Field of the invention
[0001] The invention relates to the field of antenna devices and in particular to an internal
multi-band antenna device with a parasitic element for use in communications devices.
The invention also relates to a radio communication device comprising such an antenna
device.
Background of the invention
[0002] In the art, parasitic elements have been used with multi-band antennas. The bandwidth
of an antenna is the range of frequencies over which it is effective, usually centred
around the resonance frequency. The length and shape of the parasitic element determine
its capabilities, mostly regarding the different frequencies for which it is to be
used. Typically, the parasitic elements are of elongated shape, thus having two long
sides and two short sides with a grounding point at one of the short sides. Such a
parasitic element is known from e.g.
US 6456250. Typically, in prior art, the parasitic element is arranged in a parallel relationship
to at least a first planar driven radiating element, denoted CP1 in
US 6456250 and has a grounding portion or post located at one end of the parasitic element.
Together the planar driven radiating element and the parasitic element form an electrical
capacitor, as there exists a capacitive or parasitic coupling between the planar driven
radiating element and the parasitic element.
[0003] The mechanical length of the parasitic element is closely correlated to the operating
wavelength or frequency of the parasitic element.
[0004] It has not been possible to affect the impedance match of the resonance for the different
frequencies, i.e. the Return Loss level, apart from utilizing a matching network,
i.e. lumped or distributed components (inductors and/or capacitors). Further in prior
art, the parasitic element had to be arranged close to and in parallel to the planar
driven radiating element in order to create a capacitive or parasitic coupling between
the planar driven radiating element and the parasitic element.
Summary of the invention
[0005] It is an objective of the invention to provide an antenna device having a parasitic
element for which it is possible to easily modify and influence the impedance match
of the resonance for the different frequencies, i.e. the Return Loss level.
[0006] It is further an objective of the invention to provide an antenna device having a
parasitic element that need not be arranged in parallel to the planar driven radiating
element in order to create a capacitive or parasitic coupling between the planar driven
radiating element and the parasitic element.
[0007] In accordance with the invention, an antenna device is provided which is suitable
for a communication device operable in at least two frequency intervals or frequency
bands. The antenna device comprises a generally planar driven radiating element having
a feeding point connectable to a feed device of the communication device. The antenna
device further comprises a generally planar parasitic element having a grounding portion
connectable to a ground device of the communication device. The driven radiating element
and the parasitic element are essentially coplanar and separated by a gap. The parasitic
element preferably has a general elongated shape, e.g. rectangular. The parasitic
element is connected to a grounding point, using a grounding pin or grounding portion.
The grounding portion of said parasitic element is located at a point along the elongated
parasitic element so as to virtually divide said parasitic element in a first and
a second portion with respective lengths L1 and L2, wherein L1 is longer than L2.
[0008] As the grounding point in a printed wire board (PWB) is located close to the feeding
point in the same, where the current density is relatively high, some current will
be lead up through the grounding pin to the parasitic element. At the grounding point
of the parasitic element, that is where the grounding pin is connected to the parasitic
element, some current will go into the L1 part of the parasitic element and some current
will go into the L2 part of the parasitic element. As L2 is relatively short, current
will be reflected at the end of L2. The current going through L2 will determine the
impedance of the whole parasitic antenna. This impedance will directly affect the
matching of the impedance of the resonance, i.e. the Return Loss level. Depending
on in which phase this reflected current is, when added to the current from the grounding
point of the parasitic element, the impedance is determined. The phase of the reflected
current in the grounding point of the parasitic element is directly dependant on the
length L2. Therefore, the length L2 determines the impedance of the parasitic element
and thus directly determines the impedance matching of the resonance for which frequency
the parasitic element is responsive or active. An impedance of 50 ohms will cause
the resonance depth or the Return Loss level to be infinite, provided that an interface
of 50 ohms is employed. If an interface with other impedance/resistance is employed,
the impedance matching will have to be done around that impedance accordingly. Hereinafter,
the description will be focused on the 50 ohms impedance matching by way of example.
It is therefore possible to design a parasitic element that is effective for a certain
frequency band, due to L1, and that has optimal resonance depth or Return Loss level
for that particular frequency band, due to L2.
Brief description of the drawings
[0009]
Figure 1 illustrates an overview of an antenna device in accordance with the invention.
Figure 2 illustrates schematically a frequency diagram for the antenna device of figure
1.
Figures 3a and 3b illustrate other embodiments of antenna devices in accordance with
the invention.
Figures 4a, 4b and 4c illustrate other embodiments of a parasitic element used in
an antenna device in accordance with the invention.
Detailed description of the invention
[0010] The invention will now be described with reference to the accompanying drawings.
Figure 1 illustrates an antenna device in accordance with an embodiment of the invention.
The antenna device 1 comprises a first radiating element 2, which is an active element,
also known as a driven element, and in the following denoted driven radiating element
2. The driven radiating element 2 is made of a suitable electrically conductive material,
such as a metal sheet, or a conductive flex film or the like. The driven radiating
element 2 may have any suitable shape, for example square, rectangular, thin strip,
circular, elliptical or triangular.
[0011] The driven radiating element 2 is connected to a feed portion electrically connectable
to radio frequency (RF) circuitry of an underlying printed wire board (PWB) 4 of a
communication device in which the antenna device 1 is to be used. For example, the
feed portion could be a contact pin 5 having an extension essentially perpendicular
to the plane of the driven radiating element 2. In the following a contact pin 5 is
used as an exemplary feed means, although it is noted that other feeding means could
be used. The contact pin 5 functions as a feeding point P of the driven radiating
element 2. The driven radiating element 2 is thus fed via the contact pin 5. In the
figure, a PIFA antenna arrangement is shown. As such, the driven radiating element
2 comprises a grounding portion or grounding pin 7 having an extension essentially
perpendicular to the plane of the driven radiating element 2 and the PWB.
[0012] The PWB 4 of the communication device also functions as a ground plane for the internal
antenna device 1.
[0013] The antenna device 1 further comprises a parasitic element 3. The parasitic element
3 is, like the driven radiating element 2, made of a suitable electrically conductive
material. The parasitic element 3 has a general elongated shape, e.g. rectangular
and is connected to a grounding point G in the PWB 4, using a grounding pin 6. The
grounding pin 6 of said parasitic element is located at a point G' along the parasitic
element so as to virtually divide said parasitic element in two portions with respective
lengths L1 and L2, wherein L1 is longer than L2.
[0014] The grounding point G in the PWB 4 is preferably located close to the feeding point
P, they are typically 0.5-3 cm apart. As the grounding point preferably is located
close to the feeding point, where the current density is relatively high, some current
will be conducted or directed up through the grounding pin 6 to the parasitic element
3. At the grounding point G' of the parasitic element 3, that is, where the grounding
pin 6 is connected to the parasitic element 3, some current will go into the L1 part
of the parasitic element 3 and some current will go into the L2 part of the parasitic
element 3. As L2 is relatively short, current will be reflected at the end of L2.
The current going through L2 will determine the impedance of the whole parasitic element.
This impedance will directly affect the impedance matching of the resonance, i.e.
the Return Loss level. Depending on in which phase this reflected current is, when
added to the current from the grounding point G', the impedance is determined. The
phase of the reflected current at the grounding point of the parasitic element G'
is directly dependant on the length L2. Therefore, the length L2 determines the impedance
of the parasitic element and thus directly determines the impedance matching of the
resonance, i.e. the Return Loss level, for the operating frequency of the parasitic
element. An impedance of 50 ohms will cause the resonance depth or the Return Loss
level to be infinite, provided that an interface of 50 ohm is employed. An impedance
around 50s ohms is thus desirable. It is therefore possible to design a parasitic
element that is effective for a certain frequency band, due to L1, and that has optimal
resonance depth for that particular frequency band, due to L2. By placing the grounding
point G in locations on the PWB 4 close to the feeding point P where the current density
is relatively high, it is possible to design a parasitic element with optimal resonance
depth by adjusting the length of L2 accordingly. It is noted that it is the high current
density at the grounding point G of the PWB 4 that is of importance. Thus the grounding
point G and the feeding point P need not be close if there are other locations on
the PWB 4 with high current density.
[0015] There is no need for the parasitic element 3 itself to be close to the driven radiating
element 2 as it is the current density in the PWB 4 at the grounding point G together
with the length of L2 that determine the impedance of the parasitic element 3. In
more detail, the length L2 determines the phase of the reflected current in the grounding
point G' of the parasitic element 3. It is the phase of the reflected current when
added to the current from the grounding point G' of the parasitic element 3 that determines
the impedance of the parasitic element 3. The amplitude of the current is of less
importance.
[0016] The resonance frequencies of the parasitic element 3 are dependent the length L1
of the parasitic element 3. In other words, the length L1 of a parasitic element according
to the invention corresponds to the total length of a parasitic element according
to prior art. As such, a longer L1 will make the parasitic element 3 effective for
a lower frequency band and a shorter L1 will make the parasitic element 3 effective
for a higher frequency band accordingly.
[0017] The driven radiating element 2 and the parasitic element 3 are supported by a frame
made of an electrically non-conductive material, such as plastic (not shown). By means
of the frame the radiating elements 2, 3 are easily positioned essentially parallel
to the PWB of the communication device. It is however noted that the parasitic element
3 and the driven radiating element 2 need not be in parallel according to the present
invention.
[0018] The antenna device is connected to a communication device, shown in figure 1 by a
dashed line.
[0019] The antenna device 1 is a multi-band antenna. In a way that is know per se, the driven
radiating element 2 and the parasitic element 3 can be dimensioned in order to obtain
any desired resonance frequencies. The antenna device 1 may for example be dimensioned
so as to produce a resonance at the lower bands with central frequencies substantially
at 850 MHz and 900 MHz and/or to produce a resonance at the higher frequency bands
with central frequencies substantially at 1800 MHz, 1900 MHz or 2100 MHz, making it
suitable for use in a multi-band communication device adapted for the GSM850, GSM900
and/or GSM1800/GSM1900/WCDMA2100 bands.
[0020] Figure 2 illustrates schematically a frequency diagram for the antenna device of
figure 1. The different values "b" in the diagram denote different lengths of L2.
As is shown in the diagram, the length of L2 greatly affects the Return Loss level,
for the operating frequency of the parasitic element.
[0021] Figure 3a illustrates a parasitic element in accordance with the present invention
when employed to a monopole antenna arrangement.
[0022] Figure 3b illustrates a parasitic element in accordance with the present invention
when employed to a patch antenna arrangement.
[0023] Figures 4a, 4b and 4c illustrate an embodiment of the parasitic element in accordance
with the present invention wherein the parasitic element is curved. Although the figures
illustrates just one curve, it is to be understood that the parasitic element could
be curved in other ways, e.g. in the shape of an S. The different figures 4a, 4b,
4c illustrate a curved parasitic element in accordance with the present invention
when employed to a PIFA, monopole and patch antenna arrangement respectively.
[0024] Inside the communication device, having the antenna arrangement as described above,
there is provided the printed wire board (PWB) 4 with a size essentially corresponding
to the size of the communication device. On the PWB 4 there are mounted electronic
circuits etc. (not shown) for the operation of the communication device. These circuits
are generally not part of the present invention and will not be discussed further.
However, the antenna device is to be connected to the PWB 4 and this comprises radio
frequency (RF) circuitry for operation of the antenna device. In particular, the parasitic
element 3 is connected to the grounding portion 6 extending essentially perpendicular
thereto. The grounding portion 6 is connected to a ground device of the underlying
PWB 4. The driven radiating element 2 is electrically connected to a feed device of
the PWB 4 by means of the contact pin 5.
[0025] It should be understood that also the width of the parasitic element 3 is of importance
and affects the capabilities of the parasitic element 3. A lesser width of the parasitic
element 3 would lead to a decrease of the overall length of the parasitic element
3. Accordingly, a greater width of the parasitic element 3 would lead to an increase
of the overall length of the parasitic element 3. In other words, it is the circumference
of L1 and L2 that determines their respective characteristics.
[0026] The invention has been described by means of different embodiments thereof. It is
to be noted that the invention can be modified in a number of ways. For example, the
size or length of the parasitic element 3 can be varied. Also other shapes besides
the elongated shape described here could be used without departing from the scope
of the invention. The grounding point and feeding point can be located differently
than shown in the figures. Further yet, the term radiating element should be understood
to cover any antenna element adapted to receive and/or transmit electromagnetic waves.
[0027] It is also to be understood that the invention is applicable to any communication
device having an internal multi-band antenna arrangement, both handsets and more stationary
communication devices. It is also to be understood that the impedance matching around
50 ohm is due to the impedance of the interface, which in this case would be 50 ohm.
A different interface with another impedance would require the impedance matching
of the parasitic element to be made around that impedance.
[0028] Although an antenna device for a portable radio communication device has been described
with reference to its use in a mobile phone, it will be appreciated that the inventive
idea is also applicable to other portable radio communication devices, also devices
that are portable but primarily intended for stationary use. Examples thereof could
be small clocks, such as travel alarm clocks, TV receivers, or game consoles. Yet
a possible application of the antenna device according to the invention is in personal
digital assistants (PDAs), MP3 and CD players, FM radio receivers, and laptop computers.
A further application is in cars. Thus, the term portable radio communication device
should be construed in a broad sense.
1. An antenna device for a communication device operable in at least two frequency intervals
said antenna device comprising:
- a generally planar electrically conductive driven radiating element (2) having a
feeding portion (5) connectable to a feeding point (P) of a printed wire board (4)
of said communication device and
- a generally planar elongated parasitic element (3) having a ground portion (6) connectable
to a grounding point (G) of the printed wire board (4) of said communication device,
wherein said driven radiating element (2) and said parasitic element (3) are essentially
coplanar and separated by a gap,
characterised in that
said grounding portion (6) of said parasitic element (3) is positioned at a grounding
point (G') along said parasitic element (3) so as to virtually divide said parasitic
element (3) in a first and a second portion with respective first length (L1) and
second length (L2), wherein the first portion having a first length (L1) is longer
than the second portion having a second length (L2).
2. The antenna device according to claim 1 characterised in that the position of said grounding point (G') along said parasitic element (3) virtually
dividing said parasitic element (3) in two portions is chosen so that the length of
first portion (L1) is matched so as to be responsive to a predetermined frequency
interval.
3. The antenna device according to claim 2 characterised in that the length of the second portion (L2) of the parasitic element (3) is chosen to obtain
an impedance close to 50 ohms for the parasitic element (3) for the predetermined
frequency interval determined by the length of first portion (L1).
4. The antenna device according to any of claims 1-3 characterised in that the length of the first portion (L1) of the parasitic element (3) is between 15-25
mm.
5. The antenna device according to any of claims 1-4 characterised in that the length of the second portion (L2) of the parasitic element (3) is between 1-14
mm.
6. The antenna device according to any of claims 1-5 characterised in that the width of the parasitic element is between 1-5 mm.
7. The antenna device according to any of claims 1-6 characterised in that the generally planar elongated parasitic element has a substantially rectangular
shape.
8. The antenna device according to any of claims 1-6 characterised in that the generally planar elongated parasitic element is curved.
9. The antenna device according to any of claims 1-8 characterised in that the antenna device is any of a PIFA antenna, a monopole antenna or a patch antenna.
10. A mobile communication device
characterised in that said mobile communication device comprises an antenna device comprising:
- a generally planar electrically conductive driven radiating element (2) having a
feeding portion (5) connected to a feeding point (P) of a printed circuit board (4)
of said communication device and
- a generally planar elongated parasitic element (3) having a ground portion (6) connected
to a grounding point (G) of the printed circuit board (4) of said communication device,
wherein said driven radiating element (2) and said parasitic element (3) are essentially
coplanar and separated by a gap,
wherein said grounding portion (6) of said parasitic element (3) is positioned at
a grounding point (G') along said parasitic element (3) so as to virtually divide
said parasitic element (3) into a first and a second portion respectively with a first
length (L1) and a second length (L2), wherein the first portion having a first length
(L1) is longer than the second portion having a second length (L2).