Long time, no update
It’s been a long while. I completed the first payload container using some high density insulation from B&Q. Nice to work with, very insulating, very dense. Unfortunately, also very heavy. The container came in at 600 grammes empty, more than the entire contents!
I also etched a base point for the quarter-wave ground plane antenna that I will be using for the payload. Really please with the way that came out:
In order to save some weight I’ve rebuilt the payload in 25mm polystyrene from Wickes, and it now comes in at a much more palatable 100 grammes 
Just need to make the hole for the camera. Originally I was going to put a skylight filter in front of the lens, but after discussion on the #highaltitude IRC channel I’ve changed my mind. Apparently the have a tendency to mist up, and as there is no air there is little convection so heat loss through the hole should be minimal.
Finally, I’ve spent some time playing with the resistor values in the potential divider driving the Radiometrix NTX2. Originally, I had 47k and 33k resistors, with a 10k to ground. However, this neglected to take into account that a pin pulled low is also grounded, giving a real world of 43k/43k and 53k/33k. After playing with some options, mostly around what stock of 0805 resistors I had, I ended back up at 43k and 33k, but without the 10k ground. In testing this gives a solid 350hz carrier shift. This goes against the maths, but then I’m still not sure how to calculate the effect of the ~100k input impedence on the NTX2. I was shooting for 425hz shift, but 350hz is OK. One combination of resistors gave me a mere 180hz shift!
Here’s the completed flight computer. In two parts, the left side is the main FC and transmitter, the right houses the SD card logger (which logs all output from the hardware uart to a file) and the GPSBee. It was done in 2 parts to allow flexibility; the GPS can be changed for an alternate easily, and it folds up relatively small.
