WO2008023095A1

WO2008023095A1 - Multi-resonance antenna

Info

Publication number: WO2008023095A1
Application number: PCT/FI2007/050434
Authority: WO
Inventors: Jyrki Mikkola; Petteri Annamaa
Original assignee: Pulse Finland Oy
Priority date: 2006-08-25
Filing date: 2007-07-24
Publication date: 2008-02-28
Also published as: FI20065528A0; FI119268B; FI20065528A

Abstract

An internal antenna (300) having at least two operating bands especially intended for small radio apparatuses. The antenna comprises a directly fed monopole-type basic element (310) and a short-circuited parasitic element (320). The elements are conductor patterns on the surface of a low-loss and thin substrate (301). Between them, at many different points (31, 32, 33), there is a significant electromagnetic coupling. In addition, both elements have separately adjacent portions between which there is a significant electromagnetic coupling. The elements can be branching, in which case said portions can be located in different branches. The couplings are implemented so that each element together with the other element has at least two usable resonances. The resonances of the antenna are quite independent from each other also at frequencies below 1 GHz, which enables constituting a wide operating band in the range in question covering a frequency range used by more than one system. Furthermore, the antenna is very small-sized, and its feed point (FP) and short-circuit point (SP) can be arranged in the middle area of the structure, the omnidirectionality then being good.

Description

Multi-resonance antenna

The invention relates to an internal antenna having several resonances and at least two operating bands that is especially intended for small radio apparatuses.

Models operating in two or more systems, such as different GSM systems (Global System for Mobile telecommunications), which use different frequency ranges, have become common in mobile stations. The basic condition for the operation of a mobile station is that the radiation and receiving properties of its antenna are satisfactory in the frequency bands of all the systems in use. Without limit on size, it is relatively easy to make a high-quality multi-band antenna. However, in mobile stations, especially in mobile phones, the antenna is placed for convenience of use inside the covers of the apparatus. This complicates antenna design.

Another development of mobile stations has been the decrease of their size. Thus, the space spared for the internal antenna has become ever smaller, which further makes the design more demanding. The space available especially in hight direction is naturally the smaller the flatter the apparatus is. Flatter than usual structural parts are e.g. parts of such a two-part radio apparatus which are either one on the other or one after the other as an extension of each other, depending on the use situation. In such cases is generally utilised a monopole-type antenna which does not require such a space in height direction as e.g. an otherwise generally used PIFA-type (Planar Inverted F-Antenna) planar antenna.

The use of parasitic element(s) is a known way of achieving required characteristics for the antenna. The parasitic element means a structural part which has only an electromagnetic coupling to the immediately fed antenna element, i.e. basic element. The parasitic element can be arranged so that the resonance frequency corresponding to it is the same or almost the same as the resonance frequency corresponding to the basic element or it is relatively far from the resonance frequencies corresponding to the basic element. In the former case, the characteristics of the antenna are improved in an existing operating band by means of the parasitic element, and, in the latter case, one more operating band for the antenna is formed by means of the parasitic element.

Fig. 1 shows a two-band antenna comprising also a parasitic element, known from the publication EP1432072. The circuit board PCB of a radio apparatus is seen in the figure, at one end of which board the antenna is. The main parts of the antenna are its radiators: the basic element 110 and the parasitic element 120. The basic element is here a rigid conductor wire resembling an open rectangular ring which is located slightly above the upper surface of the circuit board.

In a corner of the basic element is its feed point FP which is coupled to the antenna port of the radio apparatus. The upper surface of the circuit board is mostly of conductive signal ground GND. This does not, however, extend at the radiators, the basic element 110 then forming a monopole-type radiator. In this example, this monopole radiator is two-band. Its basic resonance frequency is arranged in a frequency range used by one radio system and the nearest harmonic frequency of the basic resonance frequency in a frequency range used by another radio system. For setting the ratio of the harmonic frequency to the basic resonance frequency to be correct, the basic element has two branches: it is divided as viewed from the feed point FP to the first branch 111 and the second, shorter branch 112. Between the outer ends of these, there is an electromagnetic coupling which reduces said ratio of the harmonic frequency to the basic resonance frequency.

The parasitic element 120 is a conductor strip on the surface of the circuit board PCB. The parasitic element is coupled from its one point GP to the signal ground GND via an inductive element 125 which is e.g. a spiral conductor strip on the lower surface of the circuit board. Using the inductive element, the matching of the whole antenna is improved. The point GP divides the parasitic element to two parts 121 , 122. The first part 121 together with the inductive element resonates in the lower operating band of the antenna, and the second part 122 together with the inductive element resonates in the upper operating band of the antenna. The parasitic element thus functions here as an auxiliary radiator and improves the antenna gain in both operating bands of the antenna.

By means of the structure according to Fig. 1 is provided a two-band antenna which fits even to a flat radio apparatus. A disadvantage is though that especially resonances corresponding to the lower operating band interfere each other so that the frequency range, in which there is an acceptable match, remains narrow. Furthermore, the number of the operating bands is limited to two.

Fig. 2 shows a second example of a two-band antenna comprising also a parasitic element, known to the applicant. A part of the circuit board PCB of a radio apparatus is seen in the figure. The basic element 210 and the parasitic element 220 of the antenna are conductor areas on the surface of a small antenna circuit board 201 , which is fastened to the circuit board PCB at a certain distance from it. The upper surface of the circuit board PCB is mostly, also below the antenna circuit board, of conductive signal ground GND. The basic element is connected to the antenna port of the radio apparatus from the feed point FP and to the ground from the first short-circuit point S1. In the basic element, there is a non-conductive slot starting at its edge so that the element is divided, as viewed from the first short-circuit point, to two branches of different lengths. The second, shorter branch 212 is straight and parallel to one side of the circuit board PCB. The first, longer branch 211 resembles a rectangular letter U circling mostly the second branch 212. The parasitic element 220 is connected to the ground from the second short- circuit point S2. Also in the parasitic element there is a non-conductive slot starting at its edge so that the element is divided, as viewed from the second short-circuit point, to two branches of different lengths, to the third and the fourth branch. The fourth, shorter branch 222 is straight and its direction is perpendicular to the direction of the second branch 212. The third, longer branch 221 resembles a rectangular letter U circling mostly the fourth branch 222. The outlines of both the basic and the parasitic element form an elongated pattern according to the main direction of the radiating branches, and the longitudinal directions of the elements are perpendicular to each other.

The feed point FP is between the first S1 and the second S2 short-circuit point relatively close to both. The starting portion of the basic element 210, as viewed from the feed point, and the starting portion of the parasitic element, as viewed from the second short-circuit point S2, are relatively close to each other. Between these starting portions, there is a slot 205, which is thus very narrow, e.g. 0.2 mm.

By means of the narrow slot, a strong enough coupling is provided between the elements despite of their perpendicular position in respect of each other.

Both the first 211 and the third 221 radiating branch together with the surrounding antenna parts form a resonator, the natural frequency of which is in the lower operating band of the antenna. Correspondingly, both the second 212 and the fourth 222 radiating branch together with the surrounding antenna parts form a resonator, the natural frequency of which is in the upper operating band of the antenna. In the structure according to Fig. 2, also the resonances in the lower operating band are obtained quite independent from each other, in which case it is possible to implement a relatively wide continuous lower operating band in addition to the wide upper operating band. A disadvantage of this solution is that acceptable operation requires the ground plane beneath the radiators, which results in a considerable space requirement for the antenna. Furthermore, also here the number of the operating bands is limited to two.

The object of the invention is to minimise said disadvantages characteristic to the antennas of prior art. The antenna according to the invention is characterised by what is presented in the independent claim 1. Some advantageous embodiments of the invention are described in the other claims.

The basic idea of the invention is the following: The internal antenna of a radio apparatus comprises two radiators: a directly fed monopole-type basic element and a short-circuited parasitic element. The elements are conductor patterns on the surface of a low-loss and thin substrate. Between them, at many different points, there is a significant electromagnetic coupling. In addition, both elements separately have adjacent portions between which there is a significant electromagnetic coupling. At least one element can be branching, in which case said portions can be located in different branches. The couplings are implemented so that each element together with the other element has at least two usable resonances.

An advantage of the invention is that the resonances of the internal multi-band antenna are quite independent from each other also at frequencies below 1 GHz. This enables constituting a wide operating band in the range in question, covering a frequency range used by more than one system. A further advantage of the invention is that the antenna can be made very small-sized. This is due to the above-mentioned electromagnetic couplings between the elements and within the elements which enlarge the electric size of the elements, and secondly to the absence of the ground plane beneath the elements, which means a flat structure. Furthermore, an advantage of the invention is that the feed point and the short- circuit point of the antenna can be arranged in the middle area of the structure, in which case its omnidirectionality is good. An additional advantage of the invention is that the efficiency of the antenna is good in respect of its size.

The invention will now be described in detail. The description refers to the accompanying drawings in which

Fig. 1 shows an example of the internal dualband antenna of a radio apparatus according to prior art,

Fig. 2 shows a second example of the internal dualband antenna of a radio apparatus according to prior art, Fig. 3 shows an example of the internal multi-resonance antenna of a radio apparatus according to the invention,

Fig. 4 shows a second example of the internal multi-resonance antenna of a radio apparatus according to the invention,

Fig. 5 shows two examples of the band characteristics of an antenna according to the invention.

Fig. 6 shows a modification to the antenna according to Fig. 3,

Fig. 7 shows a third example of the band characteristics of an antenna according to the invention,

Fig. 8 shows a modification to the antenna according to Fig. 3, related to Fig. 7,

Fig. 9 shows a fourth example of the band characteristics of an antenna according to the invention,

Fig. 10 shows a modification to the antenna according to Fig. 3, related to Fig. 9,

Fig. 11 shows a modification to the antenna according to Fig. 3, related to the third operating band of the antenna,

Fig. 12 shows an example of the efficiency of an antenna according to the invention, and

Figs. 13a,b show examples of the location of an antenna in a radio apparatus according to the invention.

Figs. 1 and 2 were already discussed in connection with the description of prior art.

Fig. 3 shows an example of the internal antenna of a radio apparatus according to the invention, which antenna has several resonances and at least two operating bands. The radiators of the antenna are the basic element 310 and the parasitic element 320 which are conductor patterns on the surface of the substrate 301. In the basic element there is the feed point FP of the antenna, and in the parasitic element there is the short-circuit point SP of the parasitic element. The substrate, the basic element with its feed point and the parasitic element with its short-circuit point form a small antenna circuit board 300 which is fastened to the radio apparatus so that there is no ground plane at the circuit board. The radiating elements are thus of monopole-type. The antenna circuit board comprises a relatively long and narrow part and a transversal part starting from around its centre so that the board resembles a wide letter T. The length of the upper part is e.g. 45 mm and the length of the transversal middle part is e.g. 8 mm. The feed point FP and the short-circuit point SP are contact pads which have a reinforced conductor coating, and they are located next to each other at the outer end of the transversal middle part. From this kind of location in the middle, as viewed in the longitudinal direction, results that the omnidirectionality of the antenna is relatively good. The shape of the antenna circuit board is partly caused by the fact that the speaker and possibly a vibrator of the radio apparatus has been designed to be positioned to the same structural module.

The basic element 310 and the parasitic element 320 are folded by shape so that both separately have relatively long parallel and close portions. Between such two close portions in the same element there is a significant electromagnetic coupling. In addition, both elements have portions which are so close to a portion of the other element that there is a significant electromagnetic coupling between such two close portions belonging to different elements. The whole radiating structure is because of said folding of the elements on such a small area that some electromagnetic coupling occurs between very many portions. However, the term "coupling area" means in this specification and claims only two portions and a slot between them over which slot there is a clearly more significant electromagnetic coupling than between two randomly selected portions on average.

The radiating elements are in this example branching. The basic element comprises, starting from the feed point FP, first its head portion 31 H which is divided to the first and the second branch. The free end of the first branch B31 is physically relatively close to the feed point but galvanically farthest from it. The second branch B32 is directed away from the head portion 31 H and the first branch B31. The parasitic element comprises, starting from the short-circuit point SP, first the head portion 32H which is then divided to the third B33 and the fourth B34 branch, the fourth branch is divided to the fifth B35 and the sixth B36 branch, and the sixth branch is divided to the seventh B37 and the eighth B38 branch. The free end of the eighth branch is electrically farthest from the short-circuit point.

In the example of Fig. 3, there are three inter-coupling areas between the basic element and the parasitic element. The first inter-coupling area 31 is next to the feed point and the short-circuit point. It comprises a total of five interlacing strips of the head portion 31 H of the basic element and the head portion 32H of the parasitic element so that the slot between the elements forms a meander pattern in the coupling area. The second inter-coupling area 32 is somewhat farther from the feed and short-circuit point, yet still between the head portions of the basic and the parasitic element. There the basic element 310 has a projection which extends at a certain distance very close to the parasitic element 320. The third inter- coupling area 33 is formed by the free ends of said second branch B32 and eighth branch B38 which ends are face to face very close to each other.

The basic element 310 has one internal coupling area 311. It is formed so that the head portion 31 H and the first branch B31 of the basic element are side by side for their whole distance. The slot between them comprises, for strengthening the coupling, a meander-shaped portion which is constituted of the area between the total of eight interlacing conductor strips. The parasitic element 320 has four internal coupling areas. Its first internal coupling area 321 is formed so that the tail end of the head portion 32H of the parasitic element travels first beside the fifth branch B35 and then beside the fourth branch B34. The second internal coupling area 322 of the parasitic element comprises the third B33 and the seventh B37 branch. They have the total of fourteen interlacing strips so that the slot between the branches forms a meander pattern. The third internal coupling area 323 of the parasitic element is formed so that the fourth branch B34 and the sixth branch B36 travel side by side. The fourth internal coupling area 324 is formed of the tail portion of the sixth branch and the head portion of the eighth branch B38 travelling side by side.

In the example of Fig. 3, the coupling areas can be arranged so that the antenna has 4-7 significant resonances in two operating bands. At least two of these resonances are located in the lower operating band and the rest in the upper operating band. Also the resonances in the lower operating band are provided quite independent from each other, in which case the lower operating band can be shaped relatively wide.

Fig. 4 shows a second example of the internal antenna of a radio apparatus according to the invention, which antenna has several resonances and at least two operating bands. The radiators of the antenna are the basic element 410 and the parasitic element 420, which are also in this example conductor patterns on the surface of an elongated substrate 401. In the basic element there is the feed point FP of the antenna, and in the parasitic element there is the short-circuit point SP of the parasitic element. The feed point and the short-circuit point are contact pads with a reinforced conductor coating, and they are located relatively close to each other at the first end of the substrate on the side of the first long side. The substrate, the basic element with its feed point and the parasitic element with its short-circuit point form a small antenna circuit board 400 which is fastened to the radio apparatus so that there is no ground plane at its place. The radiating elements are thus of monopole-type. The size of the antenna circuit board is e.g. 6 mm x 40 mm and the distance of the edge of the ground plane from it is e.g. 7 mm.

The basic element and the parasitic element are designed so that there are inter- and internal coupling areas in the antenna principally as in the structure shown in Fig. 3. The basic element 410 comprises, starting from the feed point FP, the starting half 41 H, which goes meandering towards the second end of the substrate and then the tail half 41 T, which goes following the starting half to the opposite direction arriving next to the feed point. The parasitic element 420 comprises, starting from the short-circuit point SP, first the head portion 42H directing towards the second end, which portion is then divided to the first and the second branch. The first branch B41 continues towards the second end. The second branch B42 comprises the first portion extending to the first end, the second portion directing towards the second end and circling behind the first branch, the third portion extending to the second end, the fourth portion parallel to the second end, and the fifth portion extending to the first end on the side of the second long side of the substrate.

Both the basic element and the parasitic element comprise a few conductor strips in the small area between the feed and the short-circuit point. These strips are interlaced so that the slot between the elements forms a meander pattern. The area in question between the feed point and the short-circuit point is the first inter- coupling area 41 between the basic element and the parasitic element. There can be considered to be three other inter-coupling areas. The second inter-coupling area 42 is formed of the head portion 42H of the parasitic element and the head portion of the starting half 41 H of the basic element. The third inter-coupling area 43 is formed of the first branch B41 of the parasitic element and the mid portion of the starting half 41 H of the basic element. The fourth inter-coupling area 42 is formed of the third portion of the second branch 42H of the parasitic element and the tail portion of the starting half 41 H of the basic element. Between the starting and the tail halves of the basic element 410 there is only a narrow slot for the whole distance, and thus actually the whole element excluding the conductor strips in the area between the feed and the short-circuit point forms an internal coupling area. However, at the end on the side of the feed point of the element there is the first internal coupling area 411 separating from the other part, in which area there are densely the total of ten interlacing strips so that the slot between the starting 41 H and the tail 41 T halves forms a meander pattern in this coupling area. From the joining point of the starting and the tail halves at the second end of the substrate towards the feed point FP, there is the second internal coupling area 412 of the basic element, in which area the starting and tail halves bend in turns towards each other. Also in this case the slot between the coupling parties in the coupling area is meander-shaped. The parasitic element 420 has three internal coupling areas. Its first internal coupling area 421 comprises the head portion 42H of the parasitic element and the first portion of the second branch B42 abreast with each other. The second internal coupling area 422 of the parasitic element comprises the first branch B41 of the parasitic element and the first and the second portion of the second branch B42. The third internal coupling area 423 of the parasitic element comprises the second portion of the second branch and the tail part of a fourth portion, which part goes abreast with the second portion.

Fig. 5 shows two examples of the characteristics of an antenna according to the invention. The curve 51 shows the variation of the reflection coefficient S11 as a function of frequency, when the antenna is according to Fig. 3. The smaller the reflection coefficient, the better the antenna has been matched and the better it functions as a radiator and a receiver of radiation. The antenna is seen to have two operating bands. When the reflection coefficient value of -6 dB is used as the criterion of boundary frequency, the lower operating band is about 0.8-1.0 GHz. It covers well both the frequency range of 824-894 MHz used by the American GSM system and the frequency range of 880-960 MHz used by the European EGSM (Extended GSM) system. The bandwidth is ample 200 MHz which is a notably high value. This is obtained by arranging in the frequency range in question two resonances so that they do not interfere with each other badly. The lowest i.e. the first resonance r1 is the basic resonance of the parasitic element 320 deriving thus from the electric length of the whole parasitic element. The second resonance r2 is the basic resonance of the basic element 310 resulting thus from the electric length of the whole basic element. The upper operating band corresponding to the curve 51 is about 1.45-2.05 GHz, the bandwidth being as high as about 600 MHz (34%). In the range of the upper operating band are, inter alia, the frequency range 1 ,710-1 ,880 MHz used by the GSM1800 system, the frequency range 1 ,850-1 ,990 MHz used by the GSM1900 system and the frequency 1 ,575 MHz used by the GPS (Global Positioning System) system. The upper operating band is based on at least three significant resonances. The third resonance r3 is based on the slot between the third B33 and the seventh B37 branch belonging to the parasitic element, the slot continuing between the fourth B34 and the sixth B36 branch. Its frequency is about 1.5 GHz. The fourth resonance r4 is a harmonic of the first resonance r1 and its frequency is about 1.6 GHz. The fifth resonance r5 is a harmonic of the second resonance r2 and its frequency is about 1.87 GHz. The fifth resonance is especially strong.

The curve 52 shows the variation in the reflection coefficient S11 as a function of frequency, when a change shown in Fig. 6 has been made in the antenna according to Fig. 3. There the second branch B32 belonging to the basic element 310 has been shortened by removing from it a portion RM1 extending close to the end of the sixth branch B36. In this case the coupling between the second branch and the sixth branch B36 weakens and, as a result, the electric lengths corresponding to the harmonic resonances r4 and r5 decrease and the frequencies correspondingly increase. The frequency of the fourth resonance r4, or the fourth resonance frequency, rises to about 1.8 GHz and the fifth resonance frequency to about 2.02 GHz. The latter fact means a further increase in the width of the upper operating band of the antenna, as the upper boundary shifts close to frequency 2.2 GHz. In that case the upper operating band covers also the frequency range used by the WCDMA (Wideband Code Multiple Access) system the upper part of which is 2,110-2,170 MHz. The fourth resonance is clearly stronger than in the antenna represented by the curve 51 , but an offset of this advantage is that the matching deteriorates a little at the GPS frequency 1 ,575 MHz. In the lower operating band of the antenna the changes are small; only the second resonance frequency rises a little widening the lower operating band a little.

Fig. 7 shows a third example of the band characteristics of an antenna according to the invention. For comparison, the figure includes the same reflection-coefficient curve 52 as in Fig. 5 corresponding to the antenna of Fig. 6. The curve 71 shows the variation in the reflection coefficient as a function of frequency, when a change shown in Fig. 8 has been made to the antenna according to Fig. 6. Here the basic element 310 has been shortened by short-circuiting the slot between its head portion 31 H and the first branch B31 relatively close to the closed end thereof by a conductor SHT. As a result of this, the electric length corresponding especially the second resonance r2 decreases, and the frequency of the second resonance correspondingly rises. From this further follows that the lower operating band of the antenna widens as great as to about 300 MHz. Also the frequency of the harmonic fifth resonance rises, and the fourth and the fifth resonance frequency then draw away from each other so that the matching in a part of the upper operating band falling on the band of the GSM1900 system deteriorates.

The change shown in Fig. 8 further means the strengthening of said resonances except the third resonance r3 based on the slot between the third B33 and the seventh B37 branch and between the fourth B34 and the sixth B36 branch belonging to the parasitic element, which resonance on the contrary weakens a little. As a result of this, the matching of the antenna is deteriorated a couple of decibels at the GPS frequency 1 ,575 MHz.

Fig. 9 shows a fourth example of the band characteristics of an antenna according to the invention. For comparison, the figure includes the same reflection-coefficient curve 52 as in Fig. 5 corresponding to the antenna of Fig. 6. The curve 91 shows the variation in the reflection coefficient as a function of frequency, when a change shown in Fig. 10 has been made to the antenna according to Fig. 6. Here the parasitic element 320 has been extended from the free end of the seventh branch B37 belonging to it with a conductive extension LNG. As a result of this, the electric lengths corresponding to the first resonance r1 and especially to its harmonic fourth resonance r4 increase and the frequencies of these resonances correspondingly lower. From this further follows that the matching of the antenna at the lower end of the lower operating band improves and this band also widens a little. As a result of the lowering of the fourth resonance frequency, the fourth and the fifth resonance frequency draw away from each other so that the matching especially in a part of the upper operating band falling on the band of the GSM1800 system deteriorates. On the other hand, both the fourth and the third resonance strengthen so that the matching in the frequency range 1.5-1.6 GHz improves clearly.

The antenna according to Fig. 3 has a weak resonance also in the range of, inter alia, 2.7 GHz. If this range is desired to be utilised, the resonance in question can be strengthened by shortening the fifth branch B35 belonging to the parasitic element 320 in a way shown in Fig. 11. Here a portion RM2 has been removed from the free end of the fifth branch, which portion is almost a half of the whole branch. Shortening the branch weakens the coupling between the fifth branch B35 and the head portion 32H of the parasitic element and also the coupling between the fifth and the eighth branch B38.

Using the antenna circuit board shown in Fig. 4 can be implemented an antenna the width of the lower operating band of which located below the frequency 1 GHz is e.g. 250 MHz. True, the matching is only adequate, the reflection coefficient being in the range of the whole lower operating band -5 ■■■ -7 dB. Instead, a good matching is obtained in the upper operating band e.g. in the frequency range of

1 ,710-1,880 MHz used by the GSM1800 system. One resonance of the antenna is also arrangeable at the GPS frequency 1 ,575 MHz.

Fig. 12 shows an example of the efficiency of an antenna according to the invention. The efficiency curves 01 and 02 have been measured in the antenna according to Fig. 3. It is seen that in the lower operating band the efficiency is on average about 0.42 and in the upper operating band on average about 0.65. Efficiency values are notably high.

Figs. 13a and 13b show examples of the location of an antenna according to the invention in a radio apparatus. In Fig. 13a the radio apparatus RD1 is a baton- shaped mobile station, and it is shown as a simplified longitudinal cross section. The antenna 500 is located at the other end of the circuit board PCB of the radio apparatus, on its outer side as viewed from the side of the display and keyboard of the apparatus. The antenna circuit board is supported to the circuit board PCB at e.g. 5 mm distance from it. The feed and short-circuit point are connected to the circuit board using some kind of spring contacts. The edge of the ground plane GND on the circuit board is at a certain distance from the place of the antenna circuit board.

In Fig. 13b the radio apparatus RD2 is a foldable mobile station, in which case it comprises a first turning part TP1 and a second turning part TP2 joining a hinge HG. The radio apparatus is shown as a simplified longitudinal cross section. The first turning part TP1 comprises, i.a., the keyboard and the microphone of the apparatus, and the second turning part comprises, i.a., the display and the speaker of the apparatus. The second turning part is thus the part held on the ear of the user of the apparatus. An antenna 700 is located at the opposite end of the first turning part TP1 as viewed from the hinge HG. Its arrangement is otherwise similar to the one in Fig. 13a. The dimensions of the component formed by the antenna circuit board and its supporting structure are only a few millimetres both in the longitudinal direction of the circuit board PCB and the direction of the normal of the circuit board. Thus, the antenna fits well inside a small and flat turning part.

A functioning antenna is obtained when an antenna circuit board e.g. according to Fig. 3 or 4 is attached to a radio apparatus in the above-described way so that the short-circuit point is connected to the ground plane which is away from the radiators. However, in this specification "antenna" means for simplicity also only an antenna circuit board with its radiators.

Above was described a multi-resonance antenna according to the invention. Its implementation can differ in its details from the ones described. The antenna elements can naturally be shaped in many different ways within the limitations set by the independent claim 1.

Claims

1. An internal antenna of a radio apparatus, which antenna has at least a lower and an upper operating band and which comprises as radiators a monopole type basic element (310; 410) and a monopole type parasitic element (320; 420) which are conductor patterns on a surface of a substrate (301 ; 401 ), which basic element comprises a feed point (FP) of the antenna and which parasitic element comprises its short-circuit point (SP), characterised in that it comprises at least two separate electromagnetic inter-coupling areas (31 , 32, 33; 41 , 42, 43, 44) between the basic element (310; 410) and the parasitic element (320; 420) so that each inter- coupling area consists of a portion of the basic element and a portion of the parasitic element, and at least one electromagnetic internal coupling area (321 , 322, 323) of the basic element consisting of its two different portions and at least one electromagnetic internal coupling area (311) of the parasitic element consisting of its two different portions, from which coupling areas a first inter- coupling area (31 ; 41 ) is located next to the feed point (FP) and the short-circuit point (SP) and comprises at least one conductor strip of the basic element directing towards the parasitic element and at least one conductor strip of the parasitic element directing towards the basic element, which coupling areas are arranged so that the antenna has at least two resonances (M , r2) in said lower operating band and at least two resonances (r3, r4, r5) in said upper operating band.

2. An antenna according to claim 1 , characterised in that the basic element (310) comprises, starting from the feed point (FP), first its head portion (31 H) which is divided to a first and a second branch, the free end of which first branch (B31 ) is physically relatively close to the feed point but galvanically farthest from it, and which second branch (B32) is directed away from said head portion (31 H) and the first branch (B31).

3. An antenna according to claim 2, characterised in that the head portion (31 H) and the first branch (B31) of the basic element travel side by side substantially for their whole distance, in which case they form an internal coupling area (311 ) of the basic element.

4. An antenna according to claim 3, characterised in that the head portion (31 H) and the first branch (B31) of the basic element comprise interlacing conductor strips so that a slot between the head portion and the first branch comprises a meander-shaped portion to strengthen the coupling on said internal coupling area (311 ).

5. An antenna according to claim 1 , characterised in that the parasitic element (320) comprises, starting from the short-circuit point (SP), first its head portion (32H) which is divided to a third (B33) and a fourth (B34) branch, the fourth branch is divided to a fifth (B35) and a sixth (B36) branch, and the sixth branch is divided to a seventh (B37) and an eighth (B38) branch, a free end of the eighth branch being located electrically farthest from the short-circuit point.

6. An antenna according to claim 5, characterised in that a tail end of the head portion (32H) of the parasitic element travels first beside the fifth branch (B35) and then beside the fourth branch (B34), said tail end, the fifth branch and the fourth branch then forming a first internal coupling area (321) of the parasitic element, the third (B33) and the seventh (B37) branch of the parasitic element travel side by side substantially for their whole distance, then forming a second internal coupling area (322) of the parasitic element, the fourth (B34) and the sixth (B36) branch of the parasitic element travel side by side substantially for their whole distance, then forming a third internal coupling area (323) of the parasitic element, and a tail portion of the sixth branch (B36) and a head portion of the eighth branch of the parasitic element travel side by side, then forming a fourth internal coupling area (324) of the parasitic element.

7. An antenna according to claim 6, characterised in that the third branch (B33) and the seventh branch (B37) belonging to the parasitic element, there are interlacing conductor strips so that a slot between these branches is for its substantial part meander-shaped to strengthen the coupling on said second internal coupling area (322) of the parasitic element.

8. An antenna according to claims 2 and 5, characterised in that said at least one conductor strip of the basic element (310) in the first inter-coupling area (31) belongs to the head portion (31 H) of the basic element and said at least one conductor strip of the parasitic element (320) in the first inter-coupling area belongs to the head portion (32H) of the parasitic element, and said conductor strips interlace with each other so that a slot between the elements in the first inter-coupling area is meander-shaped, and the head portion (31 H) of the basic element comprises a projection which extends so close to the head portion (32H) of the parasitic element that the projection and the part of the head portion of the parasitic element at the projection form a second inter-coupling area (32) between the basic element and the parasitic element.

9. An antenna according to claim 8, characterised in that the free ends of the second branch (B32) belonging to the basic element and the eighth branch (B38) belonging to the parasitic element are close so that they form a third inter-coupling area (33) between the basic element and the parasitic element.

10. An antenna according to claim 1 , characterised in that said first inter- coupling area (41) is located between the feed point (FP) and the short-circuit point (SP), in which case said at least one conductor strip of the basic element (410) in the first inter-coupling area is directed towards the short-circuit point, and said at least one conductor strip of the parasitic element (420) in the first inter- coupling area is directed towards the feed point, and said conductor strips interlace with each other so that a slot between the elements in the first inter- coupling area is meander-shaped.

11. An antenna according to claim 1 , characterised in that one (r1 ) of said resonances in the lower operating band is the basic resonance of the parasitic element and the other (r2) is the basic resonance of the basic element, and one (r4) of said resonances in the upper operating band is a harmonic of the basic resonance of the parasitic element and another (r5) is a harmonic of the basic resonance of the basic element.

12. An antenna according to claim 6, characterised in that one (r3) of said resonances of the upper operating band is based on a slot between the third (B33) and the seventh (B37) branch extending between the fourth (B34) and the sixth (B36) branch, these branches belonging to the parasitic element.

13. A radio apparatus (RD1 ; RD2), characterised in that it comprises an antenna (500; 700) according to claim 1.