We propose a novel architecture of a planar phased array antenna for wireless sensor network. The principle is based on wave scattering from distributed scatterers on a waveguide surface. Each scatterer radiates a portion of the incident guided wave in a location-dependent phase delay. Therefore, the directivity or the focus of the synthesized scattered wave can be tailored by tuning the spatial pattern of the scatterers. We also propose to fabricate electronically tunable scatterer pattern using semiconductor devices and verify the idea by simulation. Since it can be made in an extremely thin and light body at low cost compared to the conventional phased arrays, vast applications will be expected with respect to wireless sensor network.