A LINEAR-ARRAY ANTENNA USING BIFILAR HELICAL ELEMENTS FOR MOBILE SATELLITE-COMMUNICATIONS

In this paper, authors propose a linear array antenna using two bifila r helical antenna elements placed along the helix axis to reduce beam direction movement according to frequency change. The beam direction m ovement of this proposed array antenna is smaller than that of a conve ntional bifilar helical antenna. Also, the gain of this proposed array antenna is higher than that of the conventional helical antenna for a cross point angle of radiation patterns at the different transmit and receive (Tx and Rx) frequencies. The conventional helical antenna is suitable for vehicle antennas in mobile satellite communication system s such as the MSAT system because it owns circularly polarized omni-di rectional radiation pattern and its thin pole form. However, this ante nna has a disadvantage that the beam direction in an elevation plane m oves according to frequency change. In the proposed array antenna, the beam direction movement is about 9 degrees smaller than that of the c onventional bifilar helical antenna on condition that antenna total le ngth is 4.83 lambda(0), antenna diameter is 0.12 lambda(0), and freque ncy change is from 0.957f(0) to 1.043f(0) (f(0) is center frequency an d lambda(0) is free space wavelength at f(0)) Also, the Tx and Rx gain s of this proposed array antenna at the cross point angle between Tx a nd Rx beams are about 2 dB higher than that of the conventional bifila r helical antenna on the same condition.