High Altitude Balloon Radio

### Problem

This is a prototype for a high altitude balloon project I worked on with the Purdue Orbital rocketry team. I designed this board to test the range and power draw of our comms system and threw on a relay and microcontroller so that it can be used for triggering things on a real flight.
One of the FAA requirements is that balloons over a certain weight have four cutdown systems (two on the balloon envelope and two on the payload tether) and that these systems must be independent, meaning they are electrically disconnected from each other and have separate batteries. Many amateurs balloonists overlook this rule, but due to our close work with the FAA it is important that we follow the regulations.



## Packetized Data

Packet Specification
start address payload checksum end
1 2 3 / 4 5 / 6 7

I was able to issue commands from a master device and control an LED (and other GPIO) on a slave device as well as receive information back from the slave. From limited range tests I performed, the radios are able to transmit at least a mile with line-of-sight, but quite possibly much more than that. I ran out of space before the signal began dropping.

## Antenna Tuning

Antennas are easily the point of greatest loss in range and throughput in amateur radio, and any mismatches between transmitter and antenna impedance cause reflections and loss of transmission power. I used a network analyzer to measure and compensate my crappy dipole antenna so that it matched the 50 ohm output impedance of the radio module for greater range.
From the first Smith chart, we see that antenna has an impedance of 57+45j ohm at 144 MHz. If we can add some capacitance in series with the antenna, we can kill off the reactive component and get 57 ohm, which is "close enough".
\frac{1}{c 2 \pi j \omega} = -45j \Overrightarrow = 24pF\$