A compact two-port tunable dual-band spiral antenna for MIMO terminals

In this paper, a frequency tunable Dual Band Inverted L Spiral Antenna (DBILSA) for Multiple Input Multiple Output (MIMO) mobile terminals is presented. Two different shapes of L-shape and spiral shape were combined to produce a dual band response. Varactor diodes were used to simultaneously tune the resonant frequency from 1900 MHz to 3100 MHz (in the lower band) and from 3300 MHz to 3700 MHz (for the upper band), with a minimum reflection coefficient of -10 dB. Maximum realized gains of 2.83 dB (in the lower band) and 3.28 dB (in the upper band) are achieved. Besides that, a high isolation between the two DBILSA elements reduced the mutual coupling to result in an Envelope Correlation Coefficient (ECC) of less than 0.2. Along with its frequency tuning ability and the low ECC, the proposed MIMO antenna is deemed suitable for mobile terminal applications.