influence of the switch on the antenna efficiency depend on the length of the transmission line joining the radiating plane. That length and the lengths of the extension lines can be optimized so that the desired band displacements will be obtained at the cost of relatively small lowering of the antenna efficiency. The adjusting circuits further may comprise discrete tuning capacitors as an addition or replacing some transmission lines.

In the solution described above, the controlled displacement of two bands requires two adjusting circuits with their switches. This means a relatively complicated structure and high production costs.

The object of the invention is to implement the adjusting of a multi-band antenna by a new way, which alleviates the flaws associated with the prior art. An adjust- able multi-band antenna according to the invention is characterized in that which is specified in the independent claim 1. Some advantageous embodiments of the invention are presented in the dependent claims.

The basic idea of the invention is as follows: An adjusting circuit of an antenna, which has at least two operating bands, is galvanically connected to a point of the radiator, where the circuit can affect the places of two antenna operating bands. The adjusting circuit comprises a multi-pole switch, by which said radiator point can be connected to one of alternative transmission lines. For example, one of the two transmission lines is open and another shorted. A discrete capacitor can be located between the separate conductor of the transmission line and an output pole of the switch as an additive tuning element. The adjusting circuit further comprises an LC circuit between the radiator and the switch. Among other things, the lengths of the transmission lines, the values of the discrete components and the distance between the antenna short-circuit point and the adjusting circuit connecting point then are variables from the point of view of the antenna adjusting. Such values are calculated for these variables that each of the two antenna operation bands separately shifts to a desired other place, when the switch state is changed.

An advantage of the invention is that desired displacements for the two antenna operation bands are obtained. One of the displacements can be set as zero, too.

Another advantage of the invention is that these displacements can be imple- mented by a relatively simple adjusting circuit, which is connected to the radiator only at one point. A further advantage of the invention is that the space required for the antenna adjusting circuit is relatively small. This is due to that physically very short transmission lines are enough in the adjusting circuit according to the invention. A further advantage of the invention is that a relatively high efficiency is achieved for the antenna despite the use of a switch. A further advantage of the invention is that said LC circuit functions as an ESD protector (electro-static discharge) for the switch at the same time.

The invention is below described in detail. Reference will be made to the accompanying drawings where Fig. 1 presents an example of an adjustable antenna according to the prior art,

Fig. 2 presents an second example of an adjustable antenna according to the prior art,

Fig. 3 presents an example of the radiating plane of an adjustable antenna according to the invention, Fig. 4 presents an example of the adjusting circuit of an antenna according to the invention,

Fig. 5 presents an example of the displacement of operation bands of an antenna according to the invention,

Fig. 6 presents changes in the impedance of the antenna adjusting circuit in the exemplary case of Fig. 5,

Fig. 7 presents the antenna efficiency in the exemplary case of Fig. 5,

Fig. 8 presents another example of the adjusting circuit of an antenna according to the invention,

Fig. 9 presents another example of an antenna according to the invention, and Fig. 10 presents an example of a radio device equipped with an antenna according to the invention.

Figs. 1 and 2 were already described in conjunction with the description of the prior art.

Fig. 3 shows an example of an antenna according to the invention as seen from above, or from the side of the radiating plane. The circuit board PCB of a radio device is seen below the radiating plane 320, the conductive upper surface of the circuit board functioning as a ground plane 310 of the antenna. The antenna short- circuit conductor joins the radiating plane at the short-circuit point, or the grounding point G, and the feed conductor joins the radiating plane at the feeding point F. In addition, a conductor of the antenna adjusting circuit joins the radiating plane at the adjusting point X. In this example the radiating plane is rectangular by outline, and all three points are located at its same long side, the feeding point being located closest to a corner and the grounding point being located therebetween. The radiating plane is shaped so that the antenna of the example is a dual-band antenna; it has a lower and an upper operating band. The lower operating band is based on the PIFA structure formed by the radiating plane, the ground plane and the feed and short-circuit conductors. The upper operating band is based on the slot radiator, which slot 322 starts at the edge of the radiating plane, beside the adjusting point X, on the farther side of the point X as seen from the grounding point G. The slot 322 ends in the inner area of the radiating plane near the opposite end of the plane as seen from the feeding point. The slot naturally affects the electric length of the lower operating band radiator 320 at the same time. In the radiating plane there is also an L-shaped slot starting between the feeding and short-circuit points, by which slot the antenna matching is improved both in the lower and the upper operating bands. In addition, the radiating plane has in this example two projections being directed towards the ground plane to tune the an- tenna and to improve its matching. One projection 328 is located at the end on the side of the feeding point, and the other projection 329 is located at the side of the grounding and adjusting points, from the open end of the slot radiator 322 towards the opposite end of the plane.

Based on the location of the adjusting point X, a circuit connected to it affects both the lower and the upper operating band. If the adjusting point were connected directly to the ground plane, for example, the electric length of the antenna parts corresponding to both the lower and the upper operating band would decrease, in which case both bands would shift upwards. The adjusting circuit connected to the adjusting point is located either below the radiating plane 320 or on the opposite side of the circuit board PCB.

The electric distance between the grounding point G and the adjusting point X has a significant effect on how big the band displacements are when the adjusting circuit is controlled. In an antenna according to the invention, said distance is one variable in addition to the variables of the adjusting circuit when a desired result is seeked. An arrangement is included in the radiating plane for setting said distance. At the simplest, this arrangement means only that the direct distance between the points G and X is chosen to be suitable. In the example of Fig. 3 the arrangement comprises a notch 326 being located in the portion of the radiating plane between those points.

Fig. 4 shows an example of the adjusting circuit of an antenna according to the in- vention. The adjusting circuit 430 is galvanically connected to the antenna radiator at the adjusting point X. The adjusting circuit comprises, in order from the radiator, an input line 431 of the adjusting circuit, an LC circuit 432, a switch 433 and the tuning lines 434, 435. Each transmission line comprises a ground conductor and a conductor isolated from the ground, which conductor is also here called a separate conductor. The LC circuit 432 is on one hand for the ESD protection of the switch and on the other hand for increasing the number of the variable parameters of the adjusting circuit. It is formed of a coil L and a capacitor C1. The coil has been connected transversely to the input line 431 , that is between its separate conductor and the ground. The capacitor C1 is in series with the separate conductor of the input line, and the second terminal of the capacitor is connected to the common pole of the switch 433. The switch is a two-way switch, where the common pole can be connected to one of two other poles. These other poles are called output poles of the switch. The first output pole of the switch is connected to the head end of the separate conductor of the first tuning line 434, and the second output pole is connected, through the capacitor C2, to the head end of the separate conductor of the second tuning line 435. Thus the input line of the adjusting circuit can continue, after the LC circuit and the switch, either as the first tuning line or as the second tuning line. When the switch state is changed, the reactive impedance, which is "seen" from the adjusting point X of the radiating plane to the ground, changes. In that case the resonance frequencies of the antenna parts change and the operating bands therefore shift.

In this example the first tuning line 434 is open at its tail end, and the second tuning line 435 is short-circuited at its tail end. The tuning lines are short, usually shorter than the quarter wavelength. In that case the open line represents a cer- tain capacitance, and the short-circuited line represents a certain inductance. As known, the values of the capacitance and the inductance depend on the frequency: At the frequencies of the upper operating band they are higher than at the frequencies of the lower operating band, if the line is shorter than the quarter wavelength also in the upper band. The frequency-dependency of the capacitance in the discrete capacitor is just negligible. So the lengths of the tuning lines are used as variables in this invention when the adjusting circuit is designed. Among other things, the values of the discrete components of the adjusting circuit, the length of the input line 431 and the electric distance between the grounding point G and the adjusting point X in the radiating plane, mentioned in the description of Fig. 3, are other variables, or variable parameters,. Naturally, the starting point is the dimensioning of the antenna basic structure for part of the radiating plane. The number of the variables is high considering the simplicity of the adjusting circuit, and some variables have different frequency characteristics than some others. These facts make it possible to design the antenna with its adjusting circuit so that the displacements having desired directions and extents can be obtained for the lower and upper operating bands independently from each other. For example, if one band has to remain in its place, its displacement can be arranged as zero.

The capacitor C2 functions also as a blocking capacitor preventing the forming of a direct current circuit through the short-circuited tuning line as seen from the control circuit of the switch. On the side of the open tuning line, no blocking capacitor is needed, of course, but also there could be a discrete component for the tuning purpose.

The number of the switch operating states and of the tuning lines or circuits corresponding to those states can naturally be also more than two to implement several alternative places for an operating band. On the other hand, more than two operat- ing bands may be implemented by the radiating plane, in which case the displacements of them all can be controlled by one adjusting circuit to some extent.

Fig. 5 shows an example of the displacement of operation bands of an antenna according to the invention. The example relates to the antenna according to Fig. 3 comprising an adjusting circuit according to Fig. 4. The object has been that in one switch state the antenna's lower operating band would cover the frequency range 890-960 MHz of the GSM900 system and the upper operating band would cover the frequency range 1710-1880 MHz of the GSM1800 system, and that in the other switch state the lower operating band would cover the frequency range 824-894 MHz of the GSM850 system and the upper operating band would cover the frequency range 1850-1990 MHz of the GSM1900 system. Curve 51 shows fluctuation of the reflection coefficient as a function of frequency, when the radiator is connected to the short-circuited, very short tuning line. Curve 52 shows fluctuation of the reflection coefficient, when the radiator is connected to the tuning line, which is open at its tail end. From the curves can be seen that the above- mentioned object is fulfilled for part of the lower operating band, if the value -5 dB is considered as a criterion for the usable reflection coefficient. The object is ful- filled also for the upper operating band except for its uppermost part, where the antenna matching is only passable.

In the example of Fig. 5 the antenna adjusting circuit has been designed as follows: L=5.6nH, C1=8.2pF and C2=100pF. The first tuning line 434 is a 3 mm long planar line on the surface of circuit board material FR-4. The length of the second tuning line as well as the length of the input line 431 of the adjusting circuit is practically zero. In that case, when the radiator is connected to the short-circuited tuning line, the whole adjusting circuit is "seen" from the radiator as a very short short- circuited transmission line at the frequencies of the lower operating band. This means a low impedance. Without the capacitor C2 the adjusting circuit would represent a short-circuited transmission line with about a Ve wavelength, but a value has been searched for the capacitance C2, which shortens the electric length of the transmission line to zero. At the frequencies of the upper operating band the capacitance C2 has only a minor effect. Because the upper operating band is lo- cated at about double frequencies compared with the lower band, the adjusting circuit is "seen" from the radiator as a short-circuited transmission line with about a quarter wavelength at the frequencies of the upper operating band. This means a high impedance. On the other hand, the adjusting circuit is designed so that when the radiator is connected to the open tuning line, the whole adjusting circuit is "seen" from the radiator as a very short open transmission line at the frequencies of the lower operating band. This means a high impedance. Without the coil L the adjusting circuit would represent an open transmission line with about a Vs wavelength, but a value has been searched for the inductance L, which shortens the electric length of the transmission line to zero. At the frequencies of the upper op- erating band the inductance L has only a minor effect. For this reason the adjusting circuit is "seen" from the radiator as an open transmission line with about a quarter wavelength at the frequencies of the upper operating band. This means a low impedance. These facts explain the directions of the displacements of the operating bands.

Another alternative would be to design the adjusting circuit so that when the radiator is connected to the open tuning line, the whole adjusting circuit would be "seen" as an open transmission line with about a quarter wavelength at the frequencies of the lower operating band, and correspondingly as an open transmission line with about a half wavelength at the frequencies of the upper operating band. On the other hand, when the radiator is connected to the short-circuited tuning line, the whole adjusting circuit would be "seen" as a short-circuited transmis- sion line with about a quarter wavelength at the frequencies of the lower operating band, and correspondingly as a short-circuited transmission line with about a half wavelength at the frequencies of the upper operating band. Also in this case the impedance of the adjusting circuit would change from low to high in the lower op- erating band and from high to low in the upper operating band, when the switch state is changed. This again results in that the lower operating band shifts downwards and the upper operating band shifts upwards, as in the previous case corresponding to the exemplary design. Using discrete components according to the invention, the physical lengths of the transmission lines needed are considerably shorter, for which reason the adjusting circuit fits into a smaller space.

Fig. 6 shows as a Smith diagram an example of changes in 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. 5. Curve 61 shows fluctuation of the impedance as a function of frequency, when the radiator is connected to the short- circuited, very short tuning line, curve 62 shows fluctuation of the impedance, when the radiator is connected to the tuning line, which is open at its tail end. In a lossless case the curves would travel along the outer circle of the diagram. Now they travel only relatively close to the outer circle, which means losses of a certain level in the adjusting circuit. These losses are included in the efficiency curves of Fig. 7.

The left end of the curve 61 represents the band used by GSM900 system and the right end represents the band used by GSM 1800 system. In the previous band the adjusting circuit impedance is intended to be low, in which case particularly the resistive part of the impedance should be low. The resistive part is indeed only about 5 % of the antenna characteristics impedance. In the band used by GSM 1800 system the adjusting circuit impedance is intended to be high. In this example it is inductive and has an absolut value, which is about five times the antenna characteristics impedance. The left end of the curve 62 represents the band used by GSM1900 system and the right end represents the band used by GSM850 sys- tern. In the previous band the adjusting circuit impedance is intended to be low, in which case particularly the resistive part of the impedance should be low. The resistive part is indeed less than 10 % of the antenna characteristics impedance. In the band used by GSM850 system the adjusting circuit impedance is intended to be high. In this example it is inductive and has an absolute value, which is nearly three times the antenna characteristics impedance. Fig. 7 shows an example of the efficiency of an antenna according to the invention. The example concerns the same structure as the matching curves in Fig. 5. Curve 71 shows the fluctuation of the efficiency as a function of frequency when the radiator is connected to the short-circuited, very short tuning line. Curve 72 shows fluctua- tion of the efficiency when the radiator is connected to the tuning line, which is open at its tail end. It can be seen from the curves that the efficiency is better than 0.4 in the lower operating bands and better than 0.5 in the upper operating bands except for the very uppermost parts.

Fig. 8 shows another example of the adjusting circuit of an antenna according to the invention. The adjusting circuit 830 is galvanically connected to the antenna radiator at the adjusting point X. The adjusting circuit comprises, in order from the radiator, an input line 831 of the adjusting circuit, an LC circuit 832, a switch 833 and the tuning lines 834, 835, as in the circuit of Fig. 4. Similarly, the first output pole of the switch is connected to the head end of the separate conductor of the first tuning line 834, and the second output pole has been connected, through the capacitor C2, to the head end of the separate conductor of the second tuning line 835. Also in this example the first tuning line 834 is open at its tail end. The differences in respect of the circuit of Fig. 4 are: The tuning lines are now of equal length, the second tuning line is now terminated by a coil L2, and the capacitor C2 functions only as a blocking capacitor.

The antenna proper and the adjusting circuit are designed so that when the radiator is connected to the open tuning line, the antenna's upper operating band covers e.g. the frequency range of the GSM1800 system and the lower operating band covers e.g. the frequency range of the GSM850 system. At the frequencies of the lower operating band the adjusting circuit impedance is arranged to be relatively high. The inductance of the coil L2 is chosen so that its reactance in the upper operating band is relatively high. For this reason the adjusting circuit impedance hardly changes at the frequencies of the upper operating band when the radiator is connected to the tuning line, which is terminated by the coil L2. In that case the upper operating band remains nearly in its place. Instead, at the frequencies of the lower operating band the adjusting circuit impedance becomes lower so that the lower operating band shifts upwards for example to the range used by the GSM900 system.

Another way to limit the effect of the switch to one operating band is to implement the LC circuit between the radiator and the switch as a filter, the cut-off frequency of which is located between the lower and upper operating bands of the antenna. When the object is to displace only the upper operating band, the filter is of high- pass type, and when the object is to displace only the lower operating band, the filter is of low-pass type. The order of the filter is naturally selectable. Also this kind of filter functions at the same time as an ESD protector for the switch. For this aim a high-pass part can be added to the low-pass filter so that a bandpass filter is formed.

Fig. 9 shows another example of an antenna according to the invention as seen from above, or from the side of the radiating plane. For its inventive part the antenna is similar to the antenna presented in Fig. 3. One difference is that the an- tanna in Fig. 9 further comprises a parasitic radiator 950. This is located beside the end of the radiating plane 920 on the side of the feeding point F, and is connected to the ground plane at the grounding point G2 next to the feeding point F. Changing the resonance frequencies of the main radiator hardly affects the resonance frequency of the parasitic element because of its location. The resonance frequency of the parasitic element can be arranged e.g. into the range of 2.2 GHz so that an operating band is obtained for the antenna in the frequency range used by the WCDMA system (Wideband Code Division Multiple Access).

The antenna in Fig. 9 lacks ground plane on a relatively large area 901 below the radiating plane. This feature has nothing to do with the above-mentioned parasitic radiator: An antenna according to the invention does not require a "solid" ground plane below the radiating plane. The ground plane can be located even considerably more aside from the radiating plane than in the example of Fig. 9.

Fig. 10 shows a radio device RD, which comprises an adjustable multiband antenna AOO according to the invention with its adjusting circuit A30.

The adjustable multiband antenna according to the invention has been described above. Its structure can naturally differ from that presented. The invention does not limit the manufacturing method of the antenna. The antenna can be e.g. ceramic, in which case the radiators are conductive coatings of the ceramics. The switch used in the adjusting circuit can be of e.g. the FET (Field Effect Transistor), PHEMT (Pseudomorphic High Electron Mobility Transistor) or MEMS (Micro Electro Mechanical System) type. It is possible to use a capacitance diode as the adjusting component, too. The inventive idea can be applied in different ways within the scope defined by the independent claim 1.