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
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  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