A TEM line to double-ridged waveguide launcher and horn antenna are disclosed. The launcher uses multiple probes or one or more wide-aspect probes across the ridge gap to minimize spreading inductance and a TEM combiner or matching taper to match the impedance of the probes over a broad bandwidth. The horn uses a power-law scaling of gap height relative to the other dimensions of the horn's taper in order to provide a monotonic decrease of cutoff frequencies in all high-order modes. Both of these techniques permit the implementation of ultra-wideband designs at high frequencies where fabrication tolerances are most difficult to meet.