Antenna structure

The invention relates to an internal multiband antenna structure intended especially for small radio devices.

The available space is an important factor when designing antennas for portable radio devices. An antenna of good quality is relatively easy to make without restrictions in size. However, in portable devices, the antenna is most preferably placed inside the casing of the device in which case, when the size of the device becomes smaller, also the space available for the antenna has become ever smaller. This means that the design becomes more demanding. This is also affected by that the antenna often has to function in two or more separate frequency bands.

In practice, an antenna with satisfactory characteristics and fitting inside a small apparatus can most easily be achieved as a planar structure: The antenna com- prises a radiating plane and a ground plane parallel with it. For matching, these planes are usually connected to each other by a short-circuit conductor, in which case a PIFA-type (Planar Inverted F-Antenna) structure is created. However, when reducing the distance between the radiating plane and the ground plane from the optimal value, the characteristics of PIFA, such as bandwidth, deterio- rate. This means a problem with relatively flat radio apparatuses which are common nowadays. In addition, the fact that the apparatuses have become smaller has also resulted in a decrease in their ground plane. In this case, the capability of a PIFA lowers as the antenna resonances weaken, and because of the ground plane's own resonances that fall on the useless frequencies. In dual- band mobile station antennas, the disadvantages are bigger in the lower operating band, which is located in the range of 0.9 GHz.

The problem caused by the flatness of a radio apparatus can be reduced by making the antenna to have a monopole structure. Fig. 1 shows an example of such an internal monopole antenna of an apparatus. The drawing shows the cir- cuit board PCB of a radio apparatus, and two radiating elements 120, 130 of the antenna. The surface of the circuit board is mostly of conductive ground plane GND. The radiating element 120 is the main radiator. It is located at the end of the circuit board, almost totally outside, when seen from above the board, the ground plane thus being located beside the radiator. Seen from the feed point

FP, the main radiator 120 is divided into two branches 121 , 122 of different lengths for implementing two separate operating bands. The main radiator with its branches is mostly on a plane, which is parallel to the circuit board PCB. For lengthening the longer branch 121 and for shaping the radiating pattern, at its outer end there is a section which extends towards the geometric plane of the circuit board, for which reason the antenna has a certain height. However, this height remains smaller than the height required by all the other parts of the radio apparatus together. An inductive component can be connected to the ground near the feed point FP for improving the matching of the antenna. Another radiat- ing element 130 is a parasitic element. It is connected to the ground plane GND from the ground point GP at the other end of the element. The ground point GP and said feed point are located side by side near a corner of the circuit board, seen from above. The parasitic element 130 starts from the ground point parallel to the starting part of the main element, in this example on a slightly lower level than the main element, and turns then under the main element.

The lower operating band of the antenna is based on the resonance of the longer branch 121 of the main element and the upper operating band is based both on the resonance of the shorter branch 122 of the main element and the resonance of the parasitic element 130. The frequencies of the two latter resonances have different values, but in any event so close to each other that a united and relatively wide upper operating band is achieved.

The performance of the monopole antennas described above exceeds the one of a PIFA made to a space of equal size. However, their drawback is that the lower operating band remains relatively narrow so that it is susceptible to shift slightly aside from the planned range, for example, due to the effect of external conductive materials. The size of the ground plane of the radio apparatus is one factor affecting the width of the lower operating band; if the size of the ground plane deviates from the optimum, the bandwidth will decrease further. A further drawback is that the resonances of the main element have a weakening effect on each other, which means deterioration in the efficiency of the antenna and also in the bandwidths.

The object of the invention is to reduce said drawbacks related to the state of the art. The antenna structure according to the invention is characterised in what is disclosed in claim 1. Some advantageous embodiments of the invention are pre- sented in the other patent claims.

The basic idea of the invention is the following: The radiating structure of the antenna includes a nearly air-insulated first monopole radiator and a second mono- pole radiator on a ceramic substrate. The former one resonates in the lower operating band of the antenna and the latter in the upper operating band. The an- tenna structure also includes as a substantial part a matching circuit which, in addition to matching, also provides the second resonance for the first radiator. The ceramic substrate and the matching circuit are located on a plastic frame supporting the first radiator or on a small auxiliary plate attached to the frame so that an integrated antenna module is built. In addition, the structure can include a radiating parasitic element for widening the upper operating band.

An advantage of the invention is that a relatively wide lower operating band can be provided for a small antenna. This is due to the double resonance of the radiator of the lower operating band, generated by the matching circuit according to the invention. A further advantage of the invention is that a shared feed point can be used between the radiators of the lower and upper operating bands. This is because the matching circuit according to the invention functions at the same time as a filter, which enhances the isolation between the radiators. A further advantage of the invention is that the effect of the size of the ground plane of the radio apparatus on the width of the lower operating band is clearly lower than in the known monopole antennas. A further advantage of the invention is that the entire antenna structure can be tested as a stand-alone module so that no testing will be required after the module has been mounted to a radio apparatus.

The invention will next be described in more detail, referring to the enclosed drawings, in which

Fig. 1 shows an example of the known internal monopole antenna;

Fig. 2 shows an example of the antenna structure according to the invention;

Fig. 3a shows the antenna structure according to Fig. 2, seen from above;

Fig. 3b shows the antenna structure according to Fig. 2, seen from the side;

Fig. 4 shows an example of the matching circuit of the antenna structure ac- cording to the invention;

Fig. 5 shows an example of the band characteristics of the antenna structure according to the invention; and

Fig. 6 shows an example of the efficiency of the antenna structure according to the invention.

Fig. 1 was already explained in connection with the description of the state of the art.

Fig. 2 illustrates an example of the antenna structure according to the invention. Fig. 2 is a perspective view, illustrating only the mechanical general structure of the antenna. It shows the circuit board PCB of a radio apparatus, the upper surface of which is largely conductive ground plane GND. The antenna structure is located at the end of the circuit board, outside the end. The radiating structure comprises two monopole radiators. The first radiator 221 is a strip-like conductive element on the surface of a dielectric support frame 205. The frame 205 is attached to the end of the circuit board PCB and has in this example a vertical section, which is curved when seen from above, and follows the shape of the end of the radio apparatus, for which the antenna is intended. The first radiator 221 is attached to the outer surface of the curved section of the frame. The curved section is relatively thin, and the material of the frame is selected so that the first radiator is nearly air-insulated. The second radiator 222 is a part of the conductive coating for the ceramic substrate. In this case, the ceramic substrate 210 is an elongated rectangular piece being located, when seen from above, parallel with the end of the circuit board PCB between the first radiator 221 and the circuit board. The frame 205 comprises a horizontal section extending from its curved section towards the circuit board. The ceramic substrate 210 with its conductive coating is attached either directly onto the horizontal section of the frame, i.e. the "bottom", or onto a small dielectric auxiliary plate, which again is attached onto that bottom. The parts of the antenna structure constitute an integrated antenna module 200, which can be tested separately and then mounted into the radio apparatus.

The antenna structure according to the invention has at least two separate operating bands. The two operating bands are called lower and upper operating band. In Fig. 2, the first radiator 221 resonates in the lower operating band and the second radiator 222 in the upper operating band. The radiators have a shared feed point FP to be connected to the antenna port of the radio apparatus which the feed point is located in this example on a protruding part of the antenna structure extending onto the circuit board PCB. The electric structure of the antenna will be described in more detail below.

Fig. 3a shows an example of the antenna structure according to Fig. 2, seen from above, and Fig. 3b shows that structure from the side of the circuit board PCB, at its level. In Fig. 3a there is seen a support frame 205 of the antenna structure, a dielectric auxiliary plate 215 on the "bottom" of the support frame, a ceramic substrate 210 on the auxiliary plate, a first radiator 221 on the outer surface of the frame, a second radiator 222 on the upper surface of the substrate 210, and a matching circuit of the antenna being located on the auxiliary plate. The matching circuit is an essential part of the antenna structure according to the invention. It comprises the capacitive elements C1 and C2, and the inductive elements L1 and L2, all of which are discrete chip components in this example. Their connection appears from Fig. 4. The second radiator 222 is at its feed end considerably narrower than at its other parts. That narrow part increases the inductance of the feed circuit of the radiator, and it can be considered to be a part of the matching circuit of the antenna.

The first and second radiator have a shared ground point G1 and a shared feed point FP, which is connected galvanically to the feed position F2 of the second radiator 222 in the antenna structure. The first radiator receives its feed to its feed position F1 through the matching circuit. The ground point G1 and the feed point FP are located on a protruding part of the auxiliary plate 215, extending onto the circuit board PCB of the radio apparatus. The ground point G1 is connected to the ground plane GND of the radio apparatus via a lead-through in the auxiliary plate, and the feed point FP is connected to the antenna port of the radio apparatus via a lead-through in the auxiliary plate.

The exemplary structure further comprises a parasitic radiator 230 seen in Fig. 3a, which is a conductive strip on the surface of the auxiliary plate 215 between the substrate 210 and the outer, curved section of the frame 205. One end of the parasitic radiator 230 is connected to the ground plane GND via a second ground point G2 on the auxiliary plate. The resonance frequency for the parasitic radiator is arranged in the upper operating band of the antenna for widening it.

In Fig. 3b there are seen the components of the matching circuit, such as the capacitor C2, mounted on the auxiliary plate 215. Further, a conductive strip on the side surface of the substrate 210 is seen as a detail, which strip connects the second radiator 222 with its feed position F2 below the substrate.

Fig. 4 illustrates as a circuit diagram an example of the matching circuit of the antenna structure according to the invention. The structure and markings of the

matching circuit 240 correspond to those of the structure in Fig. 3a. Between the feed point FP, shared between the radiators, and the ground point G1 there is the first capacitor C1. The feed point is connected to the second radiator 222 via an inductance L', which corresponds to the inductance of the above-mentioned narrow conductive strip belonging to the conductive coating of the ceramic substrate. The feed position of the second radiator can be imagined to be at the lower end of the inductance L' in Fig. 4. Also a series resonance circuit is connected to the feed point FP, which circuit includes, starting from the feed point, first the second capacitor C2 and then the second coil L2. The other end of the series resonance circuit is connected via the first coil L1 to the ground point G1 and directly to the first radiator 221.

Thus, the first coil L1 is between the feed position of the first radiator and the ground point, and the impedance of the first radiator is matched using it. The impedance of the second radiator is again matched by the first capacitor C1.

Because of the series resonance circuit C2, L2 included in the matching circuit, the first radiator 221 has a double resonance instead of one basic resonance. The frequencies of these resonances can be arranged suitably close to each other so that the lower operating band becomes relatively wide.

In addition, the matching circuit forms at the same time a low pass filter between the port represented by the feed position of the first radiator and the ground point

G1 and the port represented by the feed position of the second radiator and the ground point G1. The boundary frequency of the low pass filter is between the lower and the upper operating band. This means that the isolation between the radiators will improve so that a shared feed point FP can be used between the radiators in the antenna structure.

Fig. 5 shows an example of the band characteristics of the antenna structure according to the invention. The curve shows the change in the reflection coefficient S11 as the function of frequency. It has been measured from an antenna structure, the dimensions of the ceramic substrate of which are 18x3x1.5 mm ³ and the dimensions of the entire antenna module are 40x8x6 mm ³ , the width dimension of 8 mm concerning the distance of the outer edge of the module from the edge of the ground plane. The relative dielectric constant of the substrate used is 35. The lower the reflection coefficient, the better the antenna has been matched, and the better it functions as a radiator and as a receiver of radiation. If, for ex- ample, the value -5 dB for the reflection coefficient is used as a criterion for the

boundary frequency, the lower operating band of the antenna is about 810-1030 MHz, the bandwidth being thus about 220 MHz (24 %). The lower operating band covers well the frequency range W1 plotted in the figure, which range includes both the frequency range of 824-894 MHz of the American GSM system (Global System for Mobile telecommunications) and the frequency range of 880- 960 MHz of the European EGSM system (Extended GSM). The considerable width of the lower operating band has been achieved so that two resonances r1 and r2 have been arranged in the frequency range in question by means of the matching circuit according to the invention so that those resonances do not much interfere with each other. The first resonance r1 and the second resonance r2 are resonances of the entirety formed by the first radiator 221 and the matching circuit, especially its resonance circuit.

With the criteria mentioned above, the upper operating band of the antenna is about 1.7-2.17 GHz, the bandwidth being thus about 470 MHz (24 %). The up- per operating band covers the frequency range W2 plotted in the figure, which range includes the frequency range of 1710-1880 MHz of the GSM1800 system, the frequency range of 1850-1990 MHz of the GSM1900 system, and the frequency range of 1920-2170 MHz of the WCDMA system (Wideband Code Division Multiple Access). Also the upper operating band is based on two reso- nances r3, r4. The lower of these, i.e. the third resonance r3 is the resonance of the second radiator 222 on the ceramic substrate, and the fourth resonance r4 is the resonance of the parasitic radiator 230.

Fig. 6 illustrates an example of the efficiency of the antenna structure according to the invention. The curve shows the change in efficiency as the function of fre- quency when the antenna is in a free space. In the lower operating band, the efficiency varies between the values -3.8 dB and -2.5 dB. In the upper operating band, the efficiency varies between the values -3.7 dB and -2.7 dB. The above mentioned frequency ranges W1 and W2 are here considered as the operating bands. One factor influencing the efficiency is the size of the ground plane of the radio apparatus. The results above have been obtained with a ground plane of 40x100 mm ² .

In this specification and the claims, the qualifiers "horizontal", "vertical", "lower", "upper" and the epithet "from above" refer to the position of the antenna structure, in which the circuit board of the radio apparatus and the bottom of the frame of the antenna structure are on a horizontal plane, and the ceramic substrate is above the bottom. Naturally, the use position of the apparatus can be whichever.

One antenna structure according to the invention has been described above. The shapes and the location of the parts of the structure can naturally differ from those presented in the figures. For example, the parasitic radiator can also be located on the surface of the dielectric frame. The auxiliary plate can be omitted, in which case the substrate and the matching circuit are located directly on the bottom of the frame. In the matching circuit, both the capacitive elements C1 and C2 can also be realised by mere adjacent conductive strips on a dielectric base surface. Likewise, at least the smaller one of the inductive elements L1 can be realised by a mere narrow conductive strip on a dielectric base surface. A strip with low inductance can be in series with a discrete coil, the strip being used as a tuning element by matching it for example with a laser in the testing stage. The narrow conductive strip matching the second radiator, which strip is on the surface of the substrate in Figures 3a and 3b, can also mainly be located on said auxiliary plate. The inventive idea can be applied in different ways within the limits set by the independent claim 1.