Tunable frequency-selective surfaces have received a significant amount of interest in recent years. They can be used to achieve tunable filters for multi-band reflector antennas, tunable radar absorbers, or to provide a varying transmission window in radomes. To generate a dynamic frequency behavior, the reactive characteristics of the surface must change with a tuning voltage, biasing magnetic field, or changing shape. Thus, tunable frequency-selective surfaces can be grouped into three categories: electronically, magnetically, and mechanically tunable. An electronically tunable frequency-selective surface has several benefits, such as small size, tuning speed in the nanosecond range, and low cost. A new three-dimensional (3D) frequency-selective structure (FSS) was designed by a group of researchers from China and Singapore.
The structure is based on a two-dimensional periodic array of vertical varactor-loaded microstrip line resonators and horizontal metallic plates. The biasing circuit of the individual varactor diodes consists of an RF choke and a direct-current (DC) blocking circuit. A 0.813-mm-thick Rogers 4003 substrate was used for the design. The design goals included a tuning range from 1.40 to 2.75 GHz as well as a fractional bandwidth of 14% at 2.75 GHz. With the help of the High Frequency Structure Simulator (HFSS) software, the parameters for the final layout were determined.
A silicon abrupt varactor diode with a nominal zero-bias junction capacitance between 0.5 and 0.6 pF was selected for the design. The tunable 3D FSS was measured inside a parallel-plate waveguide structure. The measured return loss was greater than 10 dB across the entire tuning range. While varying the bias voltage from 0 to 30 V, the center frequency was tuned from 1.40 to 2.75 GHz. The insertion loss across the frequency range varied from 1.0 to 4.5 dB.
See “Tunable 3-D Bandpass Frequency-Selective Structure with Wide Tuning Range,” IEEE Transactions on Antennas and Propagation, July 2015, p. 3,297.