Non-Foster Loaded Parasitic Array for Broadband Steerable Patterns

Parasitic arrays can generate steerable patterns using tunable reactive loads at the parasitic elements to provide the required phase for the scattered radiation. However, the radiation that is coupled to the parasitic elements and reflected by the attached loads has a frequency dependent phase delay, leading to a beam/null squint effect and a limited instantaneous squint-free bandwidth. In this paper, we introduce a technique for eliminating this frequency dependence using non-Foster parasitic loads, whose reflection phase has a positive phase dispersion slope that can cancel the negative phase dispersion slope associated with propagation delays. Further, by tuning the non-Foster load, we can tune the total scattered phase to achieve steerable patterns. Tunable non-Foster impedances can be designed using transistor-based negative impedance convertor (NIC) circuits. A two element parasitic array was designed for the frequencies of operation of discrete device NICs. Non-Foster impedances required for obtaining broadband or squint-free nulls at different azimuth angles were calculated using array theory and simulations. An NIC that generated the non-Foster impedance for a broadside broadband null was fabricated and attached to the parasitic antenna. Simulation and measurement results showed broadband uniform nulls from 180-350 MHz, providing about twice the null bandwidth of a passive parasitic load. Null steering across some angles of the azimuth was restricted by the stability constraints of the NIC while being tuned. With a judicious design of the parasitic antenna and the NIC, we can achieve broader bandwidths and steerable patterns with non-Foster parasitic arrays.