Easy enough to figure out -- look up the loss at the frequency of interest for the coax used, multiply by coax length then add the antenna(s) gain minus free space path loss between the downlink side antenna and the phone. As long as the total loss is less than what is lost coming through the wall you're a winner :).
H.
>> Has anyone successfully homemade a passive repeater for cell phones?
>> It ought to be simple -- just an antenna inside the shielded area
>> (which in my case is part of my house that has foil insulation) cabled
>> to an antenna just like it on the outside.
> Care to take a guess at the losses in such a system?