While I can in no way take any original credit for this idea I just wanted to share some pictures of my Tellstick in this process since the onces I have seen were of much older revisions.
Mine is a batch 10 of the original TS001 with internal antenna.
The new antenna is 164mm from the soldering point to the end. Google a bit and you will find both the math for this and lots of howtos.
As usual, click the images to blow them up a bit.
Before any modification