"Floatything" wrote ...
Thanks - that's excellent Howard - it looks just the
job. So all I need to know now are the 'basics of
direction finding'.. I presume I need to wave the
aerial about a bit until I get a stronger beep beep?
(tell me I'm on the right track? or is there some magic
method to homing in on a beeping reptile?

The basics are a directional antenna and triangulation.
Do they still sell those RDF things for watercraft which
use a loop antenna? Same principle. A directional FM
antenna (like a yagi) which you can rotate to find the
strongest signal. Then move a distance and plot the
max signal as another line on the map. Assuming your
tortise isn't moving that fast, go the the "X" on the map
and retrieve your reptile.

There are lots of miniature FM transmitter kits available,
but beware that the range of many (most?) of those
devices may be limited to a range where you could
just locate your creature by sight (particularly those
which can be powered by photo-voltaic cells :-)

Note that you could also use preassembled GPS and
cell-phone modules (like those available from one of
my favorite toy-shops: www.sparkfun.com) and create
a box which you can call on your phone and will report
the tortise's current location. You could have the first
tortise-lojack on the planet! :-)

It is rumored that many high-value items (besides autos)
have this kind of secret locator transponder devices
installed in them.