A traveling wave maser using iron doped rutile (Fe3+: TiO2) has been developed for a signal frequency of 86–88 GHz. The 3‐dB bandwidth is 40–50 MHz for an electronic gain of 36 dB and a net maser gain of 15–20 dB. Hexagonal aluminum substituted strontium ferrite was used for isolation. A unique overmoded ridge guide structure is used to match pump and signal power to the active material from 30 to 120 GHz. The maser noise temperature has been measured asTMaser=20±10 K.