The effect of substrate permittivity on UWB antenna radiation is studied. Microstrip and CPW-fed UWB monopoles are designed using substrates with different dielectric constants. The radiation patterns are measured. The transfer function of a two identical antenna system is calculated and later the received pulse is compared with the radiated pulse. Distortion produced by the antennas in every azimuth angle of radiation is quantified with the fidelity factor. The different radiation characteristics of each antenna are discussed. A conclusion is given about the feasibility to make UWB antennas using high permittivity substrates.