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.