Abstract:
An antenna arrangement for multi frequency band operation makes it possible to reduce the number of antennae on a base station antenna mast. The antenna includes a first radiator element for operation in a first frequency band and a second radiator element for operation in a second frequency band. The second element is arranged in a different plane from said first element. The first element is placed so that it symmetrically overlaps the second element. A conductive ground plane is provided with a device for feeding energy to the radiator elements, and the radiator elements are arranged for providing dual polarization. An array antenna includes groups of high and low frequency elements.

Description:
The present invention relates to an antenna arrangement for multi frequency band operation, comprising a first radiator element for operation in a first frequency band and a second radiator element for operation in a second frequency band, wherein said second element is arranged in a different plane from said first element. The invention also relates to an array antenna arrangement comprising groups of first and second elements. Also, the invention relates to the use of such an antenna arrangement. 
     BACKGROUND 
     A large number of base station antenna installations have been necessary for the operation of cellular mobile telecommunication systems. Base station antenna arrangements have to be provided all over the area that is to be covered by the cellular communication system and how they are arranged among other things depends on the quality that is required and the geographical coverage, the distribution of mobile units etc. Since radio propagation depends very much on terrain and irregularities in the landscape and the cities the base station antenna arrangements have to be arranged more or less closely. 
     However, the installation of multiple antenna base stations has caused protests among others from an esthetical point of view both on the countryside and in the cities. Also, the construction of these antenna masts is expensive, e.g. because each antenna needs to be supplied with energy via a separate, expensive feeding cable. 
     The introduction of new base station antenna arrangements would be considerably facilitated if the infrastructure that already is in place could be better used. Today various examples of microstrip antenna elements which are capable of operating in two distinct frequency bands are known. However, it is difficult to avoid grating lobes when the frequency bands are not closely spaced. 
     SUMMARY 
     An object of the invention is therefore to provide a multi frequency band antenna which does not present the above described problems. Another object of the invention is to provide an antenna which operates with different polarization states. 
     For these objects, the antenna arrangement in accordance with the invention is characterized in accordance with the accompanying independent claims. 
     Advantageous embodiments of the invention are described in the accompanying depending claims. 
     It is an advantage of the invention that the existing infrastructure already provided for the 800 or 900 MHz frequency band can be used also for new frequency bands such as about 1800 MHz or 1900 MHz. It is also an advantage of the invention that the antenna elements or the radiating elements are simple and flexible and enables a simple feeding etc. It is also an advantage that dual polarization states can be supported with a high mutual insulation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further described in the following in a non-limiting way under reference to the accompanying drawings in which: 
     FIG  1   a  is a top view of a multi frequency antenna arrangement according to the invention, 
     FIG  1   b  is a schematical cross-sectional view of the antenna of FIG. 1A along the lines  1   b—   1   b,    
     FIG. 2 a  is a top view of an alternative embodiment of an antenna according to the invention, 
     FIG. 2 b  is a schematical cross-sectional view of the antenna of FIG. 2A along the lines  2   b—   2   b,    
     FIG. 3 a  is a top view of a third embodiment of an antenna according to the invention, 
     FIG. 3 b  is a cross-sectional view of the arrangement of FIG. 3A along the lines  3 B— 3 B, and 
     FIG. 4 is a top view of an array antenna according to the invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 a  and  1   b  illustrate a first example of a microstrip antenna which is able to operate (receive/transmit) at two different frequencies or in two different frequency bands simultaneously. In FIG. 1 a , which is a top view of the antenna, a first radiating element  10  is arranged on top. The first radiating element  10  is here square shaped. A second radiating element  11  is arranged below the first radiating element. The second radiating element is symmetrically arranged in a centralized manner under the first radiating element. The first and second radiating elements  10 ,  11  respectively particularly comprise so called patch elements made of a conducting material, for example Cu. 
     The first patch element or radiating element  10  may be used for a communication system operating in frequency band of about 1800-1900 MHz whereas the second radiating element  11  may be used for a communication system operating in the frequency band of about 800-900 MHz. To facilitate this, the first and the second radiating elements have the appropriate effective resonant dimension respectively, in accordance with common practice, and in view of the effective dielectric constant of the dielectric material or medium, e.g. air which is used for insulating the first and the second patch. 
     In FIGS. 1 a ,  1   b  the first radiating element  10  is mounted on two orthogonally arranged pairs of probes  12  that are responsible for energizing this element in two directions of polarization with a mutual angle of about 90°. The probes  12  extend via holes through the second element  11  and are mounted on a first layer  13  of a ground plane that also comprises a second layer  14 . The ground plane layer  13  is provided with an electric feed network  15  for supplying the probes with energy in the two angles of polarization. 
     The lower, second radiating element, i.e. the low frequency band patch  11  is aperture fed from the second ground plane layer  14  via an aperture arrangement comprising slots  16  and  17 . The outer slots  16  are oriented according to one of the polarization angles and the inner H-shaped slot  17  is oriented according to the other angle. The polarization is perpendicular to the long dimension of the slots. The ground plane layer  14  is provided with an electric feed net  18  for supplying the slots with energy in the two angles of polarization. The above described slot configuration is only one example, many alternative slot configurations are possible, for example with crossing slots. 
     In alternative embodiments of the above described antenna, the second element may be energized by probes and this element may be provided with slot apertures for energizing the first element. The patches may have other shapes than square. The antenna may comprise any number of stacked elements for different frequencies, depending on the number of frequencies to be used in the antenna. The above described antenna module may be used in a multiple module array antenna. 
     In the above described embodiment, it is possible to feed both patches by means of the four probes  12 . In this manner, a single power feed network may be used for energizing both patches. 
     FIGS. 2 a  and  2   b  show an alternative example of a microstrip antenna which is able to operate (receive/transmit) at two different frequencies or in two different frequency bands simultaneously. The same reference numbers have been used as in FIG. 1 a  and  1   b  to designate the corresponding details. 
     As in the first embodiment of the invention, in FIG. 2 a , which is a top view of the antenna, a first radiating element  10  is arranged on top. A second radiating element  11  is arranged below the first radiating element, symmetrically arranged in a centralized manner under the first radiating element. 
     The first patch element or radiating element  10  may be used for a communication system operating in frequency band of about 1800-1900 MHz whereas the second radiating element  11  may be used for a communication system operating in the frequency band of about 800-900 MHz. 
     In FIGS. 2 a ,  2   b  the first radiating element  10  is mounted on two orthogonally arranged pairs of probes  12   a  that are responsible for energizing this element in two directions of polarization with a mutual angle of about 90. The probes  12   a  extend via holes through the second element  11  and are mounted on a first layer  13  of a ground plane that also comprises a second layer  14 . The ground plane layer  13  is provided with an electric feed net  15  for supplying the probes with energy in the two angles of polarization. 
     The lower, second radiating element, i.e. the low frequency band patch  11  is probe fed from the second ground plane layer  14  via probes  12   b . Thus, the patch  11  is mounted on two orthogonally arranged pairs of probes  12   b . One pair of probes  12   b  is oriented according to one of the polarization angles and the other pair of probes is oriented according to the other angle. The ground plane layer  14  is provided with an electric feed net  18  for supplying the probes with energy in the two angles of polarization. 
     In alternative embodiments of the above described antenna, the patches may have other shapes than square. The antenna may comprise any number-of stacked elements for different frequencies, depending on the number of frequencies to be used in the antenna. The above described antenna module may be used in a multiple module array antenna. 
     FIGS. 3 a  and  3   b  show a third example of a microstrip antenna in accordance with the invention which is able to operate (receive/transmit) at two different frequencies or in two different frequency bands simultaneously. The same reference numbers have been used as in FIGS. 1 a ,  1   b ,  2   a  and  2   b  to designate the corresponding details. 
     As in the first embodiment of the invention, in FIG. 3 a , which is a top view of the antenna, a first radiating element  10  is arranged on top. A second radiating element  11  is arranged below the first radiating element, symmetrically arranged in a centralized manner under the first radiating element. 
     The first patch element or radiating element  10  may be used for a communication system operating in frequency band of about 1800-1900 MHz whereas the second radiating element  11  may be used for a communication system operating in the frequency band of about 800-900 MHz. 
     In FIGS. 3 a ,  3   b  the first radiating element  10  is energized via aperture slots  16   a  and  17   a  in the second radiating element  11 . The outer slots  16   a  are oriented according to one of the polarization angles and the inner H-shaped slot  17   a  is oriented according to the other angle. The element  11  is provided with an electric feed net  15   a  for supplying the aperture slots with energy in the two angles of polarization. 
     The lower, second radiating element, i.e. the low frequency band patch  11  is aperture fed from the ground plane  14  via slots  16   b and  17   b . The outer slots  16   b  are oriented according to one of the polarization angles and the inner H-shaped slot  17   b  is oriented according to the other angle. The polarization is perpendicular to the long dimension of the slot. The ground plane layer  14  is provided with an electric feed net  15   b  for supplying the slots with energy in the two angles of polarization. 
     In alternative embodiments of the above described antenna, the patches may have other shapes than square. The antenna may comprise any number of stacked elements for different frequencies, depending on the number of frequencies to be used in the antenna. The above described antenna module may be used in a multiple module array antenna. The second element  11  may be designed so that it is transparent with reference to the frequency of the first element  10 , by e.g. incorporating FSS (Frequency Sensitive Surface) technics. In this way it is possible to have the slots for the two elements in a common ground plane. 
     FIG. 4 shows an array antenna in accordance with the invention, which in this example comprises three groups of elements, but any number of such groups is possible. Two of the element groups are similar to the example shown in FIGS. 1 a  and  1   b . Between these two element groups is a third element group comprising an extra element  10  of the first high frequency type. This configuration may be suitable for avoiding grating lobes. The ground plane  14   a  preferably continues below the central group of elements, and the ground plane  14   b  of the central high frequency patch  10  preferably is arranged at the same level as the second elements  11  of the two lateral groups of elements. The central high frequency patch  10  is powered by probes  12 . 
     The elements in FIG. 4 are oriented so that the polarization directions are ±45° with respect to the long dimension of the array. Any other directions, e.g. 0° and 90° may be used. The element groups of the array antenna may also be arranged in two dimensions. 
     In any of the above described antennas the two linear polarizations may be combined to form one or two circular. polarizations. 
     The invention is of course not limited to the shown embodiments but it can varied in a number of ways only being limited by the scope of the claims. For example, any number of probes may be used in the antenna as long as they are symmetrically oriented around the axes of polarization. Rectangular, circular, oval or any other form of patches may be used.