Compact omnidirectional three-port MIMO antenna with the same vertical polarization for WLAN applications

In this letter, a compact three-port MIMO antenna with omnidirectional radiation patterns is presented for indoor WLAN applications. The proposed antenna consists of five identical monopoles, one is placed at the center and the other four are sequentially rotated around the center. The central monopole is fed by a microstrip line directly. The other four monopoles are fed by a two-port feed network that can provide four outputs with equal amplitudes. The phases are progressively increasing in clockwise or anticlockwise direction, depending on the exciting port. All the three ports can generate omnidirectional radiation patterns with the same vertical polarization. The port isolation among the three ports is analyzed in detail. In 2.4 GHz band, the measured port isolation is higher than 15 dB.