It is a staple of spy movies. The hero — or sometimes the bad guy — sticks a device never any bigger than an Alka Seltzer to a vehicle or a person and then tracks it anywhere it goes in the world. Real world physics makes it hard to imagine a device like that for a lot of reasons. Tiny power supplies mean tiny lifetime and low power. Tiny antennas and low power probably add up to short range. However, [Tom’s] Hackaday.io project maybe as close as you can get to a James Bond-style tracker. You can see a video of the device, below.
The little transmitter is smaller than a thumbnail — not counting the antenna and the battery — and draws very little current (180 uA). As you might expect, the range is not great, but [Tom] says with a Yagi and an RTL-SDR he can track the transmitter on 915 MHz for about 400 meters.
As you might expect, the circuit is simple. A low frequency oscillator (about a half hertz) triggers an RF oscillator with a SAW resonator. A bipolar transistor drives a simple whip antenna. The schematic and some more details are over on Instructables.
Thanks to the half-hertz timing, you get a beep about every two seconds which is probably good for battery life and sufficient for tracking. Just don’t expect a GPS coordinate tracker that you can receive from your secret lair a half a world away and you’ll be fine.
If you have more space and power, you can get a little closer to the spy’s dream tracker, but only a little. If you want to track something with a live USB port, you can always see how much you can jam into a USB drive casing.