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A spiral shape C-patch antenna is presented. A slot between the feed and ground plane is used as an equivalent capacitor for the C-patch antenna. By controlling the size of the slot, the loaded capacitance can be changed. A high gain antenna of size less than 8 cm × 8 cm that operates at 174 MHz is needed for automotive applications. A spiral C-patch antenna is used for this design. The original operating frequency of the spiral C-patch antenna is 420 MHz. By introducing the slot between the feed structure and the ground plane, the operating frequency of the spiral C-patch can be reduced to 170 MHz or less. The gain and efficiency of the antenna is qualitatively assessed using communication quality and radiated power. By experiment, the efficiency of this spiral C-patch antenna is shown to be much higher than a 45 cm commercial dipole antenna.