Anomalous microwave properties of high-Tc superconducting microstrip antennas around the critical temperature

We report the peculiar microwave properties of high-T-c superconducting (HTS) microstrip antennas without impedance matching circuits near the critical temperature (T-c), where impedance mismatch is usually expected. The superconducting thin films used in this report were YBa2Cu3O7-x films (T-c = 88 K) with various thicknesses deposited on MgO substrates by the pulse laser deposition technique. At the temperature of 86 K the reflection coefficient rapidly drops down below -62.52 dB and the standing wave ratio approaches almost unity 1.0015 at the resonant frequency of 11.812 GHz. This implies that a HTS microstrip antenna somehow tunes itself for the optimal performance at a specific temperature, indicative of possible exemption from impedance matching circuits. (C) 1999 American Institute of Physics. [S0021-8979(99)00224-8].