There are lots of reasons for wanting to get the payload weight down – it can save money (less helium / smaller balloon), or can mean more height with the same size balloon. Also very small payloads are used for foil balloon flights which, although they only get to 4km not the 30km+ of latex flights, cost a lot less for the balloon and for the helium. Also they generally don’t need CAA permission so they can be flown at short notice.
Here’s my attempt at one. It weighs in at 16g, which is less than a quarter of just the GPS antenna/cable on CLOUD1! It could be made even lighter by integrating everything onto one PCB, and my replace the GPS antenna with a very lightweight one. With battery, antenna and some insulation added the total weight will come to around 30g, which means it could fly with a single 36″ foil balloon.
This payload uses a new radio transmitter that hasn’t yet (as far as know) been flown at high altitudes, so as a test I will probably fly this payload soon together with CLOUD3 as the main payload.
Here’s the complete tracker assembly, just needing a radio antenna (5 bits of wire) and battery (even just a single AAA cell).