That is why I will continue to make them. Glen, K9STH Website: http://k9sth.com --- On Wed, 7/20/11, Mahlon Haunschild <mahlonhaunschild at cox.net> wrote: 300 ohms is the design input impedance for the Thunderbolt, which is what the original attenuators were intended for. Thunderbolt owners will want the original design.