Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
1998-02-24
2001-06-26
Le, Hoanganh (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S725000, C343S770000
Reexamination Certificate
active
06252549
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to SE 9700667-0 filed in Sweden on Feb. 25, 1997; the entire content of which is hereby incorporated by reference.
1. Technical Field
The present invention relates to an antenna device and an antenna apparatus for transmitting and receiving radio signals, in particular one that is located on a base station in a mobile communications system.
2. Background
An important part of the planning and dimensioning of a communications system for radio signals is the properties of the antennas. These properties affect, among other things, the cell planning (size, pattern, number). One of these properties is the radio coverage area of the antenna.
Originally, only so called omni antennas were used, having a coverage in all directions seen from the base station. If a larger coverage area was necessary, a new cell was introduced adjacent to the first one and a new base station was placed in the middle of it.
Later on it was discovered that it was advantageous from a system point of view to divide the coverage area into sectors, for example, three sectors in one full circle. Antennas intended for this coverage are called sector antennas. This becomes particularly advantageous if the base station is placed in the intersection point between the cells. Each of the sector antennas then covers one cell and the base station thus serves several cells at a time.
The coverage area of a sector antenna is determined by the antenna's beam width in the horizontal plane.
Another important property of the antennas is their polarization, or rather the polarization of the signals transmitted or received by the antenna. Originally only vertical polarization was used in the base station antennas. Nowadays, often two linear polarizations are used at the same time (polarization diversity), for example in the horizontal and the vertical planes, here referred to as 0 and 90 degrees, or in the tilted planes between them, +/−45 degrees. Usually the antenna must have the same coverage for both polarizations.
The sector antennas used today for two polarizations have a beam width of approximately 60-70 degrees. At present antennas with a wide lobes can only be made with one polarization direction. Now many operators want antennas for two polarizations having beam widths of 80-90 degrees to adapt the coverage area of the base station to existing systems and the surrounding terrain.
A sector antenna comprises a column with some type of antenna element receiving and/or transmitting in one or two polarizations within a limited coverage area. These antenna elements may be implemented, for example, as so called microstrip elements. A microstrip element has a radiating body in the form of a conducting surface, often called a patch, located in front of an earth plane. The space between them may be filled with a dielectric material or air. Air has the advantages of being light, inexpensive and causing no power loss. For the microstrip element to function efficiently the length of the patch must correspond to a resonant length in the polarization direction, usually about half a wavelength.
The beam width in a certain plane of an antenna is inversely proportional to the dimension of the antenna in the same plane. Base station antennas often have a vertical beam width of 5-15 degrees, which is dictated by the topography of the surroundings of the base station. This beam width may easily be adjusted by changing the number of elements in the vertical direction of the antenna. In the horizontal direction the antenna cannot be made narrower than one element. If, for example, the polarization of the antenna is horizontal, the width of the element is determined by the resonance condition mentioned above.
A known antenna apparatus with two different polarization directions comprises a number of microstrip elements whose radiating elements have a square shape. Each radiating element has two different feeders. One feeder transmits or receives a signal having a certain polarization different from the one transmitted or received by the other feeder. This implies that the microstrip elements must be resonant in two directions (one for each polarization direction) which implies that the width of the radiating elements must correspond to half a wavelength. This in turn means that it is very difficult to generate lobes that are wider than 60-70 degrees. One known way to widen the lobe is to fill the microstrip element with a dielectric substance having a dielectric constant greater than one. This reduces the wavelength and thus also the resonant dimension of the patch. This procedure, however, causes reduced performance because of inevitable power losses in the substance as well as a higher weight and cost.
U.S. Pat. No. 5,223,848 describes an antenna comprising microstrip elements having a pair of rectangular radiating elements. Each radiating element is fed to transmit and receive with both a vertical and a horizontal polarization simultaneously. The radiating elements may be conducting surfaces or other radiating elements. Both radiating elements in the pair transmit and receive on two frequencies with different polarization directions.
SUMMARY
The present invention attacks a problem that arises when a sector antenna implemented using plane conductor technology is to be able to generate efficiently very wide antenna lobes (more than 70 degrees) simultaneously, with two different polarization directions, while at the same time being compact, light and inexpensive.
More specifically, the problem arises when the antenna elements of the antenna must be resonant in two directions to be able to transmit and receive with two polarization directions. This limits the possibility to design a compact, light and inexpensive antenna generating small losses.
A similar problem arises when a narrow sector antenna is to generate two antenna lobes of the same width, and having two different polarization directions, in the horizontal plane.
The purpose of the present invention is thus to achieve a compact, light and inexpensive antenna with small losses having two antenna lobes of substantially the same width, greater than a certain width, and having two different polarization directions.
More specifically the present invention is intended to achieve an antenna in which the width of the antenna lobes in the horizontal plane is greater than 70 degrees.
According to the invention two different types of antenna element are used in one common unit, in which the type and geometrical shape of the antenna elements enable a unit that is as compact and light as possible. Each type of antenna element is arranged to transmit or receive with one particular polarization.
More specifically, the invention relates to an antenna unit having a narrow antenna element of a first type, for example, a microstrip element, in combination with a narrow and light antenna element of a second type, for example, a slot in an earth plane. The first type of antenna element is only designed for a first polarization direction, while the second type of antenna element is only designed for a second polarization direction, different from the first polarization direction. These antenna elements may be arranged to occupy a very small surface. This means that the antenna may be built for antenna lobes greater than a certain angle, for example 70 degrees, without the antenna becoming heavy and/or expensive.
The invention also relates to an antenna apparatus comprising a certain number of said antenna units. These antenna units may, for example, be arranged in a column forming a sector antenna. The sector antenna, too, may be built for antenna lobes greater than a certain angle, for example 70 degrees, without the antenna becoming heavy and/or expensive.
One advantage of the present invention is that the antenna can have a very wide lobe (70-90 degrees) in the horizontal plane for two different polarization directions. When both antenna lobes have substantially the same width, considerable advantages are achieved from a
Burns Doane Swecker & Mathis L.L.P.
Le Hoang-anh
Nguyen Hoang
Telefonaktiebolaget LM Ericsson (publ)
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