Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2000-01-05
2001-06-26
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S702000, C343S718000, C343S725000
Reexamination Certificate
active
06252552
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to planar antenna structures. In particular the invention relates to a planar structure combining two different antenna architectures, thus operating at two clearly distinct frequencies. In addition, the invention relates to the feed arrangement of such an antenna and to a radio apparatus employing such an antenna.
2. Description of the Related Art
FIG. 1
shows a known basic design
100
of a planar inverted-F antenna (PIFA) comprising a planar electrically conductive radiating element
101
, electrically conductive ground plane
102
parallel to said radiating element, and, interconnecting 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 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 stability. Various structures have been proposed to solve this problem. European Patent document EP 484,454 discloses a PIFA structure according to
FIG. 2
wherein a radiating element
201
, ground plane
202
and a ground contact
203
interconnecting 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 stable, but the dielectric body block makes it rather heavy. Furthermore, the dielectric body decreases the impedance bandwidth of the antenna and degrades the radiation efficiency compared with an air-insulated PIFA.
A PIFA radiating element does not have to 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 slot
302
which forms a sort of strip in that portion of the radiating element which is farthest away from the feedpoint
303
and ground contact
304
. The purpose of the slot usually is to increase the electrical length of the antenna and thus affect the antenna's resonant frequency.
All the PIFA structures described above are designed such that they have a certain resonant frequency as well as an operating frequency band centering round said resonant frequency. In some cases, however, it is preferable that the antenna of a radio apparatus has two different resonant frequencies.
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 rectangular first radiating element
401
and a second radiating element
402
surrounding said first radiating element from two sides. The first radiating element has a feedpoint
403
and ground contact
404
of its own, and the second radiating element has those of its own,
405
and
406
. In
FIG. 4
b
the antenna comprises a continuous radiating element
410
which is divided into two branches by a slot
411
. The feedpoint
412
is located near the inner end of the slot
413
, i.e. the end that does not end at the edge of the radiating element, so that the branches have different directions from the feedpoint 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 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 in the figure by the dashed line, which supports the radiating element. This structure provides a somewhat wider antenna impedance band, i.e. frequency range around a particular resonant 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 indicates the amount of radiation absorbed by the user, becomes rather high, especially in the higher frequency band.
Finnish patent application FI-982366 discloses a PIFA radiating element
600
according to
FIG. 6
, in which said radiating element is divided by a non-conductive slot
601
-
602
-
603
which divides the planar radiating element into a first branch and second branch. The feedpoint
604
and ground contact
605
are located close to the inner end of the slot. So, this structure, too, has two adjacent PIFA radiating element branches on one and the same planar surface and in the vicinity of one and the same ground plane
606
. The patent application also discloses that the outer end of the branch corresponding to the higher operating frequency is located within the border line of the radiating element, surrounded by the first branch so that the SAR value will be smaller than in the arrangement of FIG.
5
.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a planar dual-frequency antenna structure which is easy to manufacture and assemble and can be easily dimensioned for the desired operating frequencies. Another object of the invention is that the impedance bandwidth of the antenna be relatively great and that its feed impedance be selectable in a desired manner. A further object of the invention is to provide a radio apparatus utilizing the antenna structure described above.
The objects of the invention are achieved by combining in a single structure a PIFA radiating element and a slotted radiating element. The objects concerning the impedance bandwidth and feed impedance are achieved by providing the combined radiating element with a capacitive feed from the antenna port of the radio apparatus.
The antenna structure according to the invention is characterized in that it has a planar radiating element which comprises a feedpoint and a ground contact near the first end of a dividing slot so that the electrical length of the conductive area divided by the slot, measured at the feedpoint, equals a quarter of the wavelength at the first operating frequency, and the electrical length of the slot equals a quarter of the wavelength at the second operating frequency.
The radio apparatus according to the invention is characterized in that a planar radiating element in its antenna structure comprises, near the first end of a certain slot a feedpoint coupled to the antenna port of the radio apparatus and a ground contact coupled to the ground potential of the radio apparatus, so that the electrical length of the conductive area divided by the slot, measured at the feedpoint, equals a quarter of the wavelength at the first operating frequency, and the electrical length of the slot equals a quarter of the wavelength at the second operating frequency.
In the PIFA structures according to the prior art, two operating frequencies are realized by two PIFA branch
Isohätälä Anne
Kivelä Sauli
Mikkola Jyrki
Tarvas Suvi
Chen Shih-Chao
Darby & Darby
Filtronic LK Oy
Wong Don
LandOfFree
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