The purpose of the antenna is communication between the satellite and the ground. The antenna takes signals from the radio and converts them into electromagnetic radiation which is broadcast back to Earth. Our ground station receives the electromagnetic radiation through another antenna, and sends it through another radio and various signal processing mechanisms to gather the important data.
We have chosen a cross-dipole for our satellite's antenna. A cross-dipole is made by placing two dipole antennas at 90 degrees from each other to achieve circular polarization. A number of design considerations lead to a cross-dipole antenna. The satellite will be operating in the amateur radio band at 437.5MHz, the cross-dipole allows us both to operate in this range (depending on the physical length of the dipoles), as well as give us circular polarization and an omni-directional radiation pattern.
The transceiver component of the radio consists of the internal parts of the Yaesu VX-3R. This radio was particularly chosen after careful consideration of the size and the flexibility to be able to program the radio via a Windows Software. The dimensions of this radio are 9 x 4.5 x 1.6 cm. The output power of will be 1W at the desired frequency of 437 MHz. After considering all the option this radio was the best choice for Illini-Sat 2.
The Communications team uses a modem to help facilitate the transmission of digital data between the ground station and the satellite. The modem transforms binary data (zeros and ones) into an analog waveform using a process called Frequency-Shift Keying (FSK). This modulation method involves representing each binary value as a different analog frequency, changing the frequency of the output waveform whenever the binary input value changes. A simple FSK modulation is illustrated in the following diagram:
In addition to the functionality on the satellite, the Communications team is responsible for the other half of the link, the ground station. The ground station setup consists of two yagi antennas on the roof of Everitt Lab, a radio, and computers to track the satellite. The overall structure is the same as on the satellite, with the computers sending a signal to the radio, which modulates it to the appropriate frequency and sends it to the satellite through the antenna. The computers utilize an open-source program called GNOME Predict (Gpredict), which tracks satellites through the sky using information from NORAD. Gpredict provides relative coordinates of the satellite compared to a point on the ground (in this case Champaign, IL), which can be used to direct the antennas at the satellite when it is within view.