An efficient Electrically Small Antenna at HF band

A capacitive top loaded and inductively coupled Electrically Small Antenna (ESA) is presented which is tunable in the mid and upper HF band with the help of a variable capacitor situated between the monopole and the top capacitive cap. The dimensions of the coupling transformer and the cap were optimized using an EM simulator. A prototype was fabricated and the measured -10 dB bandwidth was found to be in excess of 350 kHz which makes it attractive both for radar and multi channel communication applications. Efficiency of the antenna was measured to be 80% using the Wheeler Cap method. It has been shown that the Efficiency Bandwidth product (EB) of the proposed antenna is 5 times the EB of the best state of art HF ESA in open literature. Finally it has been demonstrated that the Quality factor of the proposed antenna lies close to the fundamental Chu Mclean limit hence further reduction in size of the antenna will result in inferior bandwidth performance.