Irregularities in the electron density of the ionosphere cause phase and amplitude scintillation on transionospheric VHF and UHF radar signals, particularly at lower radio frequencies. The design of radar and other transionospheric systems requires good estimates of the coherence bandwidth (CB) and coherence time (CT) imposed by a turbulent ionosphere. CB and CT measurements of the equatorial ionosphere, made using the Advanced Research Project Agency Long-range Tracking and Identification Radar 158 MHz and 422 MHz phase coherent radar located on Kwajalein (9.4°N, 167.5°E), are presented as a function of the two-way S₄ scintillation index at 422 MHz The log linear regression equations are CT = 1.46 exp(−1.40 S₄) s at 158 MHz and CT = 2.31 exp(−1.10 S₄) s at 422 MHz. CT also varies by a factor of 2–3 depending on the effective scan velocity through the ionosphere, v _eff . The CT and CB, as a function of S₄, have been compared to those from the Trans-Ionospheric Radio Propagation Simulator, a phase screen model. A close agreement is achieved using appropriate values of v _eff and midrange values of phase spectral index and outer scale. Validation of CB is, however, limited by insufficient radar chirp bandwidth. Formulating the model in terms of the two-way S₄ index (an easily measurable parameter) rather than more fundamental phase screen parameters (which are difficult to obtain), improves its utility for the systems engineer. The frequency dependencies (spectral indices) of S₄ and of CT are also presented to allow interpolation and some extrapolation of these results to other frequencies.