A Broadband Unidirectional Antenna Based on Closely Spaced Loading Method

In order to achieve broadband and unidirectional radiation, a planar printed antenna based on closely spaced loading method is proposed and fabricated. This antenna consists of a printed rectangular loop with two gaps and a metallic strip. The rectangular loop has a good direction but has a drawback of narrow band. According to the closely spaced loading theory, the impedance at the upper frequency will decrease when a metallic strip is placed at the maximum radiation direction of the rectangular loop. This improves the impedance matching. A new resonant frequency point is induced. Meanwhile, the resonant frequency point can be changed by modifying the length of the metallic strip. The impedance matching between the two resonant peaks (One peak is determined by the rectangular loop, the other is determined by the metallic strip) is improved by changing the length and the position of the metallic strip. Thus the impedance bandwidth is greatly expanded. Experimental results show good performance of the proposed antenna. It has a 40.7% measured bandwidth, ranging from 2.29 to 3.46 GHz. The measured front-to-back (F/B) ratio of the proposed antenna is larger than 10 dB in the whole frequency band.