Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2001-06-14
2003-04-08
Le, Hoanganh (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S846000
Reexamination Certificate
active
06545640
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to radio transmission systems, in particular mobile telephones, and more particularly to microstrip antennas included in such systems.
An antenna of this kind includes a patch which is typically formed by etching a metallic layer. The skilled person refers to an antenna of this kind as a “microstrip patch antenna”.
Microstrip technology is a planar technology which is used both to produce signal transmission lines and antennas which provide coupling between such lines and radiated waves. It uses conductive patches and/or strips formed on the upper surface of a thin dielectric substrate. A conductive layer extends over the bottom surface of the substrate and constitutes a ground plane for the line or the antenna. A patch of the above kind is typically wider than a strip of the above kind and its shape and dimensions constitute important characteristics of the antenna. The substrate is typically a plane rectangular sheet of constant thickness and the patch is also typically rectangular. This is no way obligatory, however. In particular, the skilled person knows that varying the thickness of the substrate can increase the bandwidth of an antenna of the above kind and that the patch can take various shapes, for example it can be circular. The electric field lines extend through the substrate between the strip or the patch and the ground plane.
The above technology differs from various other technologies which also use conductive members on a thin substrate, and in particular it differs from coplanar line technology in which the electric field is established over the upper surface of the substrate and in a symmetrical manner between a central conductive strip and two conductive areas on respective opposite sides of the strip, from which they are respectively separated by two slots. In the case of a loop slot antenna, a patch is surrounded by a continuous conductive area from which it is separated by a slot.
Antennas constructed using the above technologies typically, although not necessarily, constitute resonant structures in which standing waves provide coupling with waves radiated into space.
Various types of resonant structure can be implemented using the microstrip technology and can employ various modes of resonance, which modes are referred to more briefly hereinafter as “resonances”. Broadly speaking, each such resonance can be described as a standing wave formed by the superposition of two traveling waves propagating in two opposite directions on a common path, the two waves resulting from the same traveling electromagnetic wave being reflected alternately at each of the two ends of the path. In the context of a description of this kind, the wave is considered to propagate in an electromagnetic line comprising the ground plane, the substrate, and the patch, and which defines a linear path of zero width. In fact a wave of the above kind has wavefronts which extend transversely across the whole of the section offered to them by the antenna, which means that the above description simplifies the real-life situation in a manner that is sometimes excessive. To the extent that it can be considered linear, the path can be rectilinear or curved. It is referred to hereinafter as the “resonance path”. The resonant frequency is inversely proportional to the time taken by the above-mentioned traveling wave to travel along this path.
A first type of resonance might be referred to as “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 traveling wave referred to above. The antenna is then referred to as a “half-wave” antenna. This type of resonance can generally be defined by the presence of an electric current node at each of the two ends of a path of the above kind, whose length can therefore be equal to said half-wavelength multiplied by an integer other than one. This integer is typically odd. Coupling with the radiated waves occurs at at least one of the two ends of the path, which ends are in regions where the amplitude of the electric field in the substrate is at a maximum.
A second type of resonance that can be obtained using the same technology might be referred to as “quarter-wave” resonance. It differs from said half-wave resonance firstly in that the resonance path typically has a length substantially equal to one fourth of a wavelength, i.e. one quarter of the wavelength as defined above. For this the resonant structure must have a short-circuit at one end of the path, the expression “short-circuit” referring to a connection between the ground plane and the patch. The short-circuit must have a sufficiently low impedance to be able to impose such resonance. This type of resonance can be generally defined by the presence of an electric field node fixed by a short-circuit of the above kind in the vicinity of if an edge of the patch and by an electric current node at the other end of the resonance path. The length of the resonance path can therefore be equal to an integer number of half-wavelengths added to said quarter-wavelength. Coupling with the waves radiated into space occurs at an edge of the patch, in a region where the amplitude of the electric field through the substrate is sufficiently high.
Other types of more or less complex resonance can be established in antennas of the above kind, each resonance being characterized by a distribution of the electric and magnetic fields, which fields oscillate in an area 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 possible presence and location of short-circuits and electrical models representing those short-circuits when they are imperfect short-circuits, i.e. when they cannot be treated, even approximately, as equivalent to perfect short-circuits with zero impedance.
The presence of an imperfect short-circuit in an antenna can give rise to resonance featuring what might be referred to as a virtual node. A virtual node is produced if some of the following conditions are satisfied at the same time. If the above antenna is referred to as the “first antenna”, these conditions are as follows:
The distribution of the fields in the first antenna is substantially identical to a distribution that can be induced in an identical area of the patch of a second antenna.
The second antenna is identical to the first antenna within this area except that within this area the second antenna does not have the short-circuit in question.
The patch of the second antenna extends not only over the area already mentioned, which then constitutes a main area of the second antenna, but also over a complementary area.
Finally, the distribution of the fields in question in the main area of the second antenna is accompanied by an electric or magnetic field node in the complementary area.
When describing the resonance occurring in the first antenna, the node occurring in the second antenna may be considered also to constitute a node for the resonance of the first antenna. For an antenna such as the first antenna, a node of this kind is referred to hereinafter as a “virtual” node, because it is in an area which is outside the patch of the antenna and in which no electric or magnetic field therefore occurs whereby the presence of the node could be determined directly.
Although these “virtual nodes” are not conventionally taken into account in these terms in describing resonances, they are implicit in the distinction that is sometimes drawn between the physical or geometrical length and the so-called electrical length of the same patch. In the case of two antennas referred to above, and with regard to the patch of the first antenna, the physical or geometrical length would be that of the patch and the electrical length of the same patch would in fact be the physical or geometrical length of the second antenna.
M
Coupez Jean-Philippe
Herve Pascal
Kouam Charles Ngounou
Alcatel
Le Hoang-anh
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