The invention relates to an antenna coupling device for coup¬ ling, substantially inductively, a first antenna, e.g., situated in a vehicle and, e.g., incorporated in a hand- portable telephone, to a second antenna.

Specifically, the invention relates to an antenna coupling device for coupling inductively a first antenna, situated in a screened place, to a second antenna, working under better transmission and reception conditions. Such an antenna coupling device may be used, e.g., when travelling in a vehicle for applying onto a helical antenna of a hand-portable telephone, which is not provided with means for a galvanical connection to an external antenna. The telephone may hereby be coupled induc¬ tively via an RF coupling means and a coaxial cable to an external antenna for achieving better antenna performance.

An antenna coupling device of this kind is disclosed in US,A,4 220 955, which describes a device for coupling induc¬ tively an antenna, which is incorporated in a "transceiver", to an external antenna. The coupling device comprises a shielded housing intended to be threaded onto the antenna and containing an helical antenna in the form of a coil for the inductive coupling.

An isolated sleeve with an open end carries the coil, the one end of which is connected to the center conductor of a trans¬ mission line, which leads to an external device, and the second end which is open. The coil and the sleeve are surrounded by a conductive housing, which is coupled to the shield of the transmission line.

The above antenna coupling device is distinguished by the use of a helical antenna, i.e. a coil with one open end, for the inductive coupling. Further, the diameter of the coil is small in relation to the transferred wavelengths. This means that the helical antenna is functioning in "normal mode" . Such is also the case for the helical antenna built in the "transceiver". Hereby the short range radiation fields of the electromagnetic

radiation of the two antennas correspond, which is one of the conditions for satisfactory inductive coupling, when the antennas have the same geometrical orientation and are situated close to each other.

In order to achieve satisfactory inductive coupling in this prior art antenna coupling device, the helical antenna must also be provided with a relatively large number of turns, in respect to its required diameter and total wire length, and the turns must be distributed spaciously, i.e. the helical antenna shall have a large axial elongation.

If the helical antenna is to be compressed axially through a more compact winding this may to some extent be compensated by a greater number of turns, which gives a more complicated design and difficulties in achieving the same efficiency.

The object of the invention is to achieve an antenna coupling device that overcomes as far as possible the above mentioned drawbacks and that fulfills the demands of a high degree of coupling, simple and compact design, and uncomplicated ope¬ rating.

A loop antenna has other features than a helical antenna. The loop antenna works by both its ends, or poles, being galvani- cally coupled. Hereby it presents another radiation pattern or field. However, in the immediate proximity of antennas of the different types, i.e. within the short range fields, the radi¬ ation patterns are approximately the same. This enables a loop antenna to be designed to give excellent inductive coupling to a helical antenna, provided that these have the same geometri¬ cal orientation and that the loop antenna preferably surrounds the helical antenna.

Further, it is advantageous to use a loop antenna, since it gives a high degree of efficiency for the inductive coupling with a very simple design. An electrically well-dimensioned loop antenna including a well-tuned impedance matching unit may

in one single turn give a degree of efficiency of 50-70%. Hereby the loop antenna may be given an exceptionally small size and an easily adapted design. With this solution it is also possible to achieve an extremely broad-banded antenna function.

It is also possible to provide the loop antenna with more than one turn. This way one could, e.g., achieve an impedance matching to a cable without the use of separate reactive ele- ments or tuned conductor elements in connection with the loop antenna.

Thus, the object of the invention is achieved by an antenna coupling device of the outlined type, said antenna coupling device comprising a loop antenna with two poles, the loop antenna being arranged to surround a first antenna, and the loop antenna being connected, via both its poles, to the second antenna.

It is advantageous that the loop antenna is connected to the second antenna via an impedance matching unit, which is coupled directly to the loop antenna. Hereby the impedance of the loop antenna may easily be adapted to an arbitrary impedance, irre¬ spective of the diameter and the number of turns of the loop antenna and irrespective of the frequency range used for trans¬ mitting and receiving.

It is also possible to connect the impedance matching unit via a cable at some distance from the loop antenna. However, this decreases the degree of efficiency as a result of a high standing wave ratio in the cable.

Further, it is suitable to connect the loop antenna to the second antenna via a transmission line in order to freely place the antenna coupling device and the second antenna, respec¬ tively, in suitable positions. For example, the antenna coupling device may for this purpose be made easily accessible inside a vehicle for a user of a hand-portable telephone,

whereat the second antenna may be mounted externally on the vehicle body.

In this case, the impedance matching unit adapts the loop antenna to the impedance of the transmission line, so that the least possible losses occur and the highest possible degree of efficiency is achieved. It is advantageous to choose a coaxial cable as the transmission line, since it is well-suited for transferring RF signals.

In a preferred embodiment of the invention, the second antenna is a onopole or dipole antenna, which is in principle an RF radiating, straight conductor, or a co-linear antenna, consti¬ tuted by two or more monopoles with a coil arranged between adjacent monopoles.

In order to achieve satisfactory coupling, or high degree of efficiency, the loop antenna is arranged to surround or enclose the first antenna substantially coaxially with a small mutual radial separation.

In the first preferred embodiment, the loop antenna and the impedance matching unit are arranged within a housing that may be threaded onto the first antenna and that arranges the loop antenna substantially coaxially to the first antenna. Prefer¬ ably, the housing is non-conductive, although it may be made of conductive or metallized material, whereby it may be connected to the shield of the coaxial cable.

The housing is provided with at least one mechanical fixing means for fastening onto the first antenna. The mechanical fixing means may interlock with a groove or a shoulder, be clamped or screwed either onto the first antenna or onto the hand-portable telephone itself.

Another important advantage in the use of a loop antenna is that it may easily be arranged on a printed circuit board together with the impedance matching unit.

Further advantageous features of the invention are described in the dependent claims.

The invention allows several modifications without departing from its main principles. For example, the antenna coupling device may be used for coupling inductively a first antenna to any type of RF means.

The invention is described closer below in a preferred embodiment with references to the accompanying drawings, wherein:

Fig. 1 shows, in a partly cut away side view, a hand-portable telephone provided with a helical antenna, whereon an antenna coupling device according to the invention is applied;

Fig. 2 shows, in a top view, a principle plan of the main components of the antenna coupling device of fig. 1; and

Fig. 3 shows a suitable way of arranging the antenna coupling device of fig. 1 on a printed circuit board.

In the preferred embodiment the antenna coupling device is used inside a vehicle or a building for coupling inductively a hand- portable telephone to an external antenna, which hand-portable telephone is not provided with means for a galvanical connection to an external antenna.

The hand-portable telephone shown in fig. 1 is provided with two antenna means, one of which is a helical antenna, consti¬ tuting a first antenna, and the other of which is and extend¬ able and retractable antenna rod. The helical antenna is moulded into a substantially cylindrical element 2 projecting outward from the chassis 1 of the telephone. Fig. 1 shows only an upper knob 3 of the antenna rod which is retracted through the helical antenna.

Further, an antenna coupling device according to the invention is shown. This includes a loop antenna 4 with an impedance matching unit arranged on a printed circuit board within a housing 6, which is made of a non-conductive shell with a sub- stantially cylindrical opening on the underside adapted for the substantially cylindrical element 2 of the helical antenna. The loop antenna 4, which is fixed in the housing 6, coaxially with the substantially cylindrical opening, has an inner diameter which is slightly greater than the diameter of the substan- tially cylindrical element 2.

One end of a coaxial cable 5, which extends substantially perpendicularly to the axis of the loop antenna 4, is coupled to the loop antenna 4 and the impedance matching unit of the antenna coupling device. The other end of the coaxial cable 5 is connected to a second antenna, not shown, mounted externally on the body of the vehicle, on the outside of the building or similarly. Preferably, the second antenna is a monopole antenna or a colinear antenna.

The principle plan of fig. 2 shows the electrically active components of the antenna coupling device of fig. 1. The loop antenna is a conductive loop with two poles (ends) 15, 16 with an optional number of turns, preferably approximately one turn.

Further, an impedance matching unit 7 is shown, which is a quadripole with passive reactive elements for transformation of impedance. The poles 15, 16 of the loop antenna are connected to the one pair of the poles of the impedance matching unit 7 and the coaxial cable 5 to the other pair.

Fig. 3 shows the printed circuit board 8, on which the loop antenna 4 (fig. 1 and 2) and the impedance matching unit 7 (fig. 2) are arranged. The printed circuit board has a substan- tially rectangular shape and is provided with a circular hole 14, intended for the first antenna and being offset from the center to the one short side of the rectangle. The printed circuit board is somewhat wider around the hole 14.

The printed circuit board 8 is provided with a first conductor pattern 9 adjacent to the main portion of the periphery of the hole 14, the width of the first conductor pattern 9 being substantially even and considerably smaller than its total length. This conductor pattern constitutes the loop antenna.

At the one end of the first conductor pattern 9 there is provided a hole 11 for connecting the shield of the coaxial cable 5 (fig. 1 and 2) . A second conductor pattern is arranged between the ends of the first conductor pattern 9 and is provi¬ ded with a hole 10 for connecting the center conductor of the coaxial cable. In the two spaces 12, 13 present between the ends of the conductor patterns reactive components (not shown) forming the impedance matching unit are mounted and connected.

The printed circuit board 8 is fixed inside the housing 6 (fig. 1) and its right part is used for fixing the cable.