A frequency-reduction scheme for spiral slot antenna

We present a new technique to reduce the initial resonant frequency, enhance the bandwidth, and miniaturize the antenna size for microstrip-feeding spiral slot antennas. Two spiral slot antennas were fabricated and measured to demonstrate the capability of lowering the initial resonant frequency by adding a circular capacitive stub at the end of the feedline. According to results, the initial resonant frequency of the proposed antenna has been shifted from 1.58 to 1.28 GHz; in other words, the antenna size can be reduced by 20%, approximately. The radiation patterns are simulated and measured at 1.3 GHz and 3 GHz to examine the excitation mode. The variations of the initial resonant frequency versus the radius of the circular stub are also shown to find the optimum radius of the stub.