Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
1999-04-21
2001-04-17
Le, Hoanganh (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S846000
Reexamination Certificate
active
06218990
ABSTRACT:
The present invention is generally concerned with radiocommunication devices, in particular mobile telephones, and it is more particularly concerned with microstrip antennas for use in such devices. An antenna of this kind is known as a “microstrip patch antenna” and includes a patch which is typically formed by etching a metallic layer.
BACKGROUND OF THE INVENTION
The microstrip technique is a planar technique used to produce lines conveying signals and antennas coupling such lines and radiated waves. It uses conductive strips and/or patches formed on the top surface of a thin dielectric substrate separating them from a conductive layer on the bottom surface of the substrate and constituting a ground for the line or antenna. A patch is typically wider than a strip and its shape and dimensions are important features of the antenna. The substrate is typically a plane rectangular sheet of constant thickness and the patch is also typically rectangular. This is not obligatory, however. In particular, varying the thickness of the substrate, for example exponentially, widens the bandwidth of an antenna of the above kind, and the shape of the patch can in particular be circular. The electric field lines extend between the strip or patch and the ground layer through the substrate.
The technique differs from various other techniques that also employ conductive elements on a thin substrate, in particular the coplanar line technique in which the electric field is established on the top surface of the substrate and symmetrically between a central conductive strip and two circular areas on respective opposite sides of the strip, from which they are separated by respective slots. In the case of an antenna, a patch is surrounded by a continuous conductive area from which it is separated by a slot.
Antennas using the above techniques are typically, although not exclusively, resonant structures which are the site of standing waves enabling coupling with radiated waves.
A broad distinction can be drawn between diverse types of resonant structure that can be made using the microstrip technique and which correspond to respective modes of resonance of the structure. A first type is the commonest one and might be called the “half-wave” type. Taking one dimension of the patch as its length, in a direction called the longitudinal direction, the length is typically substantially equal to one half-wave, i.e. to half the wavelength of an electromagnetic wave propagating in that direction in the line comprising the ground, the substrate and the patch. The antenna is then called a “half-wave” antenna. That type of resonance can be generally defined by the presence of an electric current node at each of the two ends of the length, which can therefore also be equal to said half-wave multiplied by an integer other than 1. This number is typically an odd number. Coupling with radiated waves occurs at the ends of the length, in regions where the amplitude of the electric field in the substrate is maximum.
A second type of resonant structure that can be produced using the same technique might be called the “quarter-wave” type. It differs from the half-wave type in that the patch typically has a length substantially equal to one-quarter wave, i.e. to one-fourth of a wavelength, the length of the patch and the wavelength being defined as above. In this case the antenna is called a “quarter-wave” antenna. It is also different in that there is a clear short-circuit at one end of the length between the ground and the patch in order to impose a “quarter-wave” resonance mode. This type of resonance can be generally defined by the presence of an electric field node fixed by the short-circuit at one end of the length of the patch and by an electric current node at the other end of the length. The length can therefore be equal to an integer number of half-waves added to said quarter-wave. Coupling with the radiated waves occurs at the other end of the length, in the region in which the amplitude of the electric field through the substrate is maximum.
Other resonance modes can be established in planar antennas. They depend in particular on:
the configuration of the patches, which can incorporate slots, possibly radiating slots,
the possible presence and location of short-circuits and electric models representative of short-circuits, which are not always equivalent, even approximately, to perfect short-circuits, for which the impedance would be zero, and
the possible presence and location of coupling devices in the antennas for coupling their resonant structures to a signal processing unit such as a transmitter.
There may be a plurality of resonance modes for a given antenna configuration, enabling the antenna to be used at a plurality of frequencies corresponding to those modes.
The present invention is particularly characterized by choosing certain “resonance paths”, as explained hereinafter. The meaning of the expression “resonance path” as used hereinafter will now be defined:
Each resonance mode can be described as the result of superposing two waves propagating in opposite directions on the same path and reflected at the two ends of the path alternately. The path is imposed by the components of the antenna. It constitutes the “resonance path” for this resonance mode. It is rectilinear and longitudinal in the case of the half-wave and quarter-wave antennas previously mentioned. However, it can also be a curved radiating slot. In all cases the resonant frequency is inversely proportional to the time for which a traveling wave travels along the resonance path (see above). The expression “resonance mode” is sometimes replaced below by the term “resonance”.
An antenna is typically coupled to a single processor unit such as a transmitter by a connection system including a coupling device incorporated in the antenna and a connection line external to the antenna connecting the coupling device to the signal processor unit.
In the case of a resonant structure transmit antenna, the respective functions of the coupling device, the connection line and the antenna are as follows: the function of the connection line is to convey a radio frequency or microwave frequency signal from the transmitter to the terminals of the antenna. The signal propagates along the whole of a line of this kind in the form of a traveling wave without its characteristics being significantly modified, at least in theory. The function of the coupling device is to convert the signal supplied by the connection line into a form in which it excites a resonance of the antenna, i.e. so that the energy of the traveling wave conveying the signal is transferred to a standing wave established in the antenna with characteristics defined by the antenna. Transfer is generally imperfect, i.e. the coupling device reflects some of the energy towards the connection line, which causes an unwanted standing wave in the line. The corresponding standing wave ratio varies as a function of frequency and the diagram of that variation defines the bandwidth(s) of the antenna. The antenna transfers energy from the standing wave to a wave radiated in space. The signal supplied by the transmitter is transformed a first time from a traveling wave to a standing wave and a second time into a radiated wave. In the case of a receive antenna, the signal takes the same forms in the same units but in the reverse order.
The coupling device and the connection line can be implemented using a technique other than the microstrip technique, for example in the form of coaxial or coplanar lines. To limit unwanted reflections their nature and dimensions are chosen to match the impedance of the various units through which the signals travel.
A transmit antenna connecting system is often referred to as an antenna feed line.
The present invention concerns antennas which can be included in various types of device, including mobile telephones, base transceiver stations for mobile telephones, motor vehicles, aircraft and airborne missiles. In the case of a mobile telephone the continuous nature of the bottom
Grangeat Christophe
Herve Pascal
Kouam Charles Ngounou
Lorcy Laurence
Alcatel
Le Hoang-anh
Sughrue Mion Zinn Macpeak & Seas, PLLC
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