A double-acting antenna and a mobile phone comprising such an antenna

Abstract: The invention relates to a double-acting antenna of a radio communication device, whereby the antenna can be pushed partly inside the cover (4) of the radio communication device and whereby it function as an antenna both in the extracted or active position (Fig.3a) and in the retracted or passive position (Fig.3b). In the passive position that section (2) of the antenna which is pushed inside the cover (4) of the radio communication device functions as a part of the matching circuit (19), which adapts the impedance of the section left outside so that it corresponds to the impedance of the antenna port (11) of the radio communication device. For this purpose the arrangement comprises connecting means (10) to connect one end of the retracted section (2) to the ground potential the electrical length of the retracted section (2) is substantially shorter than a quarter of the used wave-length.

Description: The invention relates to an omnidirectional antenna according to the preamble of claim 1, which is intended to transmit radio communication on radio frequencies, and which can be partly retracted within the case of a radio device in order to save space, and which functions as an antenna both in the retracted and in the extended position. The development of portable communications equipment has resulted in that we try to make the transceiver devices operating on radio frequencies, such as mobile phones ever smaller and lighter. This makes high demands on the antenna structures, because the user requires that the antenna does not substantially increase the outer dimensions of the otherwise small-sized radio device, particularly when the device is not in use but carried e.g. in the pocket or in a briefcase. The reliability of the communication require on the other hand good electrical properties of the antenna, and a mobile station must be able to receive paging messages transmitted by a base station also in the transport position. More detailed information about messages and power levels, which the antenna must be able to transmit and receive, are included in the specifications of the respective communication system, of which we mention the specifications of the GSM system in the publication "M.R.L. Hodges, The GSM radio interface, British Telecom Technological Journal", Vol. 8, No. 1, 1990, pp. 31-43. Below we use a mobile phone as an example but the discussion is also applicable more generally to transceiver devices, on which we make demands regarding both size and performance. A common solution is to provide the mobile phone with a double-acting antenna, of which a main part is retracted within the telephone's case in the transport and storage position, and which the user can extend when required. These two positions are called the passive position and the active position, respectively. The antenna is designed so that even in the passive position a functional antenna section remains outside the telephone case, through which the telephone is able to receive paging messages. However, the electrical performance of the antenna is essentially better in the active position, so that in order to have a successful telephone connection the user is supposed to extend the antenna to the active position when he initiates a call. In the design of a double-acting antenna we meet with the problem how to get the antenna to operate in an optimal way, both in the active and in the passive position, and yet take as little space as possible. One double-acting antenna solution is presented in the patent US-5 204 687 A. There the double-acting antenna is formed by two conducting elements according to the figures 1a and 1b, which elements are located one after another in an elongated antenna structure without a galvanic connection to each other. At the top of the antenna there is a conductor wound as a cylindrical coil, or a so called helix element 1, which in the longitudinal direction of the antenna is substantially shorter than the straight conductor functioning as the shaft of the antenna, the so called whip element 2. When the antenna is in its active position (figure la) the telephone's transceiver unit is connected through the sliding contact 3 to the bottom end of the antenna, and it uses only the whip element as an antenna. In the passive position (figure 1b) the whip element 2 is wholly retracted in the telephone's case 4, and the transceiver unit is connected to the helix element 1 through the "neck" of the antenna. The matching circuit 9 adapts the respective antenna impedance so that it corresponds to the impedance of the antenna port 11. The disadvantage of this arrangement is the fact, that when one of these is in use the other is in a way unnecessary, because the whip element and the helix element are not electrically connected. An arrangement like this will result in quite modest space savings. The telescope antenna is an antenna solution which was already used for a long time in portable radios, whereby the antenna comprises cylindrical elements within each other which slide against each other. The telescopic structure is expensive and rather difficult to manufacture, and will hardly withstand mechanical stresses, so it has not gained wide acceptance in mobile phones. The patent publication WO-92/16980 A presents, according to the figures 2a, 2b and 2c, a double-acting solution, which comprises a helix part 1 and a whip part 2 which are arranged one after another as in the antenna presented in the publication US-5 204 687 A, but which in this case are connected to each other with an electrically conducting joint. The idea of the invention is to dimension the whip part 2 and its retraction case 5 so that in the passive position (figures 2b and 2c) the whip part is seen as a very high impedance and will not have an effect on the helix part's function as an antenna. An incorrectly dimensioned whip part would in the retracted position cause undesired reflections or unnecessary attenuation in the signal. In the design of the publication WO-92/16980 A the whip part 2 is preferably dimensioned with a length of a half-wave. The dimensioning of the helix is not essential to the invention presented in the publication WO-92/16980 A, but its electrical length can be e.g. a quarter-wave or a half-wave. The matching circuit 9 adapts the antenna impedance to the impedance of the antenna port 11. The publication also discloses an alternative embodiment, in which a sliding contact 6 is arranged in the middle of the retraction case 5 and in the passive position forms a ground contact to the whip part, so that the distance from the antenna feeding point 3 to the grounding point 6 is a quarter of the wave-length. Figure 2d illustrates in a circuit diagram the inventive idea of the publication WO-92/1 6980 A. The circuit diagram of the figure shows the helix part 1 functioning as the antenna and the matching circuit 9 in an exemplary embodiment when the antenna is in the passive position (cf. Figures 2b and 2c). The signal will not see the whip part 2 of the antenna as a branch leaving the feeding point, and thus it will not cause detrimental reflections or attenuation, because a transmission line, which is open at one end and has the length of a half-wave, and a transmission line, which is shorted at one end and has the length of a quarter-wave, appear on the radio frequency as an infinite impedance. However, the whip part 2 can not be made shorter than a quarter-wave, because a shorter transmission line can not be made to "vanish" from the view of the signal. The publication WO-92/16980 A does not even present a whip part with the length of a quarter-wave, but only the grounding of a half-wave whip part at the middle through the slide contact 6. A half of the wavelength at 450 MHz is about 30 cm, and at 900 MHz about 15 cm, so the half-wave whip element according to the publication WO-92/16980 is still quite long considering modern radiotelephones. It is obvious that the antenna design should provide still shorter solutions. Relevant background art for the invention is also disclosed in publications EP 0 613 206 A, WO 94/17565 A and EP 0 522 806 A that present respectively a straight antenna rod and a loading coil, certain combinations of helical and whip antenna elements. and the use of several alternative feeding points for a retractable whip+coil antenna combination. The object of this invention is to present an antenna structure, which both in the retracted position and in the extended position functions in the way required by a communication system, preferably a mobile phone system, and which has a very small size. The design must be simple to manufacture, and regarding manufacturing costs it should be suitable for series production of mobile stations. The object is achieved with the double-acting antenna structure, which comprises a first section and a second section. When the antenna is in the extracted position the first section and the second section function as an omnidirectional antenna, and when the antenna is in the retracted position the second section functions as a part of the impedance matching for the first section. The antenna structure comprises further a matching part, which comprises means to be connected to the antenna and to adapt its impedance to the correct value both in the extracted and in the retracted positions. The antenna structure according to the invention is characterized by the features recited in the characterizing portion of claim 1. The invention is based on the perception that in the retracted or the passive position we do not try to make the second section of the antenna invisible for the signal as disclosed in the publication WO-92/16980 A, but its electrical characteristics are utilized by connecting it as an inductance between the feeding point of the antenna and the ground potential. Then the electrical length of the second section can be substantially shorter than a quarter-wave. In the extracted or the active position the first and second sections form a combined antenna structure, whose electrical length is sufficient to achieve an effective antenna function. The matching part belonging to the antenna structure is designed in a way presented further below so that in both operating positions of the antenna it matches the antenna in an optimal way to the transceiver part of the radio device. The invention will now be described in more detail with reference to the enclosed figure, in which figure 1a shows a double-acting antenna structure known from the patent US-5 204 687 A in its extracted position, figure 1b shows the double-acting antenna structure known from the patent US-5 204 687 A in its retracted position, figure 2a shows a double-acting antenna structure known from the patent publication WO-92/16980 in its extracted position, figure 2b shows a double-acting antenna structure known from the patent publication WO-92/16980 in its retracted position, figure 2c shows a modification of the design of figure 2b, figure 2d shows the situation of figures 2b and 2c as a circuit diagram, figure 3a shows the antenna structure according to the invention when the antenna is extracted, figure 3b shows the antenna structure according to the invention when the antenna is retracted, and figure 3c shows the situation of figure 3b as a circuit diagram. The prior art was described above with reference to the figures 1a to 2d, so below we describe the invention with reference mainly to the figures 3a, 3b and 3c. Corresponding parts are marked with the same reference numerals in all figures. The description of the invention uses a mobile phone as an illustrative radio device. The antenna structure according to the invention is shown in figure 3a when the antenna is extracted, and it comprises a first section, preferably a helix 1, and a second section, preferably a whip 2. In the following we discuss as an example the helix as the first section and the whip as the second section. Both are made of conducting material, preferably a metal conductor or strip. They can be made as one body or as different bodies, whereby they are connected by a electrically conducting joint. The electrical length of the first section is preferably a quarter of the wave-length for which the antenna is intended. The electrical length of the second section is considerably shorter than one quarter of the wave-length, preferably about one eighth of the wave-length. Within the case of the mobile phone there is a directing tube 5, into which the whip section 2 withdraws when the antenna is retracted. The directing tube may be made of plastic or metal. If it is made of metal it must be isolated from the antenna. A directing tube is not necessary if the correct movement of the antenna between the outer and inner positions or the active and passive positions can be reliably guaranteed by other means. The matching part 7 of the antenna structure comprises connecting means 8 for a connection to the antenna, an impedance matching circuit 9 with which the impedance of the antenna arrangement is adapted so that it corresponds to the impedance of the antenna port 11 of the mobile phone's transceiver, and an ground contact 10, which grounds the lower end of the whip section when the antenna is pushed into the retracted position, so that the whip section will function as the inductance according to the inventive idea. Figure 3 shows two alternative embodiments 10a and 10b of the grounding circuit, which will be described below. The RF feeding point of the antenna is located at the connecting means 8, both in the active and in the passive positions. In the active position the radiating antenna element is formed by the series connection of the whip and helix sections, the series having an electrical length which is the sum of the helix and whip sections, preferably 3λ/8. In the passive position the radiating antenna element is formed by the helix section having an electrical length of about λ/4. Because the electrical length of the radiating antenna element changes between the active and the passive positions, the feeding impedance of the antenna gets different values, which results in that the same impedance matching circuit 9 will not function optimally in both cases. According to the present invention the whip section 2 and the ground contact 10 form in the passive position an inductance circuit from the antenna feeding point to the ground potential, which is illustrated in figure 3c. This inductance circuit functions as a part of the impedance matching of the helix section 1 when the antenna is in the passive position. The impedance matching circuit 9 is typically a low-pass structure realized with an inductive element L1 and a capacitive element C1. A high-pass structure, a strip element structure, or any other antenna matching circuit known by a person skilled in the art may also come into question. The circuit is dimension according to the active position, i.e. it adapts the impedance of the series connected whip section 2 and helix section 1 to be 50 Ω. Due to the grounding 10 in the passive position the whip section is connected between the antenna feeding point 8 and the ground potential, i.e. in the figure as a parallel inductance in parallel with the capacitive element C1, whereby the combined circuit of the whip section and the matching circuit 9 should adapt the impedance of the helix section to be 50 Ω. If the parallel inductance of the whip section is not exactly correct, then we can add to the grounding contact a series capacitance C2 according to the alternative 10a, a series inductance L2 according to the alternative 10b, or any other matching means obvious to a person skilled in the art. The antenna structure according to the invention is remarkably small sized, because the whip element has a length of only one eighth of the wave-length. However, the antenna provides a good performance, because in the active position its electrical length is longer than the length of either part alone, and the impedance matching is optimal both in the active and in the passive positions. The design is simple and mechanically strong, and its manufacturing costs are low. Although we described the antenna according to the invention with an example, its application possibilities will not be limited to the embodiments shown above. The antenna design according to the invention can be used in all transceiver devices, where the antenna preferably is small sized and retractable.

Claims: An antenna arrangement for the communication by a radio communication device comprising an antenna port (11) to transfer the antenna signal and a cover (4), and which antenna arrangement comprises: an antenna, which comprises a first section (1) and a second section (2), whereby the antenna can be moved relative to the cover of the radio communication device between a first extreme position and a second extreme position, whereby said first and second sections in the first extreme position are substantially outside said cover, and whereby said second section in the second extreme position is substantially within said cover, and a matching part (7) to adapt the impedance of the antenna so that it corresponds to the impedance of said antenna port (11), whereby the matching part comprises an impedance matching circuit (9) between said antenna and said antenna port, characterized in that the electrical length of said second antenna section (2) is substantially shorter than a quarter of the wave-length at which the antenna is intended to be used, and that in the second extreme position the second antenna section forms together with the matching part (7) a connection in which the second antenna section (2) is a part of a circuit that is arranged to function as a part of the impedance matching of the first antenna section (1), to adapt the impedance of the first antenna section (1) so that it corresponds to the impedance of said antenna port (11). An antenna arrangement according to claim 1, characterized in that the electrical length of the second antenna section (2) is substantially one eighth of the wave-length at which the antenna is intended to be used. An antenna arrangement according to claim 1 or 2, characterized in that said second antenna section (2) comprises a first end and a second end, of which the first end is connected to the said first antenna section (1), and that said matching part (7) comprises means to arrange a connection (10) between said second end and the ground potential when the antenna is in said second extreme position. An antenna arrangement according to claim 3, characterized in that said second antenna section (2) and said connection (10) form a substantially inductive connection (10b) between said first antenna section (1) and the ground potential (Fig. 3b). An antenna arrangement according to claim 3 or 4, characterized in that said connection (10b) between the second end of the antenna and the ground potential comprises a series inductance (L2) (Fig. 3b). An antenna arrangement according to claim 3, 4 or 5, characterized in that said connection (10a) between the second end of the antenna and the ground potential comprises a series capacitance (C2). An antenna arrangement according to any of the previous claims, characterized in that said first antenna section (1) is a conductor wound to a cylindrical coil. An antenna arrangement according to any of the previous claims, characterized in that said second antenna section (2) is a straight conductor. A mobile phone for communication in a mobile network, the mobile phone comprising a cover, a transceiver antenna port, and an antenna arrangement, whereby the antenna arrangement comprises: an antenna, which comprises a first section (1) and a second section (2), whereby the antenna can be moved relative to said cover of the mobile phone between a first extreme position and a second extreme position, whereby said first and second sections in the first extreme position are substantially outside said cover, and whereby said second section in the second extreme position is substantially within said cover, and a matching part (7) to adapt the impedance of the antenna so that it corresponds to the impedance of said antenna port, whereby the matching part comprises an impedance matching circuit (9) between said antenna and said antenna port, characterized in that the electrical length of said second antenna section (2) is substantially shorter than a quarter of the wave-length at which the antenna is intended to be used, and that in the second extreme position the second antenna section forms together with the matching part (7) a connection in which the second antenna section (2) is a part of a circuit that is arranged to function as a part of the impedance matching of the first antenna section (1), to adapt the impedance of the first antenna section (1) so that it corresponds to the impedance of said antenna port (11). The use of the antenna arrangement according to claim 1 in a mobile phone.

Inventors: HUUSKO RISTO, HUUSKO, RISTO
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1996
Main classification: H01Q1
IPC codes: H01Q1, H03H7