colling of power rf transistor

i have a blf278 power rf transistor for 400w ch.7 transmitter.
and in normal operation the temp. of this device is very high and air cooling not response. any one have an idea for cooling this transistors?

See picture on page 11 of the BLFs data sheet:
http://www.semiconductors.philips.co...s/BLF278_4.pdf
What is under it is a flat heatsink that cools down this transistor ..

This transistor is designed to be mounted on heatsinks (with heat compound to esure low thermal resistance between its base and heatsink .. and this is the only way of cooling it down ..

Regards.
IanP

i use 30cm*90cm heatsink but this device is very hot. and temp. of heatsink about 90 degree.

The heatsink that you use, is it just 30x80 plate?
For powers like this you will need at least 10mm thick plate to distribute heat within the heatsink ..
Regards,
IanP

RF power amplifiers of this power class requires a hefty heatsink, supported by a fan blower. See picture below, which is an example from http://www.pcs-electronics.com/en/catalog.php

An improvement on the above photo is to house the amplifier in a box with fans on both ends of the heat sink at the box wall. I have seen one design that has a copper (higher thermal conductivity than aluminum) plate between the transistor and the heat sink to spread the heat over more area of the heat sink.

Another trick is to use lower power transistors on separate heat sinks and use power splitters and combiners at the input and output of the group. This spreads the heat over more heat sink area.