Beam forming networks of optimum size and compactness for multibeam antennas at 900 MHz

Size and compactness are the main aspects to be considered in the design of microstrip networks to feed multibeam antennas operating around 1-2 GHz, as in the case of the 900 MHz GSM system. A significant reduction in size along with a good compactness can be achieved only by an accurate choice of the network configuration. This configuration has also to guarantee an optimum mechanical reliability and the phase distribution at the output of the network to correctly feed the elements of a linear or two-dimensional microstrip array. We propose practical solutions for implementing beam forming networks to feed a 4-beam or 8-beam smart antenna for GSM, operating over the 880-960 MHz frequency band. These solutions assure acceptable global dimensions for the microstrip networks also at 900 MHz, along with an optimum mechanical implementation and a very interesting electrical performance.