Tissue elasticity μ1 and viscosity μ2 can be estimated by evaluating dispersion of shear wave propagation velocity over a range of frequencies. Alternatively, μ1 and μ2 can be calculated from shear wave attenuation αs and velocity cs at a single frequency. For shear waves generated by a focused ultrasound beam, attenuation due to geometric spreading makes it difficult to estimate αs correctly. In this study, we use a wide unfocused beam to generate quasi-planar radiation force (minimal diffraction) and monitor the out-of-plane (elevation direction) shear wave propagation using another transducer. Frequency dependent cs and αs values are calculated from the 2D Fourier transform (k-space) of the spatiotemporal shear wave data, using peak extraction and a full-width-of-half-maximum (FWHM) method. Simulation and experiment studies show good agreement between the results using the proposed method and the theoretical or independent measurement results. With the two probes placed on one side of the target or integrated into a 2D array probe, the proposed method could be applied to study in vivo tissue viscoelastic properties.