| Frequency- and polarization-agile antennas are of great interest for multi-function, software-defined, and cognitive radios. Frequency tuning over an octave is challenging because the radiation conductance changes as the antennas are tuned. In this work, single- and dual-polarized antennas are tuned over frequency ranges of up to 2:1 and 1.7:1, respectively, while remaining impedance matched. The impedance match over tuning is maintained by a properly chosen reactive element in series with the antenna. Both polarizations of the dual-polarized antennas are tuned independently, enabling either simultaneous frequency- and polarization-agility, or operation at two independent frequencies on orthogonal polarizations.;First, the tunability of the slot-ring antenna is demonstrated with fixed-capacitor loading, and then, single and dual-polarized slot-ring antennas are tuned from 0.95-1.8 GHz and 0.93-1.6 GHz, respectively, using varactor diodes. Microstrip reactive loads are developed to maintain >20 dB port-to-port isolation over most of the tuning range of the dual-polarized antenna. This may be the first dual-polarized tunable antenna with independent control of both polarizations over a 1.7:1 frequency range.;Single-sided radiation is desirable for many applications, and varactor-tuned cavity-backed slots are investigated for this purpose. A shallow cavity-backed slot is tuned from 1.0 to 1.9 GHz with <-20 dB return loss. The antenna operates either in free-space or conformally mounted with minimal change in input impedance.;Finally, a dual-polarized cavity-backed patch antenna is tuned from 0.6-1 GHz. Differential feeds are employed to achieve >30 dB port-to-port isolation (differential- differential mode) over the tuning range, and the measured cross-polarization level at 0.65, 0.8, and 0.95 GHz is <-25 dB. This may be the first demonstration of dual-polarized independent tuning over a 1.7:1 frequency range with single-sided radiation. |
