An electronically tunable planar dipole grid deposited on a dielectric substrate is described. Such dipole grids can be used as active frequency selective (FSS) or polarization sensitive (PSS) surfaces in reflector antenna systems. It is shown how the resonance frequency of the surface can be tuned by changing the steering voltage or current of electronically tunable components such as varactor diodes or YIG films, respectively, implemented with each of the dipoles. The analysis of the problem is based upon a Floquet theory approach for double periodic structures. The resulting integral equation for the current distribution is numerically solved by the method of moment with Galerkin's weighting. The experimental investigations of the reflection- and transmission coefficient were performed by a waveguide simulation technique operating from 7 to 16 GHz. A dipole grid of size 40×25 mm² is deposited inside a tapered waveguide. Results show that the resonance frequency can be tuned around 9.85 GHz by a band-width of 7% and around 10.1 GHz by 14% in case of capacitive or inductive tuning, respectively.