The objective of this study was to investigate the feasibility of exploiting the anisotropy of liquid crystals to create a phase agile reflectarray antenna by applying a low bias voltage between the resonant elements and the ground plane. A change in the resonant frequency and hence the phase of the reflected signals is shown to occur when the permittivity of the tunable substrate varies due to the change in the orientation of the organic molecules. Phase agility from two element reflectarray cells constructed on commercially available liquid crystals (K15, BL037 and BL006) is demonstrated to occur at X-band when a bias voltage in the range 0 V to 20 V, is applied between the patches and the ground plane. The Flomerics MICROSTRIPES7 .o and Ansoft HFSS computer models have been employed to study the scattering behaviour of periodic arrays of grounded resonant elements printed on tunable dielectrics. These results are compared with experimental data to extract important electrical design parameters which include the values of Ell, f,..!.., tan 011, and tan Ool for each of the three liquid crystal (LC) specimens. The trade off between the dynamic phase range and the reflection loss is studied using different thickness ratios of non tunable and tunable dielectric layers. Distortion of the phase curves which is observed for reflectarray cells constructed on K15 LC substrates for low bias voltage states is investigated. Numerical and experimental studies show that the phase distortion can be removed by using a thicker low loss non tunable dielectric layer thus demonstrating the importance of minimizing the loss tangent value of the LC substrate in order to increase the efficiency of the reflectarray antenna and to remove phase distortion across the aperture. A new novel mixture which could satisfy the demanding requirements of reflectarray antenna design Le. <1 dB reflection loss and >300° dynamic phase range is specified using the HFSS computer model. Predicted results generated by MICROSTRIPES7.0 show that the size of the patch elements can be reduced by using slots integrated into the conductors thus creating the potential to manufacture a more robust structure which is suitable for practical applications. The trade off which exists between the signal reflection loss and the decrease in the static bandwidth and the patch size reduction is discussed. A novel design technique to create a reconfigurable monopulse reflectarray antenna that uses only half of the dynamic phase range which is available from commercially available liquid crystals is presented. A numerical simulation tool based on the Method of Moments (MoM) was used to design the antenna to demonstrate the possibility of switching between sum and difference radiation patterns at 10 GHz. A robust 244 element active LC reflectarray antenna has been constructed using non tunable supporting spacer material with voids to contain the BL006liquid crystals beneath the individual ofpatch elements of the periodic array. The exp'erimental results at 9.4 GHz and 9.6 GHz are close to the computed results and moreover demonstrate that the sum and difference radiation patterns can be reconfigured when the bias voltages that are applied to the two halves of the aperture are varied in the range 0 V and 22 V. Furthermore the measured radiation patterns obtained at 9.4 GHz, in the temperature range' 20°C and 30°C, show that the radiation pattern is not strongly influenced by the temperature, thus confirming that this is within the operating range of the nematic liquid crystals.