Antenna for radio telephone.

0001The present invention concerns a small-size antenna for radio telephone, comprising two parallel plates disposed at a space from one another, provided with an insulator therebetween.

0002The object of the invention is to provide a small-size antenna for a radio telephone, the matching of which with the antenna input lead is of higher order and wide-band, and said matching being maximally compact.

0003In conjunction with radio telephones is known in prior art the so-called PIFA antenna (Planar Inverted F-Anten­na), also referred to as resonator antenna. The PIFA an­tenna is composed of two parallel conducive plates inter­connected by a conductor of appropriate width so that the capacitance between the plates is in resonance with the inductance of this conductor. The antenna input lead is carried at an appropriate distance from said conductor through one of the plates to the opposite plate. The an­tenna is matched to the input lead by selecting an appro­priate spacing between the input lead and the conductor connecting the plates. The input impedance is determined by said spacing. The difficulty in implementing the above described PIFA antenna of prior art accordingly is that the antenna element has to be brought at the desired frequency into internal resonance which is thus determined by the capacitance of the plates and the inductance of the conductor connecting them. In case the size of the antenna element is restricted, as owing to the other dimensions of the radio telephone, the endeavour to achieve resonance at a given frequency implies that the length or width of the conductor connecting the plates may become impractical.

0004Another drawback of the PIFA antenna is that the resonant circuit constituted by the plates and the conductor con­necting them is of the first order, and it therefore has narrow band character.

0005The object of the present invention is to provide an an­tenna which eliminates the drawbacks encountered in con­junction with the PIFA antenna and at the same time, maintains the small antenna size. This object is mainly achieved in that the first plate is an electrically con­ducive plate and the second plate is an insulator plate, on one surface of which, that which is opposite with re­ference to the first plate, is provided a substantially coherent metallic layer acting as ground level, and on the other surface facing the first plate is disposed a matching circuit in such manner that it matches the antenna imped­ance to the impedance of the antenna input lead of the radio telephone.

0006It is thus understood that, according to the invention, the conductor connecting the plates of the PIFA antenna is eliminated, whereby the antenna element need not be in resonance and one of the antenna plates is replaced by a circuit board on which a higher-order, and therefore wide­band, matching circuit can be placed.

0007The invention is more closely described referring to the figures attached in which <ul id="ul0001" list-style="none"><li>Fig. 1 presents the design drawing of the PIFA antenna of prior art,</li><li>Fig. 2 presents the design drawing of the antenna of the invention,</li><li>Fig. 3 presents a schematic design of the matching circuit.</li></ul>

0008In Fig. 1 is presented the construction of the PIFA antenna embodying the state of art. The antenna comprises two condu­cive plates 1 and 2, wherebetween is placed an insulating layer 3. The thickness of the insulating layer is l_e. The conducing plates 1 and 2 are interconnected with a conductor 4, its width being l_j. To the antenna is supplied a signal by the coaxial cable 5 serving as the feeder lead, passing through the second plate 2 and the insulating layer 3 being connected to the conducive plate 1. The space between the feeder lead 5 and the lead connecting the plates is l. The desired feed impedance can be selected by varying said distance.

0009In the antenna of the invention, the conductor 4 connect­ing the plates in the antenna design of Fig. 1 has been eliminated. One of the conducive antenna plates has been replaced with a thin glass fiber circuit board 22, of which the surface lying farther away from the insulator between the plates 21 and 22 has been coated with metal. Said sur­face, which is pointing downwards in Fig. 2, acts as a ground level. On that surface of the plate 22 which faces the insulator 23 between the plates 21 and 22, i.e. the electrically conducive plate 21, has been disposed a mat­ching circuit S including chips and its microconductor strips. The matching circuit S is advantageously of one of the second order but the higher-order matching circuits may equally be used. From the matching circuit S the sig­nal is carried to the antenna plate 21 through a flat-lead 24. The purpose of the insulation 23 between the antenna plates is to brace the plates 21 and 22 and to keep them appropriately spaced. For insulation material, so-called honeycomb paper has been used, which is aramide fibre paper saturated with phenolic resin and embossed in honeycomb configuration. Other appropriate insulations may equally be used. The circuit board 22 is also provided with a con­nector L by which the antenna is connected to the radio components of the radio telephone.

0010The matching circuit on the plate 22 may be implemented e.g. as shown in Fig. 3. Here, the antenna feed impedance has been matched to 50 ohms with the second-order match­ing circuit of bandpass type, composed of two capacitances C1 and C2 and reactances w₁L1,w₂L2. The electrical equiv­alent connection of the antenna plate comprises a resist­ance R and a capacitance C connected in parallel and in­ductance L in series therewith. In a free space, the band­width of a test specimen constructed, when connected to the telephone chassis, was measured and found to be 860-­990 MHz with the standing wave ratio SWR &lt; 2.

0011The distance of the antenna from the conducive chassis of the radio telephone affects the matching of the antenna, whereby when the chassis is essentially altered, the matching circuit has also to be redesigned. The matching circuit of Fig. 3 is primarily exemplary. The matching circuit and the plastic housing of the antenna, as well as the plastic materials of the radio telephone housing cause some signal attenuation.

0012With the antenna matching of the invention, a higher-­order, and therefore a greater, bandwidth matching is ob­tained than with the PIFA antenna, the external dimensions of the antenna being unchanged. In addition, the matching is compact because the matching circuit is accomodated within the antenna. For instance, it may be mentioned that as regards bandwidth, the antenna is at the same time usable in the 900 MHz NMT network and in the E-TACS-network with 52 x 30 x 7.5 mm antenna size.

0013The antenna of the invention has also the additional ad­vantage that when the telephone is held in hand the matching improves and the useful frequency range increas­es implying that the antenna is not greatly sensitive to environmental effect.

0014The antenna constituting the object of the invention may also be so implemented that the antenna is integrated within the housing of the radio telephone in conjunction with the radio components so that no separate antenna con­nector is needed. The matching circuit may then be disposed on the circuit board containing the radio components. Such integration is advantageous because it is simple and there is no plastic material causing losses between the antenna element and the chassis of the radio telephone. The shape designing of the radio telephone is moreover easier as there are less external components.