Paul, I haven't seen a reply by anyone so I will takle this with what I know. If you replacing coax cable (an impedence cable) then the flex should also be the same impedence, especially at that frequency. Now I spoke with a flex board house that said they could do Multilayer flex with controlled impedence. They were 1 of few. Ensure that it is at least a 3 layer (shield/ground, signal, shield/gound) also required to controll the impedence. The only problem I forsee is the connection from the flex to the board. ML flex will probabbly use offset connections for each layer so the point of connection will be of concern. May want to work with the board house to create off set layer connections. A pain to connect to the board but continues the impedence. From reading, there is a similar problem with the coax connection to the board anyway. Hope this helps, Chris Robertson [log in to unmask] Hello all, I have an application for a flex circuit that has to run at 10 gig or above at a 1/4 watt. The circuit will be about 3 inches long. This will be used in place of coaxial cable. Is this possible in a flex circuit and if so what design considerations must be taken? Thank you in advance, Paul Gerlach TRI-C Design Inc. 1-507-663-1745