What sort of antennas can you get on a satellite that is only a 10cm cube? Good question! If you can arrange to have a deployment mechanism, and if you can make your antennas as thin and as light as possible, the answer as far as FUNcube is concerned, is a dipole on 145 MHz and a separate dipole on 435 MHz!
Pictured is the Antenna Deployment system that will be used by FUNcube. It is supplied by ISIS BV. It is very small and light. The actual antennas are made from material similar to that used in roll up metal tape measures, such that when released, they will spring out automatically.
The release mechanism is triggered by the process of burning through a very small length of nylon wire, using electric current from the satellite’s battery. Each burn (a total of four, one for each element)) take a few seconds, and draws something between .5 and 1 amps from the battery. The antennas will be deployed approximately 5 minutes after ejection from the rocket. This is to ensure that we are at a sufficient distance from the other payloads so we don’t hit anything!
Of course, deploying the antennas is a very critical step when commissioning the satellite. It might just be possible to uplink to it with the antennas still stowed, but it would require a very powerful transmitter and some very high gain antennas.
We have recently had some interesting discussions on how the antennas should be commanded to deploy. There are two schools of thought, either;
- The Antennas deployment system acts entirely autonomously and simply deploys the antennas 5-15 minutes after the satellite is powered up (i.e. after it has been ejected), 0r
- The Micro Processor Unit (MCU) on board the satellite, commands the Antenna Deployment system to deploy. It does this by communicating with it via the satellite I2C bus.
One of the disadvantages of method 1 above, is that whenever the satellite is powered up during testing, integration, etc., there is a risk that the antennas might deploy ‘accidentally’. Apart from the possible danger to personnel next to the satellite, the rearming of the burning mechanism is not a simple task, special tools are required, and the task takes several hours. Additionally, of course, the more times the mechanism is used, the more there is chance of wear, etc.
So we have decided on the following scheme. The MCU will command the Antenna Deployer, but it will be inhibited from doing this with two mechanisms. The first of these is by having a “Add Before Flight” plug, and the second is by having a special status string in the EEPROM of the MCU which it will read when it powers up, and which, before launch, will tell the MCU “You are still on the ground”. This string will be changed remotely to tell the MCU that it is in space, just before launch.