Abstract ~ Chris Culpepper, Jerome Glick, Syed Ali Kazi, Damodar Adhikari ~ The is a small self-contained device that allows an amateur radio operator to conveniently connect to distant repeater nodes using their hand-held HT radio. The operator is not required to use a computer or mobile device. It also improves over previous designs that require connecting the HT to a computer using a proprietary cable, thus limiting the mobility of the HT. This unit features a Raspberry Pi with Ethernet connection, built-in USB hub, audio chip, and an on-board radio transceiver that acts like a local repeater to the operator s HT. It can transmit at low (1 mw) or high (1 W) power modes, has a range of 1000 feet, and is powered by 12 volts. The unit is developed under an open-source model and is accessible to the average amateur radio hobbyist. Background Amateur ( ham ) radio exists for hobbyists as a provision for two-way wireless communications over certain frequency bands allocated for this purpose. The transmission format can include many different forms, such as voice, Morse code, RTTY, PSK31, Packet, ARPS, SSTV, etc. The distance of communication can also vary widely from a few miles to thousands of miles, depending on the equipment and output power used. For the purposes of this discussion, ham radio communications can be broken down into two basic categories: HF (High Frequency), typically associated with longer transmission distances (hundreds to thousands of miles), and VHF/UHF (Very High Frequency/Ultra High Frequency), typically associated with shorter transmission distances (ones to tens of miles). This discussion will focus on the voice mode of communication, and on the second category, VHF/UHF. Ham radio operators typically use hand-held transceivers (or handy-talkies (HTs)) with an output power of about 1 to 5 Watts to communicate by voice over these frequency ranges. In this scenario, a single HT has a broadcast coverage radius of a few miles. Repeaters are often utilized to boost this range up to 50 miles. A repeater is a device set up at central location (often high in elevation to optimize radio reception and transmission) that listens for an incoming signal broadcast from HTs at a specified receive frequency and simultaneously rebroadcasts that signal at high power on another send frequency.
Many repeaters around the world are digitally linked such that audio can be sent and received from one to another in real time through the Internet. EchoLink and AllStar are examples of networks and applications that facilitate this. Each repeater is called a node and identified by a unique node number. Using a computer or mobile device with an Internet connection, ham operators can talk over any repeater in the world that is linked in this manner just by specifying the node number for the repeater of interest. But this requires that the ham operator use a computer or mobile device instead of an HT. Efforts have been made to enable amateur radio operators to accomplish both processes using one device the hand-held HT radio to talk over both local and distant repeaters utilizing the Internet node links. Such set-ups have involved connecting the HT to a computer using a proprietary cable that limits the physical mobility of the HT. This project aims to restore the physical mobility of the HT and improve the user experience under these circumstances, making the process of talking to a distant repeater as simple and familiar as switching to different transmit and receive frequencies. Process One goal of this project involves restoring the physical mobility of the hand-held HT radio when it is used for the purpose of talking to distant repeaters over Internet node links. Instead of connecting the HT to a computer using a proprietary cable, the connection should be wireless. In other words, the HT should transmit and receive over its VHF/UHF frequencies as usual. This design goal points towards a RF-IP hotspot transceiver. Eliminate the general-purpose PC and replace it with a small unit that includes an on-board low-power radio transceiver and is run by a microcontroller on a single board. The on-board radio acts like a local repeater to the ham s HT, which makes the process of talking to a distant repeater in this manner no more complicated than setting the HT to transmit and receive on different frequencies. The board is connected to the Internet via an RJ-45 Ethernet cable. Requirements include that the unit be able to receive and transmit at a range of 1000 feet using low (100 mw) and high (1 W) power modes over the VHF and UHF amateur radio bands. It should allow the user to set the on-board radio s frequencies of use (so as not to interfere with other local repeaters). It should be powered by a standard 12-volt connection for ease of use with car batteries. The microcontroller of choice for this project is the Raspberry Pi, which includes Ethernet and USB connections and is powered by 5 volts. The unit is divided into multiple subsystems: radio, audio, USB hub/serial, and power.
The radio subsystem includes a DRA818 chip which outputs the RF signal to an on-board antenna and passes audio in both directions to the USB audio subsystem. A low-pass filter is placed on the antenna output to filter undesired RF harmonics. Filters are also placed on the audio paths between the radio and audio subsystems to reduce noise outside of the audio band of interest. A serial connection to the radio subsystem from the USB serial chip provides the user a means of programming the radio to use specific frequencies. The USB audio subsystem includes a CM108 chip which performs A/D and D/A conversion and sends the data via a USB connection to the USB hub. The USB hub is connected to the Raspberry Pi via another USB connection. The Pi runs the application to connect with the AllStar node of interest. When a remote ham operator speaks, the on-board radio is signaled to transmit to the user s HT via a PTT ( Push-to-Talk ) GPIO signal from the USB audio chip. The power subsystem includes a protection circuit and ensures that power is converted from 12 volts down to the 5 volts needed by all subsystems (except the radio), and the 3.3 volts needed by the radio. 12V-5V conversion utilizes the TPS62130 chip and 12V-3.3V conversion utilizes the TPS62160 chip. These chips were carefully selected so each has output short protection along with over temperature protection integrated within the chip. Each chip has a wide range of input and output voltages (i.e. 3-17 V input and 0.9-6 V output). Since the 12-5V converter was used to power the Raspberry PI and USB subsystem, the chip is capable of providing up to 3A output current. The 12-3.3V converter used to power the DRA818 chip for the radio subsystem can provide an output current up to 1A. During the testing phase, resistor values were adjusted according to get the desired output voltage. Designs began by identifying the subsystems needed to fulfill the performance requirements and then identifying key components for each subsystem (e.g. chips). Circuit design and PCB design was performed using KiCAD software. Each team member was in charge of a different subsystem. Once the subsystems were individually designed, a tally of components was taken
and purchases made through Digi-Key. PCB boards were ordered and manufactured through OshPark. Upon receipt of all components, respective team members assembled their boards via manual soldering at RIT s SMT lab, the proceeded to test for functionality. During testing it was found that not all features worked according to plan, so necessary modifications were made to the circuit designs. At this stage all subsystems were integrated into a single board. The final PCB design was again sent to OshPark for manufacture of the integrated board. Results/Discussion Multiple issues arose during the subsystem testing phase. Due to certain team members inexperience with the process of SMT soldering, it was found that many traces were shorted together on the initial USB audio subsystem board. It had to be resoldered multiple times until integrity of continuity was guaranteed and no shorts remained. Communication between the CM108 audio chip and the Raspberry Pi failed on initial tests. It was determined that the CM108 chip was not operating due to lack of a crystal in the board circuit, which was added in the final revision. The other major problem was that the AllStar node didn t work as intended. Although the software was able to detect all the components, it wasn t able to detect changes in the input. The root cause of this was the fact that the COS pin wasn t configured in the schematics. There were two possible fixes to this problem that were explored. The first solution was to hand solder the components needed to fix the COS line. This did not work and therefore the other solution was explored. This involved making certain changes to the software and creating a custom configuration for USB-Radio rather than using the pre-configured default USB-Radio setup. After some experimenting with the hardware as well as software, the project mostly works as expected. Conclusions/Recommendations The project mostly works as expected. Solving the unexpected problems served as a valuable learning experience to members of the team. One issue that remains to be solved is the fact that the radio seems to be stuck in transmission mode. This is the only requirement that hasn t been met so far. This can be fixed with another revision to the final board. However, due to time and budgetary constraints, it is not possible for our team to do another revision of the final board. Instead, this is a good goal for a future team to meet if the project is continued in a further iteration.
References and Acknowledgements MSD Team 17311 would like to thank Dr. Louis Beato (team guide) and Jim Stefano (project customer) for their logistical and technical guidance and feedback throughout the entire design and construction process. Thanks also goes to Dr. Elizabeth DeBartolo, MSD lead instructor, for her instructive guidance and support. Team Members: Chris Culpepper USB Hub/Serial Subsystem Jerome Glick USB Audio Subsystem, Abstract, Background, Process, Results & Discussion, References & Acknowledgements Syed Ali Kazi Radio Subsystem, Results/Discussion, Conclusions Damodar Adhikari Power Subsystem, Process