Miniaturization of a class of ultra-wideband antennas using dual-mode radiating structures

Antennas are essential components in all communication and radar systems. In recent years, there has been an increased interest in ultra-wideband antennas, which find applications in high data rate communication systems. Antennas used in such applications are required to be impedance matched and have good radiation and polarization characteristics over a large frequency span. Traditional ultra-wideband (UWB) antennas such as frequency-independent antennas and travelling-wave antennas can be very large in size particularly if they have to operate low frequencies. As the current trends in electronics technology continue to push for more miniaturized and integrated systems, the requirements on the size, weight, and power consumption (SWAP) of wireless systems has become more stringent. This prohibits the use of traditional ultra-wideband antennas in low-frequency (e.g. VHF) applications, where size and weight are of prime importance. Different miniaturization techniques such as inductive and capacitive loading have been used to realize compact ultra-wideband antennas. Recently, the concept of closely-coupled, dual-mode radiators was introduced. This new concept can be used, in conjunction with existing antenna miniaturization approaches, to develop compact, low-profile, ultra-broadband antennas. By making use of two distinct modes of a radiating structure, which have complementary frequency bands of operation, the lowest frequency of operation of an ultra-wideband antenna is significantly reduced. In this paper, we further investigate this concept and design a frequency-dependent feed network that automatically feeds the antenna structure in its correct mode of operation. Utilizing such a feed network, an ultra-wideband antenna is demonstrated to achieve a larger overall bandwidth compared to what is obtained using each mode individually. In particular, the lowest frequency of operation of the antenna is considerably reduced without increasing its occupied volume. A prototype of the designed antenna is fabricated and measurement results are presented.