A Compact, Capacitively Fed UWB Antenna With Monopole-Like Radiation Characteristics

A compact, ultrawideband (UWB) antenna occupying a cylindrical volume and providing omnidirectional monopole-like radiation is introduced. The antenna consists of two closely placed 3-D loops, two parasitic small loops, and a circular top hat. Two strip-shaped probes, each coupling with one loop, are used to feed the antenna in common mode. UWB operation is achieved by taking advantage of the magnetic coupling of the parallel-fed 3-D loops. Small electrical dimensions of the antenna are due to the special capacitive proximity feeding scheme and the presence of the top hat. A prototype with 4.1:1 bandwidth is designed, fabricated, and characterized. The antenna demonstrates a VSWR better than 2: 1 and monopole-like omnidirectional radiation characteristics across this entire band. It occupies a cylindrical volume with a diameter of 0.14 lambda min and a height of 0.09 lambda min, where lambda min is the free-space wavelength at the lowest frequency of operation. The antenna has a k(min)a = 0.73, where k(min) is the wavenumber at its lowest frequency of operation. The fabricated prototype is demonstrated to have a lowest frequency of operation that is only 10% above the theoretical limits on UWB antennas. Further miniaturization of the antenna using a spherical top hat is also examined and demonstrated to be capable of reducing k(min)a values down to 0.60.