Ultrafast optics find more and more applications to microwave and millimeter wave technology through picosecond optoelectronics. THOMSON CSF is working on sequential RF waveform generation using a frozen-wave generator and picosecond photoconductive switches. Such a generator produces unique wide-band and high—power microwave pulses. A critical issue consists in radiating that pulse. A solution is a quasi-TEM horn terminating the feeding line. A two-dimensional, boundary-element numerical simulation has been developped to calculate the reflection coefficients of the dominant modes. The standard case of open-ended parallel-plane waveguide enables us to validate the code. Then, the code is applied to exponentially tapered videband horns, showing that a 1:10 bandwidth is theoretically reachable. Work is in progress to calculate the radiation pattern, the fluence of the electromagnetic field inside the horn, and to extend the code to the case of horns partially filled with dielectric.