How to build my VHF tags from the scratch?

Hi all,

I was wondering if anybody has experience in building VHF-tags from the scratch. I couldn't find useful instructions online, so maybe anybody wants to share own experience...I am working on bats and sometimes I simply need to locate a roost of a bat, which I caught foraging. 150€ for a commercially avaliable tag is quite a lot for this simple application. 

all the best,