Do you have any type of receiver that can detect and/or
demodulate the signal you are looking for? If not, you
may have to build a down converter.
Is the signal (relatively) continuous, or just intermittent?
As long as the signal is on the air enough then a beam
antenna of some sort attached to whatever receiver you
have should do the job. There should be a number of
antenna designs on the web. A high gain antenna will
give greater detection range and more accurate bearings,
but needs to be turned more slowly when the signal is
intermittent due to the narrow beamwidth.