Low-Profile Tunable Dipole Antenna Using Barium Strontium Titanate Varactors

In this paper, a 2.4 GHz low-profile dipole antenna that uses a frequency-selective surface (FSS) with interdigital barium strontium titanate (BST) varactor-tuned unit cells is presented. The tunable unit cell is a square patch with a small aperture on either side to accommodate the BST devices. The Ba_0.6Sr_0.4TiO₃ varactors were fabricated on alumina substrates and demonstrate capacitance tuning of 1.5:1 (33%) at 90 V. The varactor chips were placed only along one dimension of the FSS to avoid the use of vias and simplify the dc-bias network. The measured data of the antenna demonstrate tunability from 2.23 to 2.55 GHz with a peak gain at broadside that ranges from 3 to 3.7 dBi, and instantaneous bandwidths of 50 to 160 MHz within the tuning range. The total antenna thickness is approximately λ/47.