+40dBm ODU@18GHz, in case exist,it should have a dissipation factor like a water heater. Definitively right approach is with the antenna size approach..has no sens to ask a 40Db Radio just for asking ..do you have antenna size constrains?do you have budget constrains? hop lenght constrains?
First idea : you can use compressor for Ethernet( for exemple) in order to reduce the channel requirements (110-56-->28MHZ it gives you 3 6dBm) . In the past I heard a company that produce a radio amplifier to put in front of radio standard flange but ..stupid approach IMHO..Linearity problem , contact losses etc.....if you really are interested to have 40 dB maybe you should ask for this strange arrangements (it is like final amplifier, external)