Antenna for high frequency radio signal transmission

[0002] According to the article “Investigations of antennas for an indoor wideband communication system at 60 GHz,” Zimmermann, MMMCOM, Dresden, 12-13 May 1997, it is known that an antenna for communication between a base station and a plurality of mobile stations in an indoor space may be designed as a lens antenna. The goal of this antenna is to set up radio links from a base station arranged beneath the ceiling to a plurality of mobile stations located in an indoor space, as part of a system for high-bit-rate data transmission in the 60 GHz frequency range. Using the antenna, the base station's high-frequency signal, which is present at the antenna's input, is radiated into the space for which coverage is to be provided. Thanks to the antenna's radiation pattern, even coverage can be provided over the entire indoor space at a defined working height. Among other things, greater transmitting power is supplied to the more distant mobile stations than to the mobile stations that are in close proximity beneath the transmitting antenna. The power level of the signal directed exactly perpendicular to the floor is lower than that of the signal radiated against the boundary walls of the indoor space. When a signal is transmitted between the base station and a given mobile station, reflections are avoided thanks to multipath propagation. Otherwise, individual waves are superimposed on one another at the receiving location, so that total field intensity may be lost entirely due to interference, depending on the phase position. The proposed antenna for radiating the high-frequency signal from the base station includes a lens-shaped plexiglass shaped element fed by a waveguide. The geometry of the outer shell of the lens is tailored to the characteristics of the indoor space for which high-frequency signal coverage is to be provided. The radio signals which are radiated are linear polarized. Due to the geometry of the lens's outer shell, reflection losses occur during the transition from the lens material to the air. In addition, the antennas of the mobile participating units must be oriented so that they receive the linear polarized signals in a suitable manner.

[0003] By contrast, the antenna according to the present invention has the advantage that the geometry of the dielectric lens's inner shell is tailored to the indoor space, while the outer shell includes a hemisphere. As a result it is easier to apply an anti-reflection layer and to avoid reflection losses during the transition from the lens material to the air.

[0004] By using a primary radiator which includes a waveguide having a helix antenna, one can create lenses having a lower and having small dimensions. For example, this allows one to use polyethylene as the lens material. One can also achieve this advantage if the primary radiator includes a waveguide having a patch antenna.

[0005] Using a primary radiator of this kind has the advantage that the radio signals can be rendered circularly polarized. As a result, the antennas of the mobile stations no longer need to be oriented in a particular direction. In addition, using circularly polarized radio signals diminishes the effects associated with multipath propagation. Thus one can minimize interference effects. It is advantageous if the electric lens includes suitable antireflection features. To this end, it is advantageous if a λ/4 layer made of a suitable dielectric is applied or achieved via grooves.

[0006] FIG. 1 shows the communication system.

[0007] FIG. 2 shows the antenna according to the present invention.

[0008] FIG. 1 shows a base station 1 and a plurality of mobile stations 2, which communicate with one another via radio signals. Mobile stations 2 are located in an indoor space, the boundaries of which are formed by a wall 4 and a ceiling 3. The radio signals radiated by the base station are formed so as to give radiation cone 5. Note that the radiation cone is formed so that as far as possible reflections on wall 4 are avoided. Transmitting power varies within the radiation cone: it is higher on the cone's surface, so that the more distant mobile stations can be supplied with transmitting power, and lower in the middle of the radiation cone.

[0009] FIG. 2 shows antenna 6 according to the present invention, which includes a primary radiator 13 and a dielectric lens 12. Primary radiator 13 includes a waveguide 7, on which a helix antenna or patch antenna 8 is arranged. The primary radiator protrudes into the inner shell of dielectric lens 12. Outer shell 10 of dielectric lens 12 is hemispherical in shape. Anti-reflection layer 11 is located on the hemispherical surface of outer shell 10.

[0010] The antenna of the base station includes a primary radiator and the dielectric lens. Primary radiator 13 is excited directly by the waveguide, and as a result no transitions or additional interfaces are required. The primary radiator generates a 60°-wide radiated field pattern having circular polarization which is formed by dielectric lens 12 so as to give the desired pattern. The shape of the dielectric lens is based on the spatial geometry and can be tailored to any indoor space. As the outer and the inner shell of the lens can be used for beamforming, there are two degrees of freedom. To ensure that one can implement the simple anti-reflection layer, the wavefronts of a high-frequency signal must be as parallel to the lens surface as possible when they issue from the material of outer shell 10. Hemispherical geometry is therefore used for the outer shell. Inner, rotationally symmetrical shell 9 can be tailored to different indoor spaces. The lens itself is made from dielectric material that is easy to manufacture. For example, one may use polyethylene for which ε_r=2.14. Symmetrical grooves 23 are milled into the material of the lens as the λ/4 antireflection layer for the dielectric-air transition. These grooves must be smaller than the wavelength in the substrate. One can achieve a simple anti-reflection layer by using grooves of this kind having a suitable depth in conjunction with a suitable pulse-duty ratio, and as a result no additional coating is required. For example, given a pulse-duty ratio of 1:1, 0.5-mm-wide, 1-mm-deep grooves are cut into the lens. As a result, reflection losses are avoided and the antenna's efficacy is improved. In addition, the antenna's radiation pattern is flattened.