A method of designing a tri-band double-dipole quasi-yagi antenna (DDQYA) using a double co-directional split-ring resonator (SRR) is suggested. A pair of double co-directional SRRs are added on a second dipole of the DDQYA and generate two new resonances at a low frequency for a tri-band operation. Each size of the inside SRR and outside SSR of the double co-directional SRRs and an interval between the inside SRR and outside SSR determine the position of a newly generated resonant frequency. In an operational band of the DDQYA before adding the double co-directional SRRs, the double co-directional SRRs operate as a director and improve bandwidth and gain. A pair of SRRs are added to a ground surface reflector of the DDQYA to prevent the main beam′s direction from being reversed at 180 degrees since the length of a dipole is longer than the length of the ground surface reflector in the lowest resonant frequency.
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