Arrangement for antenna matching

The invention relates to an arrangement for matching transmitting antennas in mobile stations, especially in mobile stations applying new transmission technologies, as well as to an arrangement for protecting a power amplifier feeding an antenna. In order to match an antenna ( 260 ), the strength of the field (re) reflected from the antenna is continually measured. The measurement data is used in controlling an antenna matching circuit ( 250 ) so that the strength of the reflected field is minimized. The measurement is carried out using a circulator ( 220 ) which also provides isolation, i.e. prevents the field reflected from the antenna from propagating back to the power amplifier (PA). In a multi-band apparatus, the band in use is also taken into account in the matching. The structure according to the invention saves space, as the number of relatively large components becomes smaller. Furthermore, the attenuation of the transmission path to the antenna is reduced compared to corresponding structures according to the prior art.

FIG. 1 was already discussed in connection with the description of the prior art. FIG. 2 shows an example of the matching and attenuating arrangement according to the invention. There are, connected in series, in the direction of the propagation of the signal, a radio-frequency power amplifier 210 , a circulator 220 , a matching circuit 250 and an antenna 260 . In addition the structure includes a control unit 240 controlling the matching circuit. The matching adjustment principle is the same as in the known structure according to FIG. 1 : The reactance of at least one capacitive or inductive element in the matching circuit is changed in order to minimize the strength of the field reflected from the antenna. The variable capacitance may be realized using a conventional capacitance diode or e.g. MEMS (Microelectro-Mechanical System) switches, which are known per se, and capacitors with a relatively high Q factor. The capacitances to be utilized may be included in the MEMS switch structure itself, in which case there are no separate capacitors. Let such a variable component be called a MEMS capacitor. The difference from the structure of FIG. 1 is that the directional coupler and isolator are replaced by a single component, namely the circulator 220 . A circulator is a waveguide having e.g. three or four ports. The shape of the waveguide and the piece of ferrite inside it give the variable fields propagating in the waveguide phase shifts such that a field fed into a given port can only leave through a certain other port but not through the rest of the ports. This is realized in a circulatory manner: For example, the circulator in FIG. 2 has three ports. A field fed into a first port p 1 can go out only through a second port p 2 , a field fed into the second port can only go out through a third port p 3 , and a field fed into the third port can only go out through the first port. In practice there is naturally a small leakage in the prohibited direction, but the attenuation in that direction is, however, several tens of decibels. In FIG. 2 , an output of the power amplifier 210 is connected to the first port p 1 of the circulator, and the second port p 2 is connected to the matching circuit, so a relatively high-power signal ff can go to the antenna via this route. The third port p 3 of the circulator is connected to an input of the control unit 240 . A field re reflected from the antenna is thus directed almost unattenuated through ports p 2 and p 3 to the control unit where a field strength detector produces a voltage proportional to it. The reflected field re, however, cannot return through the first port to the power amplifier, so the circulator 220 also provides isolation. FIG. 3 shows a second example of the matching and attenuating arrangement according to the invention. There are, connected in series, in the direction of the propagation of the signal, a radio-frequency power amplifier 310 , a circulator 320 , a matching circuit 350 and an antenna 360 . In addition the structure includes a control unit 340 of the matching circuit. The difference from the structure of FIG. 2 is that the antenna matching adjustment is meant to function in more than one frequency band, say both in the GSM900 and GSM1800 systems. If there is a clear difference between the two alternative bands, as in the case of the GSM systems mentioned above, the antenna has to have two bands. A change in the resonating structure in the antenna may in that case require a substantial change in the matching circuit 350 , too. To that end, a signal BS (Band Select) indicating the band in use is brought to the control unit 340 from the baseband control unit CU of the radio apparatus, as shown in FIG. 3 . Moreover, a signal PW′ proportional to the output power of the power amplifier PA is brought to the control unit of the matching circuit from the control unit CU of the whole radio apparatus. This is used to determine the relative strength of the field re reflected from the antenna in order to generate a suitable control signal to the matching circuit. The signal PW′ is obtained from the output power measurement signal PW generated in the power amplifier and used primarily for output power control for the purpose of keeping the radiation power of the antenna within the limits given in the specifications of the system in question. In FIG. 3 , the output power of the power amplifier is controlled by means of a power control signal PWC generated in the control unit CU. FIG. 4 shows a mobile station MS. It includes a component 420 according to the invention for separating the field reflected from the antenna to a path of its own and preventing the reflected field from entering the radio-frequency power amplifier. Above solutions according to the invention were described. The invention does not limit the type of antenna used in the radio apparatus, nor the ways in which the adjustment of the matching is realized. Neither does the invention limit the method of implementation of the component for directing radio-frequency fields. The inventive idea can be applied in different ways within the limits defined by the independent claim 1.