Carrying your radio along is nothing new, since a small and portable radio set is readily available. However, there is a different charm in carrying your single board computer with you while it is playing software-defined radio, and this is exactly what Miller Jacobson did with his Raspberry Pi (RBPi). There are two aspects to this project, the hardware and the software. While the software is simple enough, the hardware is somewhat involved.
To carry your RBPi around means to free it from the wall-mounted power supply unit. Power to the RBPi then has to come from batteries. Jacobson used lithium ion cells placed in an imported battery box. The enclosure uses four cells and can recharge USB devices and cell phones. It also incorporated charging voltage regulation and protection. He salvaged cells from a dead laptop battery unit, since these generally have quite a few good cells with only one or two cells bad, which render the entire unit useless.
To save space, Jacobson did not use connectors between the RBPi and the battery box. Instead, he soldered wires directly to the DC power jack on the battery box. The other end of the wires he soldered to the +5V and GND pins at the GPIO pins of the RBPi. He took the video output from the RCA port and used right-angled connectors everywhere he could, so that space used was at a minimum.
Jacobson used a second enclosure box, which was nearly the same size as the battery box, and fitted the two boxes one on top of the other, arranging to screw them together. Within the second enclosure box, he drilled holes to mount the RBPi board. He also cut holes on the sidewalls to take out the projecting wires and make the USB and HDMI ports accessible. Some ventilation holes in the enclosure allowed cooling.
For the display, Jacobson used a Nyxio Venture head mounted display. This is a cheap MMV or Mobile Media Viewer with a composite input. It features a slim profile with a full-sized image and has 2GB flash memory for storage. The virtual display simulates a huge virtual 62-inch screen in a 16:9 wide format on a non-radiation LCD panel.
For an input device, Jacobson used a generic wireless mini keyboard and trackpad. These are generally used for giving presentations and connect via a wireless interface. The tiny wireless receiver connects to the USB port of the RBPi.
Jacobson uses a Realtek RTL2832U based TV tuner as the front-end for the Software Defined Radio or SDR. The tuner covers a huge chunk of spectrum in the VHF and UHF range. The processor in the RBPi board was quite capable of handling the processing of the signals from the tuner.
GNU Radio, GNU Radio Companion and multimon are some of the software being used to receive and decode the APRS packets from the tuner. Jacobson is using Python for some of the other components such as for filtering the signals from the noise, extracting the raw audio and removing the DC offset, etc.
