Design of electrically small Hilbert fractal NFRP magnetic monopole antennas

Electrically small, Hilbert fractal, near-field resonant parasitic (NFRP) antennas are reported and investigated in this paper. This type of electrically small antennas (ESAs) consists of Hilbert fractal curves as the NFRP resonators and small vertical monopoles as the driven elements. By offsetting the driven monopole off the centre position, good impedance matching can be easily realized. The current distributions on the fractal NFRP elements and the driven monopoles manifest that strong electric coupling could excite loop-mode currents along the Hilbert fractal NFRP structures, and thereby horizontal magnetic monopole patterns with broadside radiation property are realized. The results show that the 2nd-step evolution Hilbert fractal NFRP antenna witnesses an electrically size reduction of 25% compared to ka = 0.9622 of the square capacitively loaded loop (CLL) NFRP antenna, while maintaining its original broadside radiation. It is also demonstrated that there is a performance trade-off, i.e. higher fractal iteration orders leading to smaller resonant frequencies, narrower bandwidth and lower radiation efficiencies.