Every year, Shelly the tortoise gets lost or manages to escape the garden.
It can take days to find her. I would like to build a locating device so
that I can track her down.
The basic requirements a

a) that the 'transmitter (transponder?) on the tortoise is as small as
possible.

b) that it can be made by an electronics numpty. I have basic soldering
skills, and can eventually work out a circuit diagram.

Can anyone point me in the right direction?

TIA
Floatything (an escapee from uk.rec.sailing)