Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
1999-10-29
2002-04-02
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S702000
Reexamination Certificate
active
06366243
ABSTRACT:
The invention relates in general to antenna structures in radio apparatuses. In particular the invention relates to a planar inverted-F antenna (PIFA) structure that has two resonating frequencies.
FIG. 1
shows a known basic model of a planar inverted-F antenna
100
comprising a planar electrically conductive radiating element
101
, electrically conductive ground plane
102
parallel to said radiating element, and, connecting these two, a ground contact
103
which is substantially perpendicular to the radiating element and ground plane. The structure further includes a feed electrode
104
which also is substantially perpendicular to the radiating element and ground plane and which can be coupled to an antenna port (not shown) of a radio apparatus. In the structure of
FIG. 1
the radiating element
101
, ground contact
103
and the feed electrode
104
are usually manufactured by cutting a thin metal sheet into a suitable rectangular shape which has got two protrusions bent to a right angle. The ground plane
102
may be composed of a metallized area on the surface of a printed circuit board so that the ground contact
103
and feed electrode are easily connected to holes on the printed circuit board. The electrical characteristics of the antenna
100
are affected in general by the dimensions of its elements and in particular by the size of the radiating element
101
and its distance from the ground plane
102
.
A disadvantage of the antenna structure depicted in
FIG. 1
is its poor mechanical sturdiness. Various solutions have been proposed to this problem. European Patent document No. 484,454 discloses a PIFA structure according to
FIG. 2
wherein a radiating element
201
, ground plane
202
and a ground contact
203
connecting these two are realized as metal platings on surfaces of a solid dielectric body
204
. The antenna is fed through a coupling element
205
which does not touch the radiating element
201
. An electromagnetic coupling exists between the coupling element
205
and radiating element
201
, and the coupling element extends over the edge of the dielectric body
204
to a point that can be coupled to the antenna port of a radio apparatus. The structure is mechanically sturdy, but the dielectric body block makes it rather heavy. Furthermore, the dielectric body makes the impedance bandwidth of the antenna narrower and degrades the radiation efficiency as compared to an air-insulated PIFA structure.
The radiating element of a planar inverted-F antenna need not be a simple rectangle as in
FIGS. 1 and 2
.
FIG. 3
shows a known PIFA radiating element
301
design. The rectangular shape is broken by a gap
302
which forms a sort of strip in that portion of the radiating element which is farthest away from the feed point
303
and ground contact
304
. The purpose of the gap usually is to increase the electrical length of the antenna and thus affect the antenna's resonating frequency.
All the PIFA structures described above are designed such that they have a certain resonating frequency as well as an operating frequency band centering round said resonating frequency. In some cases, however, it is preferable that the antenna of a radio apparatus have two different resonating frequencies. An example of such a case is a cellular radio system terminal which has to be capable of operating in two different cellular radio systems or in two different frequency ranges of a single cellular radio system. The difference of the frequencies may be considerable as at the moment of writing this patent application the frequency areas of currently existing cellular radio systems range from about 400 MHz to about 1900 MHz, and it is probable that even higher frequencies will be taken into use in the future.
FIGS. 4
a
and
4
b
show dual-frequency PIFA radiating elements known from the publication “Dual-Frequency Planar Inverted-F Antenna” by Z.D. Liu P.S. Hall, D. Wake, IEEE Transactions on Antennas and Propagation, Vol. 45, No. 10, October 1997, pp. 1451-1457. In
FIG. 4
a
the antenna comprises a rectangle-shaped first radiating element
401
and a second radiating element
402
surrounding said first radiating element from two sides. The first radiating element has got a feed point
403
and ground contact
404
of its own, and the second radiating element has got those of its own,
405
and
406
. In
FIG. 4
b
the antenna comprises a continuous radiating element
410
which is split into two branches by a gap
411
. The feed point
412
is located near the inner end of the gap
411
so that it can be said that the branches have different directions from the feed point on. Both branches have electrical lengths of their own which differ from each other considerably. The ground contacts
413
are located near the edge of the structure.
It is further known a dual-frequency PIFA radiating element
501
according to
FIG. 5
which has got two branches in the same manner as the radiating element in
FIG. 4
b
. In
FIG. 5
, the outermost ends of both branches extend to the edge of the printed circuit board, depicted by the broken line, which supports the radiating element. This structure provides a somewhat wider antenna impedance band, i.e. frequency range around a particular resonating frequency in which the antenna impedance matching to the antenna port of the radio apparatus is good. At the same time, however, the SAR value, which represents the amount of radiation absorbed by the user, becomes rather high, especially in the higher frequency band.
An object of the present invention is to provide a planar antenna with at least two resonating frequencies. Another object of the present invention is that the planar antenna according to it can be tuned in a versatile manner. Yet another object of the invention is that the antenna according to it has a relatively low SAR value.
These and other objects of the invention are achieved by a planar antenna structure which has an outer branch and an inner branch such that the outermost end of the inner branch is for the most part surrounded by the outer branch.
The planar antenna according to the invention comprises a planar radiating element formed of a conductive area confined within a substantially continuous border line, said conductive area being split by a non-conductive gap which divides the planar radiating element into a first branch and second branch such that both the first and the second branch have an outermost end, and which has a head end at said sub-stantially continuous border line and a tail end within the conductive area. The planar antenna according to the invention is characterized in that at its head end the gap has a certain first direction and at another point of the gap it has a certain second direction which differs more than 90 degrees from the first direction when the directions are defined from the head end to the tail end of the gap, whereby the outermost end of the second branch, confined by the gap, is located within the continues border line, surrounded by the first branch.
The invention is also directed to a radio apparatus. It is characterized in that it comprises a planar radiating element like the one described above and a ground plane which is substantially parallel to said radiating element and located with respect to the planar radiating element such that in the typical operating position of the radio apparatus it is between the planar radiating element and the user of the radio apparatus.
The planar antenna according to the invention comprises a planar radiating element split into at least two branches by a gap. The electrical lengths of the branches are chosen such that the first branch efficiently operates as an antenna at a first operating frequency of the structure and, respectively, the second branch efficiently operates as an antenna at a second operating frequency of the structure. An advantageous method is to choose the electrical lengths such that the electrical length of each branch corresponds to a quarter of a wavelength at the desired operating frequency. The feed point
Antila Kimmo
Isohätälä Anne
Kivelä Sauli
Mikkola Jyrki
Tarvas Suvi
Darby & Darby
Dinh Trinh Vo
Filtronic Lk Oy
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