A tuneable electrically-small antenna operating in the ´DC´ mode

The introduction of a shorting pin to the centre of an annular slot results in an additional, ´DC´, mode being excited. Operation in this mode results in an electrically small structure, which is inherently narrow-band. Through the addition of a simple varactor diode we show that this narrow-band ´DC´ mode may essentially be made wide-band through frequency tuning. The results presented in this paper consider the input response, 3-D radiation pattern, gain, directivity and efficiency.