Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2000-05-09
2001-10-02
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S846000
Reexamination Certificate
active
06297776
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to compact antenna systems, in particular to antenna constructions operating on a plurality of frequency bands.
BACKGROUND OF THE INVENTION
A conventional microstrip antenna comprises a ground plane and a radiator isolated therefrom by a dielectric layer. The resonance frequency of the microstrip antenna depends on the dimensions of the radiator and on the distances between the radiator and ground plane. Microstrip antenna constructions are described in general e.g. in the “Handbook of Microstrip Antennas” by J. R. James and P. S. Hall (Eds.), Vol 1, Peter Peregrinus Ltd, London 1989 and in “Analysis, Design, and Measurement of Small and Low-Profile Antennas” by K. Hirasawa and M. Haneishi, Artech House, Boston 1992. From the prior art it is known microstrip antenna constructions in which one edge of the radiator is short-circuited to the ground plane. Using such an arrangement a given resonance frequency can be achieved with considerably smaller physical dimensions than the simplest microstrip antenna described above.
FIG. 1
illustrates such a microstrip antenna in cross section.
FIG. 1
shows a ground plane
20
, radiator
10
and a feed line
30
. The radiator
10
is short-circuited at its first end to the ground plane
20
through a short-circuiting part
11
. The second end of the radiator is open.
FIG. 1
does not specifically show the dielectric medium which may be air, for example. Microstrip antennas are often implemented on printed circuit boards, in which case there is the usual dielectric pcb material between the radiator
10
and ground plane
20
.
A typical problem with planar antenna constructions according to the prior art is their thickness and narrow band. Antennas used in personal mobile communications devices must be small in size. However, making the microstrip antenna thinner makes the usable frequency band of the antenna narrower. Many mobile communications systems require a relatively wide frequency band, e.g. the DCS-1800 system requires a 10% frequency band, approximately, relative to the center frequency.
In the GSM system, for example, the transmit and receive bands are spaced at 45 MHz from each other, the transmit band being 890-915 MHz and the receive band 935-960 MHz. With an antenna of a single resonance the frequency band should be considerably wide, at least 890-960 MHz in the case of GSM. Because of manufacturing tolerances and objects near the antenna, such as e.g. the hand of the user, which affect the resonance frequency, the bandwidth must be even wider than in the ideal case.
A second approach is to realize an antenna with two frequency bands such that the first frequency band corresponds to the transmit band and the second frequency band corresponds to the receive band. In that case the frequency bands of the antenna need not be as wide as those of a single-band antenna. Such dual-band antennas may comprise e.g. two helix antennas tuned to different frequencies or a combination of a rod antenna and a helix, where the rod and helix are tuned to different frequency ranges. Such constructions are described e.g. in Finnish patent application no. 952780. However, such helix antenna constructions are difficult to realize inside the housing of a mobile communications device. Furthermore, these arrangements only operate on two frequency bands. However, future multimode mobile communications devices operating in more than one mobile communications system require antenna constructions operating in more than two separate frequency bands.
Microstrip constructions can be used to realize many different antenna solutions, say, constructions with more than one operating band.
FIG. 2
shows an example of such a construction.
FIG. 2
shows a ground plane
20
, radiator
10
and a feed line
30
. A gap
15
divides the radiator
10
in two parts having different resonance frequencies. The radiator may also have more gaps and more parts in which case there are several resonance frequencies as well.
Planar dual-band antenna constructions are disclosed e.g. in US Pat. No. 5,124,733. Said patent publication discloses a microstrip antenna construction which has in addition to a ground plane one active radiating element and a second passive element. The elements are quarter-wave elements short-circuited to the ground plane through one edge. The elements have differing resonance frequencies so that the antenna construction has two separate operating frequency bands. A disadvantage of such a solution is the thickness of the two stacked antenna elements. Furthermore, this solution, too, allows for operation on two frequency bands only.
FIG. 2
shows only one feed line
30
. It is also known to use more than one feed point and feed line so that the properties of the antenna, such as the resonance frequency, directivity and diversity characteristics, for instance, can be influenced by choosing the feed point used. The characteristics of the antenna construction can also be influenced by the shape and size of the radiator in the antenna construction and by the size difference and distance between the radiator and ground plane, for example.
SUMMARY OF THE INVENTION
An object of the invention is to provide an antenna construction which is adaptable and modifiable in many ways. Another object of the invention is provide said antenna construction which is also simple to manufacture. A further object of the invention is to provide an antenna construction the characteristics of which can be electronically controlled during operation.
The objects of the invention are achieved by realizing a microstrip antenna construction having a matching element capacitively coupled to the ground plane. The characteristics of the antenna construction can be controlled in a very versatile manner by controlling the strength of the capacitive coupling of the matching element and the location of the matching element.
The antenna construction according to the invention is characterized by what is expressed in the characterizing part of the independent claim directed to the antenna construction. The mobile communications device according to the invention is characterized by what is expressed in the characterizing part of the independent claim directed to the mobile communications device. Other preferred embodiments of the invention are disclosed in the dependent claims.
The antenna construction according to the invention has a radiator, ground plane and at least one matching element. The matching element is capacitively coupled to a ground potential. The characteristics of the antenna construction, such as the number of resonance frequencies, resonance frequencies and the radiator impedance at the feed point, can be controlled in a very versatile manner by controlling the number and location and the strength of the capacitive coupling of the matching elements.
REFERENCES:
patent: 5124733 (1992-06-01), Haneishi
patent: 5585807 (1996-12-01), Takei
patent: 6008762 (1999-12-01), Nghiem
patent: 6133881 (2000-10-01), Kushihi et al.
patent: 99219 (1997-07-01), None
“Handbook Of Microstrip Antennas”, Clarricoats et al., IEE Electromagnetic Waves Series 28, vol. 1, 1989.
“Analysis, Design, And Measurement Of Small And Low-Profile Antennas”, Hirasawa et al., Artech House, 1992.
Ho Tan
Nokia Mobile Phones Ltd.
Perman & Grren, LLP
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