A Broadband Planar Modified Quasi-Yagi Using Log-Periodic Antenna

In this paper, a broadband planar modified quasi-Yagi antenna using a two-element log-periodic dipole array as a driven element is proposed. To feed the two-element log-periodic dipole array, a simple microstrip to stripline transition as a balun is designed, which converts the unbalanced input to balanced output. The antenna is fabricated on a low cost glass epoxy FR4 substrate with dielectric constant = 4.4, substrate thickness = 1.6 mm, and loss tangent = 0.02. The overall size of the antenna is 84 mm x 111 mm, which is 0.41 lambda(o) x 0.54 lambda(o) at the center frequency of 1.45 GHz. Measured results show a bandwidth of 41.4% for VSWR <= 2. A gain of 6.5 dBi +/- 0.5 dB and front to back ratio (F/B) of better than 20 dB are achieved over the bandwidth. Measured results are in good agreement with the simulated ones. This antenna is useful for RFID, portable direction finding, spectrum monitoring systems, etc.