146 MHz filters
170MHz Low pass transmit Filter
Minikits in Australia make a really good low pass filter which is ideal for use with the Portsdown or other 146 Mhz DATV transmitters.
Or there are a number of published designs which are easy to build using a minimum of tools:
Low pass 7 pole 144 MHz filter
A simple DIY filter was originally published in the RSGB VHF manual and described in CQ-TV 250
144 MHz band pass filter by YU1LM
This is a nice easy filter to build and can be used on the output of the Portsdown transmitter before any power amplifier stages.
There is also some discussion on the Funcube dongle website of the YU1LM filter http://www.funcubedongle.com/?p=456
Manufacturers of 144 - 148 MHz filters
- OCI is a Canadian manufacturer [1]