Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2002-01-04
2003-08-12
Phan, Tho G. (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S846000
Reexamination Certificate
active
06606062
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based on French Patent Application No. 01 00 139 filed Jan. 5, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. §119.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to radio transmitter devices, in particular to mobile telephones, and more particularly to microstrip antennas included in such devices.
2. Description of the Prior Art
A microstrip antenna includes a patch that is typically obtained by etching a metal layer. This kind of antenna is known as a microstrip patch antenna.
The microstrip technique is a planar technique that has applications in producing lines and antennas providing coupling between lines transmitting signals and radiated waves. It uses conductive strips and/or patches formed on the top surface of a thin dielectric substrate. A conductive layer on the bottom surface of the substrate constitutes a ground of the line and the antenna. The patch is typically wider than the strip and its shape and dimensions constitute important characteristics of the antenna. The shape of the substrates is typically that of a rectangular plane sheet of constant thickness, and the patch is also typically rectangular. However, varying the thickness of the substrate can widen the pass-band of the antenna and its patch can be various shapes, for example circular. The electric field lines between the strip or the patch and the ground layer pass through the substrate.
Antennas constructed in accordance with these techniques typically, although not necessarily, constitute resonant structures adapted to support standing waves providing a coupling with waves radiated into space.
Various types of resonant structure can be produced using the microstrip technique and can support various resonant modes, which for succinctness are referred to hereinafter as “resonances”. Broadly speaking, each resonance can be described as consisting of a standing wave formed by the superposition of two travelling waves propagating in two opposite directions along the same path, these two waves resulting from the alternating reflection of the same travelling electromagnetic wave at the two ends of that path. Using this mode of description, the latter wave propagates in an electromagnetic line consisting of the ground, the substrate and the patch and which defines a linear path of zero width. In fact this kind of wave has wave surfaces that extend transversely over the whole of the section that is offered to them by the antenna, and thus this mode of description simplifies the real life situation, to a degree that is sometimes excessive. To the extent that it can be considered to be linear, the path can be rectilinear or curved. It will be referred to hereinafter as a “resonance path”. The frequency of the resonance is inversely proportional to the time taken by the progressive wave referred to above to travel the length of that path.
A first type of resonance might be called “half-wave” resonance. In this type of resonance the length of the resonance path is typically substantially equal to one half-wavelength, i.e. to half the wavelength of the travelling wave referred to above. The antenna is then referred to as a “half-wave” antenna. This type of resonance can be generally defined by the presence of an electrical current node at each of the two ends of the path, whose length can therefore be equal to said half-wavelength multiplied by an integer other than 1. That integer is typically an odd number. Coupling with radiated waves is obtained at one end of the path at least, the ends of the path being situated in regions in which the electric field in the substrate has a maximum amplitude.
A second type of resonance that can be obtained using the same technique might be referred to as a “quarter-wave” resonance. It differs from a half-wave resonance, firstly, in that the resonance path typically has a length substantially equal to one quarter-wavelength, i.e. one quarter of the wavelength defined above. To this end the resonant structure must include a short circuit at one end of the path, the term “short circuit” referring to a connection between the patch and ground. Also, the short circuit must have an impedance that is sufficiently low to impose such resonance. This type of resonance can be generally defined by the presence of an electrical field node fixed by this kind of short circuit in the vicinity of an edge of the patch and by an electrical current node situated at the other end of the resonance path. The length of the resonance path can therefore also be equal to said quarter-wavelength plus an integer number of half-wavelengths. Coupling with the waves radiated into space is obtained at an edge of the patch in a region in which the electric field through the substrate has a sufficiently large amplitude.
Resonances of other, more or less complex, types can be obtained in antennas of this kind, each resonance being characterized by a distribution of the electric and magnetic fields that oscillate in an region of space including the antenna and its immediate vicinity. They depend in particular on the configuration of the patches, which can in particular incorporate slots, possibly radiating slots. They also depend on the presence and location of any short circuits and on electrical models representing the short circuits if they are imperfect, i.e. if they cannot be regarded, even approximately, as perfect short circuits of zero impedance.
The present invention finds an application in diverse types of devices, such as mobile telephones, base stations for mobile telephones, automobile vehicles, aircraft and missiles. In the case of a mobile telephone, the continuous nature of the bottom ground layer of a microstrip antenna limits the radiation that is intercepted by the body of the user of the device when it is transmitting. In the case of automotive vehicles, and above all in the case of aircraft or missiles whose external surface is made of metal and has a curved profile to achieve low aerodynamic drag, the antenna can be conformed to the profile so as not to cause any troublesome additional aerodynamic drag.
The present invention relates more particularly to the situation in which a microstrip antenna must have the following qualities:
it must be a dual frequency antenna, i.e. it must be able to transmit and/or receive efficiently radiated waves on two frequencies separated by a large spectral gap, p
1
it must be possible to connect it to a signal processor unit by means of a single connecting line for all operating frequencies of a transmitter device without giving rise to a troublesome spurious standing wave ratio on that line, and
it must not be necessary to use a frequency multiplexer or demultiplexer to achieve this result.
Many prior art microstrip antennas that have the above three qualities have been produced or proposed. They differ in terms of the means employed to obtain a plurality of resonant frequencies. Three such antennas will be examined:
A first prior art antenna of the above kind is described in U.S. Pat. No. 4,766,440 (Gegan). The patch
10
of this antenna is generally rectangular in shape and the antenna has two half-wave resonances with resonance paths along a length and a width of the patch. It also includes a U-shaped curved slot which is entirely inside the patch. The slot is a radiating slot and produces a supplementary resonance along another resonance path. By appropriately choosing its shape and its dimensions, the slot produces required values of the frequencies of the resonances, which provides the facility to transmit a circularly polarized wave by associating two modes having the same frequency and crossed linear polarizations with a relative phase of 90°. The coupling device takes the form of a microstrip line which is also coplanar in that the microstrip is in the plane of the patch and penetrates between two notches of the patch. The device includes i
Coupez Jean-Philippe
Kouam Charles Ngounou
Alcatel
Phan Tho G.
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