Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
1999-07-16
2001-11-13
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S702000, C343S846000
Reexamination Certificate
active
06317083
ABSTRACT:
This invention relates to antennas and in particular to flat plate or planar antennas.
As electronics and communications technologies have advanced, there has been a drive to increase the performance and decrease the size of consumer devices. In particular, in the field of mobile communications, there has been continual demand for increasingly smaller communications devices, such as telephones, computers and personal organisers, but without a decrease in performance.
One area in which size and weight design goals may be counter to performance design goals is in the design of antennas. The performance of an antenna can be measured by various parameters such as gain, specific absorption rate (SAR), impedance bandwidth and input impedance. Conventionally, mobile telephones have been provided with a rod antenna. These provide good performance relative to cost. However, since the antennas extend from the housing of the device, they are prone to breakage. Furthermore, as the size of a rod antenna decreases, the gain also decreases which is undesirable. As communication devices become smaller, rod antennas are therefore unlikely to provide a convenient antenna solution.
It is desirable therefore to develop an antenna which could be located within the device. An example of such an antenna is a flat plate or low profile antenna such as planar inverted-F antennas (PIFAs) which are well known in antenna art. A PIFA comprises a flat conductive sheet supported a height above a reference voltage plane such as a ground plane. The sheet may be separated from the reference voltage plane by an air dielectric or supported by a solid dielectric. A corner of the sheet is coupled to the ground via a grounding stub and provides an inductive load to the sheet. The sheet is designed to have an electrical length of &pgr;/4 at the desired operating frequency. A feed is coupled to an edge of the flat sheet adjacent the grounded corner. The feed may comprise the inner conductor of a coaxial line. The outer conductor of the coaxial line terminates on and is coupled to the ground plane. The inner conductor extends through the ground plane, through the dielectric (if present) and to the radiating sheet. As such the feed is shielded by the outer conductor as far as the ground plane but then extends, unshielded, to the radiating sheet.
The PIFA forms a resonant circuit having a capacitance and inductance per unit length. The feed point is positioned on the sheet a distance from the corner such that the impedance of the antenna at that point matches the output impedance of the feed line, which is typically 50 ohms. The main mode of resonance for the PIFA is between the short circuit and the open circuit edge. Thus the resonant frequency supported by the PIFA is dependent on the length of the sides of the sheet and to a lesser extent the distance and the thickness of the sheet.
Planar inverted-F antennas have found particular applications in portable radio devices, e.g. radio telephones, personal organisers and laptop computers. Their high gain and omni-directional radiation patterns are particularly suitable. Planar antennas are also suitable for applications where good frequency selectivity is required. Additionally, since the antennas are relatively small at radio frequencies, the antennas can be incorporated into the housing of a device, thereby not distracting from the overall aesthetic appearance of the device. In addition, placing the antenna inside the housing means that the antenna is less likely to be damaged.
However it is difficult to design a planar antenna that offers performance comparable to that of a rod antenna, in particular as far as the bandwidth characteristics of the device are concerned. Loss in an antenna is generally due to two sources: radiation, which is required; and energy which is stored in the antenna, which is undesirable. Planar antennas have an undesirably low impedance bandwidth.
In accordance with the invention there is provided an antenna comprising a reference plane, a conductive polygonal lamina disposed opposing the reference plane; and a feed section coupled to the reference plane and the lamina, the feed section being arranged as a transmission line.
Since the feed section is arranged as a transmission line (otherwise known as a waveguide), energy is contained and guided between the conductors of the transmission line. This results in a low Q factor and hence a higher impedance bandwidth compared with conventionally-fed planar antennas. The bandwidth is increased considerably while retaining the efficiency, size and ease of manufacture of planar antennas. The feed section should be as low-loss as possible.
At the end of the feed section adjacent the reference plane, the feed section preferably has an impedance which matches the impedance of the feed (typically a 50 &OHgr; line). At the end of the feed section adjacent the lamina, the feed section preferably has an impedance which matches the impedance of the antenna. Thus the feed section acts as an impedance transformer, matching the impedance characteristics of the feed at one end and the characteristics of the radiating lamina at the other. The feed section generally has a graded impedance characteristic along its length and provides an inductive load for the antenna. The impedance advantageously varies along the length of the feed section in a uniform manner.
The feed section generally comprises a first conductor for providing the feed signal to the conductive lamina and a second conductor connected to the reference plane, the first and second conductors together forming a transmission line. Thus the conductors of the feed section are e.m. coupled and operate as a waveguide. The energy is guided along the two conductors rather than being stored in the shorting post connected to the reference plane as is the case with conventional planar antennas. Thus the resulting antenna is very efficient compared with known antennas.
Preferably the width of the two conductors are of a similar order of magnitude.
Preferably the feed section comprises a microstrip line and/or a coplanar strip. In a particularly preferred embodiment, the feed section comprises a first part comprising a microstrip line parallel to the reference plane and a second part comprising a coplanar strip which extends at an angle from the reference plane to the conductive lamina. However, other transmission lines may be used e.g. coaxial line.
Thus an antenna according to the invention has an increased impedance bandwidth compared with known planar antennas without a sacrifice in efficiency. There is little radiation from the feed section because the energy is guided along the conductors of the transmission line feed section. In addition the resulting antenna is easy, and therefore relatively inexpensive, to manufacture.
The first conductor provides an inductive load to the conductive lamina.
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patent: 5631660 (1997-05-01), Higashiguchi
patent: 5764190 (1998-06-01), Murch et al.
patent: 5896109 (1999-04-01), Hachiga et al.
patent: 6034636 (2000-03-01), Saitoh
patent: 6081728 (2000-06-01), Stein et al.
patent: 720252A1 (1996-07-01), None
patent: 2191045A (1987-12-01), None
Johnson Alan
Modro Joseph
Alemu Ephrem
Antonelli Terry Stout & Kraus LLP
Nokia Mobile Phones Limited
Wong Don
LandOfFree
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