The TH‐3 radio system employs all solid state components with one exception. Because of the need for output power not obtainable presently from solid state devices, the sole exception is the traveling‐wave tube power amplifier. This amplifier furnishes 40.5 dBm output power at 33.5 dB gain over any channel of the 5.925‐ to 6.425‐GHz common carrier band. This tube employs a long‐life cathode made of zirconium‐additive nickel.* Also, it gives higher power output than traveling‐wave tubes previously designed2 for Bell System radio relay systems. This new tube and periodic permanent magnet circuit are packaged as an integral unit. It does not require focusing or other mechanical adjustments during the life of the tube.