EP0576531B1

EP0576531B1 - An antenna construction with an extensible antenna element

Info

Publication number: EP0576531B1
Application number: EP92907567A
Authority: EP
Inventors: Erik Bech
Original assignee: DANCALL TELECOM AS
Current assignee: DANCALL TELECOM A/S
Priority date: 1991-03-19
Filing date: 1992-03-18
Publication date: 1995-03-01
Anticipated expiration: 2012-03-18
Also published as: ATE119318T1; GR3015692T3; WO1992016980A1; EP0576531A1; DK49191A; DK168346B1; AU1452992A; DE69201556D1; DK49191D0; ES2071501T3; DE69201556T2

Abstract

An antenna construction with a rod-shaped antenna element is adapted for use preferably in portable telephones. The rod-shaped antenna element is adapted to be slidably moveable in a tube between a first position in which substantially the entire rod-shaped antenna element protrudes from the tube and contributes to radiation, and a second position where just part of the rod-shaped antenna element protrudes from the tube and contributes to radiation. The rod-shaped antenna element is adapted to be received in the tube to provide a coaxial transmission line. The coaxial transmission line is adapted such that in the second position where part of the antenna element is received in the tube, it has a length and is terminated in a manner such that the impedance of the transmission line is high seen from the protruding part of the antenna element and thus does not affect this part. The coaxial transmission line may e.g. be an open half-wave transmission line. However, the transmission line may also be realized in the form of a short circuited quarter-wave transmission line or an open, shortened half-wave transmission line.

Description

The invention concerns an antenna construction, preferably for use in a portable telephone. The antenna construction is of the type comprising a rod-shaped antenna element which is slidably moveable between two positions.
As the development in the field of portable telephones entails that the models become more handy while being made smaller, the consumers demand that the antenna of a portable telephone must not contribute significantly to the overall size of the telephone. This is the case in particular when the telephone is not in the transmit mode. However, it must still be possible to receive call signals even though the portable telephone is in a passive mode.
European patent application No. 323 726 A discloses a telescopic antenna having several rods with a length of 1/2 wavelength. There is an impedance adjustment between the antenna cable and the antenna. A stationary hollow antenna rod accepts the other antenna elements when the antenna is retracted. The stationary antenna rod is placed coaxially in a conducting outer pipe which is connected to the screen of the antenna cable.
The US Patent Specification 4 890 114 tries to overcome precisely these problems by providing an antenna construction with an extensible antenna element. This antenna element is slidably moveable between two extreme positions, where in a first, active position for the telephone it is present substantially outside the case of the portable telephone. In a second position passive for the portable telephone, a considerable part of the antenna element is present within the case of the portable telephone, so that only the outermost, conductive tip protrudes from the case of the portable telephone. It is stated in this patent specification that this is sufficient to achieve a predetermined reception sensitivity. However, the signal received by the short part of the antenna will be disturbed by reflections from the long, retracted part of the antenna. This reduces the reception sensitivity of the antenna in the passive mode of the portable telephone, thereby impairing reception.
Others have tried to compensate for this unfortunate coupling between the antenna element part retracted into the case of the portable telephone and the part protruding outside the case to receive possible calls. This has been done by dividing the antenna into two separate antenna elements, which are arranged in elongation of each other and are mutually insulated. A relatively short antenna element at the outermost end of the antenna then provides for the monitoring function in the passive mode of the portable telephone, while a longer antenna element provides for the antenna function in the active mode of the portable telephone. This has the drawback that the actual antenna protrudes longer from the case of the portable telephone when the portable telephone is in the active mode, since the outermost part of the antenna does not contribute to radiation, but is insulated from the active part of the antenna and can thus be considered as being merely dummy in this state.
The object of the invention is to provide an antenna construction of the type stated in the opening paragraph, where an antenna element is slidably moveable between two extreme positions, which antenna construction must be adapted to be able to receive calls in both of these positions, without the antenna part introduced into the transmit/receive unit disturbing the active part of the antenna.
This object is achieved by the antenna construction stated in the characterizing portion of claim 1. The length of the antenna element is chosen such that the part moved into the housing of the transmit/receive unit serves as a coaxial transmission line with a high impedance seen from the feed point of the short, active part of the antenna element.
As stated in claim 2, the electrical length of the antenna element may be chosen such that the long, inserted part of the antenna element corresponds to an open half-wave transmission line. Claim 3 defines an alternative embodiment where the inserted part is constructed as a quarter-wave transmission line which is short-circuited. This may be realized e.g. by mounting a sliding sleeve at a distance from the feed point of the antenna corresponding to a quarter-wavelength, said sliding sleeve being capable of establishing electrical short-circuit between the antenna element and the coaxially arranged conductive tube. The impedance of the antenna element part received in the housing of the transmit/receive unit will hereby have a high impedance seen from the feed point of the antenna. In the retracted position, the part received in the housing will thus not affect the relatively short part that protrudes from the housing.
The invention will be explained more fully below in connection with preferred embodiments and with reference to the drawing, in which

fig. 1 shows the principle of a preferred embodiment of an antenna construction according to the invention, the antenna being in the extended position,
fig. 2 shows the antenna construction of fig. 1, with the antenna element in the retracted position,
fig. 3 shows an equivalent diagram of the antenna construction shown in fig. 1 as well as the adaptation of it,
fig. 4 shows an alternative embodiment of an antenna construction according to the invention,
fig. 5 shows how the antenna construction of fig. 1 may be realized in practice, and
fig. 6 schematically shows how an alternative embodiment with a shortened antenna element may be realized.

The principles of a preferred embodiment of the invention are shown in figs. 1 and 2, the antenna construction comprising an antenna element 1 which is slidably moveable between two extreme positions. One is shown in fig. 1, and the antenna element 1 is here extended from the housing of the portable telephone, so that the portable telephone can transmit with maximum power and receive with maximum sensitivity, the portable telephone being hereby capable of satisfying the requirements made by the authorities for NMT system approval. Fig. 2 shows how the antenna element 1 is retracted in the housing of the portable telephone (not shown in the figures) and is received in an electrically conducting tube 2, said tube 2 forming a coaxial transmission line together with the antenna element 1. In its feed point the antenna element 1 is electrically coupled to the transmit/receive parts (not shown) of the portable telephone through a coaxial cable 4, whose central conductor is connected to a pipe section 3, which is arranged coaxially with the antenna element 1 and coupled to the antenna element 1 through a capacitive coupling. The tube section 3 is electrically insulated from the tube 2 that is connected to earth, which takes place through the screen or outer conductor of the coaxial cable 4 in the preferred embodiment. The high frequency signal can hereby be coupled to the antenna element 1 without the use of physical contact arrangements.
As will be seen from figs. 1 and 2, the antenna element 1 is terminated by a coil 5. In the preferred embodiment, the part L₁ of the antenna element 1 protruding from the tube 2 is shorter than a quarter-wavelength in the passive mode of the portable telephone. Such an antenna is capacitive and may e.g. be tuned by means of an extension coil arranged in the center or at the top of the antenna. The use of a top coil is most practical since it can be concealed and protected in an antenna knob, as will be explained in connection with fig. 5.
When the antenna is fully extended, it has a length L₁ + L₂, which may e.g. be 5/8 wavelength or 3/4 wavelength. With an overall antenna length of 5/8 wavelength the antenna will have a capacitive impedance and can therefore be tuned with a series coil. In the preferred embodiment an antenna element with an overall length of 3/4 wavelength is perferred, the antenna impedance being thereby substantially real, so that there is no need for a tuning link in the antenna construction. In the extended position, this antenna has current maximum at the feed point.
The use of the expression "short and long antenna" below refers to the active part of the antenna when the portable telephone is in the passive mode and in the active mode, respectively.
As will appear from the following, it is desirable that the feed impedance of the short antenna part is low, and it is preferred that tuning is performed with the top coil, as mentioned before. In the preferred embodiment the long antenna is chosen to be precisely 1/2 wavelength longer than the short antenna and is tuned with the same top coil. Thus, there is no need for a tuning link in the feed point. Without use of insulation the long antenna will have an electrical length that corresponds to the physical length. Fig. 3 shows an electrical equivalent diagram of the antenna construction according to the preferred embodiment when the antenna element 1 is inserted into the tube 2 so that just the tip protrudes. As mentioned before, the short antenna is tuned with a top coil 5, whose physical embodiment is important since the coil does not serve as a simple impedance transformation link, but as part of the antenna radiator which has a longer electrical than physical length owing to the helixshape. Seen from the base point A of the antenna, the antenna has a low impedance, while the antenna part received by the tube with a length corresponding to 1/2 wavelength has a high impedance. As will be seen, there are no other tuning links in the antenna construction.
When the antenna element is pushed into the apparatus, a portion of the length L₂ will be received in a thin and preferably cylindrical metal tube, to take up as little space as possible in the apparatus. The antenna element may be selected according to the demands made on its function, and will preferably be an unbroken metal wire with some flexibility. Since the inserted part L₂ of the metal wire is formed with an electrical length of 1/2 wavelength and forms, together with the tube 2, an open coaxial half-wave transmission line, the impedance seen from the base point will be high, so that the short antenna part of low impedance is not affected. For optimum utilization of this principle, the short antenna is fed in a current maximum.
As an alternative to the above-mentioned open half-wave transmission line, the coaxial transmission line may be constructed as a short circuited quater-wave transmission line. This is realized by arranging a sliding contact 6 in the tube 2 at a distance form the feed point corresponding to a quarter-wavelength, which is shown in fig. 4. The part inserted into the tube 2 will have a high impedance seen from the feed point and will therefore not affect the function of the short antenna. This high impedance is independent upon the antenna element part present below the sliding contact 6, so that the physical length of the inserted part may be chosen freely. The antenna element may e.g. be constructed as an elastic element whose end is connected to e.g. a motor, so that the antenna element may automatically be moved into and out of the housing of the portable telephone controlled by the user.
The antenna element part present in the tube 2 has the same electrical and physical length when the antenna element is not surrounded by a dielectric material. If the antenna element is surrounded by a dielectric the physical length will be equal to the electrical length multiplied by the square root of the effective dielectricity constant. This is given by the following expression: ${L₂ = (λ/2)(ε}_{eff})^{1/2}$

where ε_eff is found from the expression: $ε_{eff} {= ln(r₂/r)(ε}_{r} {/(ln(r₁/r) + ε}_{r} ln(r₂/r)))$

where r₂ is the radius of the tube 2,
r is the radius of the antenna element 1,
r₁ is the radius of the dielectric surrounding the antenna element 1, and
ε_r is the relative dielectricity constant of the dielectric.
Fig. 5 shows a possible embodiment of an antenna construction according to the invention, the antenna construction comprising an antenna element 1 which comprises a rod 28 of conductive material, which is coated with an insulation layer 29 that may e.g. be plastics. A knob 30 is provided at the top of the antenna element, containing the previously mentioned extension coil 31. The antenna element 1 is extended almost completely from the housing of the portable telephone, the antenna element 1 being slidable through an opening in a wall 10 in said housing. The antenna construction is connected to the transmit/receive unit in the portable telephone through a coaxial cable 20, whose outer conductor 21 is attached to a ring 25 which connects the outer conductor 21 through a connector 26 to a tube 27, which is adapted to receive the antenna element 1 as it is inserted into the housing of the portable telephone. The inner conductor 23 of the coaxial cable 20, which is insulated from the outer conductor 21 through the insulation 22, is connected to the capacitive coupling tube, shown in fig. 1, through another connector 24, said capacitive coupling tube having e.g. the shape of the corresponding coupling tube 17 shown in fig. 5. As described before, the coupling tube 17 serves to transfer electromagnetic energy to the antenna element 1, but is here also constructed so as to serve to control the movements of the antenna element 1 with repect to the tube 27. The coupling tube 17 is therefore provided with shoulders engaging the internal side of the housing wall 10 through an insulation ring 14. The part of the coupling tube 17 protruding from the housing serves as an attachment part and is therefore provided with external threads, by means of which the coupling tube 17 is fixed with respect to the housing by clamping of a nut 15. The antenna element 1 is thus adapted to be slidably received in the coupling tube 17, where a sealing ring 16 ensures that water does not leak into the tube 27 upon displacement of the antenna element 1. The internal part of the attachment part on the coupling tube 17 is constructed so that the antenna element 1 is allowed to move transversely to some degree. The bottom of the antenna element 1 is provided with a stop 19 which, when the antenna element is extended to the extended position, is caused to engage the end of the coupling tube 17. This prevents the antenna element 1 from being moved completely out of the housing of the portable telephone.
The antenna construction illustrated in fig. 5 has a total length of 3/4 wavelength, and thereby a total length of about 20 cm when used in connection with an NMT system. Thus, when the portable telephone is in the active mode, the long antenna will have a length of 3/4 wavelength (including top coil). When the portable telephone is in the passive mode, the short antenna will be a quarter-wave antenna with top coil, while the inserted part of the antenna element will serve as a half-wave resonator in the tube 27. No tuning link is necessary for this antenna construction. The length of the tube part 17 will typically be of the order of 25-50 mm.
As mentioned in connection with fig. 4, the antenna construction shown in fig. 5 may be modified by arranging a sliding contact interiorly in the tube 27, said sliding contact being then positioned at a distance corresponding to 1/4 wavelength from the feed point of the antenna. Here too, the long antenna will be provided with a length of 3/4 wavelength (including top coil), which corresponds to about 20 cm. The short antenna will be a quarter-wave antenna with top coil, the antenna element part received in the tube 27 serving as a quarter-wave resonator. However, to provide electrical contact between the sliding contact 6 (fig. 4) and the conducting part of the antenna element 1, the antenna element must be stripped at least over part of its longitudinal extent.
It was assumed in the first of the two above embodiments that the thickness of the insulation material layer 29 was small with respect to the radius of the tube 27. An increase in the insulation material layer for the antenna construction shown in fig. 5 provides the advantage that the physical length of the antenna element can be reduced while maintaining its electrical length. Thus, the various radii may be adapted with respect to the effective dielectricity constant according to expression 2, and it is possible to use an antenna element 1 with a physical length of e.g. 16 cm. Thus, the long antenna will have a length of 5/8 wavelength, while the short antenna will still be a quarter-wave antenna with top coil, the antenna element part received in the tube 27 constituting a shortened half-wave resonator. However, a tuning link will now be necessary between the central conductor 23 of the coaxial cable 20 and the connector 24. This link must be capable of being connected and disconnected from the path of the high frequency signals in response to the mode of the portable telephone.
Fig. 6 shows how such an antenna construction may be realized. Only the most necessary parts to illustrate the principle are included in fig. 6. The housing of the portable telephone has a wall 110 with an opening in which a coupling tube 117 is mounted. The antenna element 1, which consists of a conducting part 128 with surrounding insulation material 129, is displaceable in the coupling tube 117 between two extreme positions. A permanent magnet 140 is arranged in elongation of the conducting part 128 of the antenna element. This magnet 140 does not contribute to the radiation function of the antenna, and is thus insulated from the conducting antenna element 128. As will be explained later, the magnet 140 contributes to connecting and disconnecting an antenna tuning link. The antenna element is adapted to be received in an electrically conducting tube 127, which is connected to ground via the outer conductor 121 of a coaxial cable 120. The coaxial cable 120 couples the high frequency signal to the antenna element 1 through the coupling tube 117, which is connected to the central conductor 123 of the coaxial cable 120 through the tuning link. An antenna tuning link is provided between the coaxial cable 121 and the coupling tube 117, said antenna tuning link comprising an inductance L_s1 which is connected to the central conductor 123 of the coaxial cable 120 and which is also connected to ground through a second inductance L_s2 and to the coupling tube 117 through a third inductance L_s3. A Reed contact consisting of a contact part 142 and a permanent magnet 141 is arranged in parallel over the first inductance L_s1. As will be seen, the two magnets 140 and 141 are of opposite polarity. It is hereby ensured that the contact 142 is made when the antenna is moved into the housing of the portable telephone, the magnet 140 being moved away from the contact. The magnet 141 will hereby dominate the Reed contact and ensure that it is made. When the antenna is extended again, the magnet 140 is positioned close to the magnet 141, the magnetic field, which affects the contact element 142, being thereby reduced because of the opposite polarity of the two magnets 140 and 141. The contact 142 is hereby broken. The series inductance L_s1 is dimensioned so that the antenna element, which has a length of 5/8 wavelength, is tuned in extended position. The other tuning inductances L_s2 and L_s3 tune unavoidable parallel capacities and the series capacity, respectively, in the coupling tube. These inductances are dimensioned accordingly.

Claims

An antenna construction with a rod-shaped antenna element (1) and preferably for use in portable telephones, said rod-shaped antenna element (1) being adapted to be received and slidably moveable in a tube (2;27;127) between a first position (fig. 1) where substantially the entire rod-shaped antenna element (1) protrudes from the tube (2;27;127) and contributes to radiation, and a second position (fig. 2) where just part of the rod-shaped antenna element (1) protrudes from the tube (2;27;127) and contributes to radiation, characterized in that a coaxial transmission line is formed by the tube (2;27;127) and the moveable antenna element (1) when said antenna element (1) is received in the tube, in that the feeding point (A) of the antenna is placed between the radiating part (L₁) of the movable antenna element (1) and the upper end of the tube being part of the coaxial transmission line and in that the coaxial transmission line in the second position of the element (1) is terminated in a manner such that the impedance of the transmission line is high seen from the protruding part (L₁) of the antenna element and thus does not affect it.
An antenna construction according to claim 1, characterized in that the transmission line is an open half-wave transmission line.
An antenna construction according to claim 1, characterized in that the transmission line is a short circuited quarter-wave transmission line.
An antenna construction according to claim 1 or 2, characterized in that the transmission line is an open, shortened half-wave transmission line.
An antenna construction according to claims 1-4, characterized in that the antenna element is fed through a capacitive, resonant coupling (3).