Antenna arrangement and a radio apparatus including the antenna arrangement

An antenna arrangement comprises a ground plane (14) and a planar antenna element (30) mounted spaced from and parallel to the ground plane. An open-ended slot (16) is provided in the ground plane (14), the slot being coextensive with an edge portion of the ground plane and having a first end (18) opening into the edge portion of the ground plane and a second closed end (20). An antenna feed (22) is coupled to the slot at a location intermediate the first and second ends. The planar antenna element is connected by an electrically conductive wall (28) to the edge portion of the ground plane, the wall (28) being coextensive with the slot (16). The combination of the slot shape, slot location and the wall serves to increase the bandwidth of the antenna arrangement.

The present invention relates to an antenna arrangement and to a radio apparatus including the antenna arrangement. The present invention has particular, but not exclusive application, to multiband cellular telephones.

Modern compact cellular telephones normally include an internal antenna of which a planar inverted-F antenna (PIFA) is an example. Typically a PIFA comprises a ground plane and a planar antenna element mounted spaced from, and parallel to, the ground plane. The planar antenna element has a ground connection coupled to the ground plane and a separate RF feed connection coupled to a RF output/input coupling of an RF section of the cellular telephone. An open ended slot may be provided in the planar antenna element at a location intermediate the ground and RF feed connections to enable the antenna arrangement to have two resonances in order to facilitate dual band operation.

An object of the present invention is to widen the operating bandwidth of a planar antenna arrangement.

According to a first aspect of the present invention there is provided an antenna arrangement comprising a ground plane and a planar antenna element mounted spaced from and parallel to the ground plane, wherein a slot is provided in the ground plane, the slot being coextensive with an edge portion of the ground plane and having a first end opening into the edge portion of the ground plane and a second closed end, an antenna feed coupled to the slot at a location intermediate the first and second ends, and wherein the planar antenna element is connected by an electrically conductive wall to the edge portion of the ground plane, the wall being co-extensive with the slot.

According to a second aspect of the present invention there is provided a radio apparatus having an antenna arrangement comprising a ground plane and a planar antenna element mounted spaced from and parallel to the ground plane, wherein a slot is provided in the ground plane, the slot being coextensive with an edge portion of the ground plane and having a first end opening into the edge portion of the ground plane and a second closed end, an antenna feed coupled to the slot at a location intermediate the first and second ends, wherein the planar antenna element is connected by an electrically conductive wall to the edge portion of the ground plane, the wall being co-extensive with the slot and wherein a RF module is provided, the RF module having an output/input coupling for connection to the antenna feed.

If desired the slot in the ground plane may be a L-shaped parallel sided slot having a shorter portion and a longer portion with the first end is provided in the shorter portion. Alternatively the slot may have a shaped portion which converges from its first end towards its second end. The open first end of the slot may be located in a region of the edge of the ground plane away from the marginal portions of the ground plane. For convenience the edge portion of the ground plane is straight.

In an embodiment of the present invention the length of the slot between its first and second ends is substantially a quarter of a wavelength of a frequency in a frequency band of interest.

The slot may be tunable by antenna tuning means.

The ground plane may comprise a printed circuit board (PCB) on which the RF module comprising transmitting/receiving circuitry is provided.

In another embodiment of the present invention a second wall is connected to the ground plane and extends from the opposite side of the first end of the slot.

By providing the electrically conductive wall, for example a metallic wall, in a close vicinity of the open ended slot antenna, the wall substantially increases the bandwidth of the slot antenna to an extent that a larger total bandwidth coverage can be achieved than with a conventional slot antenna.

By providing the slot in the ground plane a simpler and more practical antenna tuning circuit can be used if it is desired to cover a very wide band with frequency tuning.

Specific shaping of the slot has been found to increase the bandwidth potential of the antenna arrangement made in accordance with the present invention. Additionally the antenna arrangement may be implemented at various locations of a mobile phone PCB other than only at the corners of or at the top or bottom of the PCB. A more simplified tuning circuit may be used.

The wall does not operate similarly to and is not designed the same way as a conventional resonant parasitic element.

If desired the wall may be implemented by integrating it as part of a mobile telephone mechanics/chassis.

The antenna arrangement made in accordance with the present invention provides a good compromise between bandwidth and Specific Absorption Rate (SAR). This is attributed to the co-design of the slot shape, slot location and the electrically conductive wall.

In the drawings the same reference numerals having been used to indicate the same features.

FIG. 1shows a simplified view of a radio apparatus10, for example a cellular telephone, comprising a housing12containing a ground plane14in the form of a printed circuit board (PCB). For the sake of clarity normal features of a cellular telephone, such as a keypad, display screen, microphone port and earphone port, have not been shown. A quarter wavelength open ended L-shaped antenna slot16is provided in the ground plane14. The antenna slot16comprises a shorter portion16S and a longer portion16L. The longer portion16L extends substantially parallel to, but is spaced from, an edge portion of the ground plane. The shorter portion16S has an opening18in the edge of the ground plane located in the central region of the edge. The longer portion16L has a closed end20. An antenna feed22is provided across the longer portion16L of the slot16at a location spaced from the closed end20. A RF module24comprising transmitting and receiving circuitry (not shown) is mounted on the ground plane and is coupled to the feed point22by way of a coaxial line or stripline26. A planar antenna element30is mounted above the slot16in the ground plane14by means of an electrically conductive, for example metallic, wall28attached to the edge portion of the ground plane14adjacent the slot16. For convenience of manufacture the edge portion of the ground plane is straight but it could be curved.

FIG. 2illustrates the ground plane14having a quarter wavelength L-shaped open ended slot16and the wall28having a length which is greater than that of the longer portion16L of the slot. In operation, RF energy supplied to the feed point22causes a current to flow around the closed end20of the slot16. This will produce a first resonance. The wall28removes a constraint on the current flowing around the closed end20of the slot16and the current flowing in the wall provides a second resonance.

FIG. 3illustrates a variant ofFIG. 2in that the shape of the slot has been given a shaped design different from the L-shape shown inFIG. 2. In the example shown inFIG. 3the slot converges continuously from its open end18towards the feed point28and thereafter it is parallel to the closed end20. The slot has a length equal to a quarter wave of the frequency of interest. As will be described with reference toFIGS. 4 and 5shaping of the slot16enhances the bandwidth of the antenna arrangement.

FIGS. 4 and 5show the performance of the L-shaped slot as a continuous line and that of the shaped slot as a broken line. In the case ofFIG. 4which is a plot of bandwidth potential in percent against frequency between 0 Ghz and 10 GHz the frequency response of the shaped slot extends to a higher frequency and has a higher percentage bandwidth overall. In the case of the Smith chart show inFIG. 5the impedance locus is clearly smaller with the shaped slot design as compared to the L-shaped version.

FIG. 6illustrates an embodiment of a sub-assembly32comprising the wall28and the planar antenna element30. The planar antenna element30is of rectangular shape having a length of 18 mm and a width of 5.5 mm. The wall is 18 mm long and has a height of 6 mm. A narrow in-turned edge34of a width 1.9 mm provides a means for mounting the sub-assembly32on the ground plane14.

FIG. 7shows the sub-assembly32mounted along the top edge of the ground plane14and the open end18of the L-shaped slot16being located in the central portion of the ground plane14. In the illustrated example the overall length of the slot16is 20 mm with the shorter portion16S being 3 mm and the longer the portion16L being 17 mm.

FIG. 8illustrates an antenna arrangement which is tunable by components34coupled to the antenna feed. Typically the tuning is effected by means of Micro Electromagnetic Systems (MEMS) switches controlled by digital switches36causing devices such as capacitances and/or inductances to be connected to the antenna feed. Each of the digital switches36is connected to a respective MEMS device by a respective dc line38located in an elongate slot in the ground plane. Metal bars ground the elongate slot at intervals to prevent it from acting as an antenna slot.

FIG. 9illustrates a test piece for investigating the bandwidth enhancement using one or two walls along the edge of the ground plane14. As shown the slot16is a L-shaped slot and a first or lower wall28extends alongside the longer portion16L of the slot. A second or upper wall40extends from the opposite side of the open end18of the slot16in a direction away from the open end18. The dimensions of the upper wall40corresponds to those of the wall28, namely, 20 mm long and 5 mm high. The ground plane14comprises a rectangular 0.8 mm thick PCB: R04003C having a length of 100 mm and a width of 40 mm. The S parameter Smith chart shown inFIG. 10shows the impedance characteristics over a frequency range 1.5 GHz to 6.5 GHz and the S11 chart shown inFIG. 11show the different impedance characteristics over a frequency range of 2 GHz to 7 GHz. The respective lines are as follows: dotted—no walls; continuous—one lower wall28; broken—one upper wall40; and chain/dot—both walls28,40. The results show that the lower wall28increases the antenna bandwidth and no additional advantage is achieved by adding the upper wall40.

FinallyFIGS. 12 and 13relate to the results of an investigation into the location of the slot16. In this investigation the slot was 14 mm long and extended orthogonally to the edge of the ground plane14. Different positions of the slot in the vertical Z plane with reference to a reference point were considered.FIG. 13is a graph of fractional bandwidth in percent plotted against slot location, 0 representing the reference point. It will be deduced fromFIG. 13that in order to obtain an acceptable fractional bandwidth the open end18of the slot16should be located in a region of the edge portion of the ground plane14away from the marginal portions of the ground plane, that is not near the corners.

In the present specification and claims the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. Further, the word “comprising” does not exclude the presence of other elements or steps than those listed. The use of any reference signs placed between parentheses in the claims shall not be construed as limiting the scope of the claims.