Fragmented Antenna Realization Using Coupled Small Radiating Elements

Abstract

This paper reports on the feasibility of formation of an electrically large antenna at low frequencies using a number of miniaturized antennas through electromagnetic coupling for achieving high bandwidth. The proposed fragmented antenna system is intended for a swarm of small unmanned aerial vehicles in a linear flight formation. Inductively end-loaded folded dipole antennas are used as radiating elements. The small antennas are designed so that they can individually radiate with small bandwidth when isolated. The overall dimensions and the total mass of these fabricated elements are $12 \times 10\times 10$ cm ( $0.096\lambda _{0}\times 0.08\lambda _{0}\times 0.08\lambda _{0}$ at 240 MHz) and 18 g (including the matching network), respectively. Each miniaturized antenna can only provide 2.4 MHz (~1%) bandwidth and $25~\Omega $ input impedance. Having proper electromagnetic coupling between a cluster of three of such elements, measurement results demonstrate that the middle element can provide 18.4 MHz bandwidth (7.7%, 7.7 fold improvement) and $126~\Omega $ input impedance when the other two elements are loaded with optimal reactive elements. Moreover, the proposed configuration shows 1 dB gain improvement at boresight direction. The effects of flight formation fluctuations on the coupled antennas are also investigated, and it is shown that as the separation distance between the coupled antennas increases, the input impedance and bandwidth enhancement decrease. To overcome this problem, a tunable matching circuit containing a single varactor is proposed. Measurement results are shown to be in good agreement with the simulation predictions.