SBHT is a data/digital voice radio capable of greater than 4 watts of power initially on the VHF Ham bands. This project represents a comprehensive approach to building a handheld digital radio system with support for multiple modulation schemes and protocols.
Project Overview
Core Concept
The SBHT (Small Battery Handheld Transceiver) is designed as a compact, battery-powered VHF digital radio system. Based on the STM32F4 microcontroller and Analog Devices ADF7021-N transceiver, it provides a platform for digital voice and data communications in the amateur radio bands.
Key Features
- STM32F4 Based - Powerful ARM Cortex-M4 processor for digital signal processing
- ADF7021-N Transceiver - Low-power RF transceiver with flexible modulation support
- 4+ Watts Output - Capable of greater than 4 watts of RF power output
- VHF Band Operation - Designed for 144MHz - 148MHz amateur radio band
- Digital Voice Support - Compatible with DSTAR, Nexedge, DMR protocols
- Data Modem Capability - Support for telemetry and data transmission
- Battery Powered - Compact design suitable for handheld operation
Technical Specifications
Hardware Components
- Microcontroller: STM32F405VG ARM Cortex-M4
- Transceiver: Analog Devices ADF7021-N
- Power Amplifier: RA07M1317M VHF PA (10W capable)
- Display: OLED SSD1306 for user interface
- GPS: Neo GPS module for location services
- Storage: SPI Flash for firmware and data storage
RF Characteristics
- Frequency Range: 144MHz - 148MHz (VHF amateur band)
- Output Power: >4 watts (limited by phase noise to ~3 watts in testing)
- Modulation: GMSK, C4FM, 4FSK support
- Protocols: DSTAR, Nexedge, DMR (with external AMBE codec)
- Antenna: Standard VHF antenna connector
Power Management
- Battery Operation - Designed for handheld battery power
- Low Power Design - ADF7021-N optimized for battery operation
- Adjustable PA - Power output controlled by STM32F4
- Efficient Design - Compromise between size and performance
Design Philosophy
Size vs Performance
The SBHT design represents a careful balance between compact size and radio performance. Key design decisions include:
- Integrated Design - All major components on a single PCB
- Modular Approach - Initially built and tested as separate modules
- Battery Optimization - Low power consumption for extended operation
- Handheld Form Factor - Small enough to carry comfortably
Protocol Flexibility
The radio supports multiple digital protocols and modulation schemes:
- DSTAR - Digital Smart Technologies for Amateur Radio
- Nexedge - NXDN digital protocol
- DMR - Digital Mobile Radio (requires external AMBE codec)
- Custom Protocols - TDMA-based repeater and experimental protocols
- Data Modes - Telemetry and general data transmission
Development Process
Hardware Development
The project progressed through several phases:
- Module Testing - Individual components tested separately
- PCB Design - Complete integrated design in KiCad
- Board Assembly - Reflow soldering with custom stencil
- Firmware Development - STM32 HAL-based software
- Testing and Optimization - RF performance validation
Firmware Architecture
- STM32 HAL Integration - Modified Cube library for specific requirements
- ADF7021-N Driver - Complete transceiver control software
- OLED Display Driver - User interface with fonts and bitmaps
- GPS Integration - Location data embedded in transmissions
- Protocol Support - Multiple digital voice and data protocols
Technical Challenges
RF Design Considerations
- Phase Noise - Limited output power due to ADF7021-N phase noise
- Spurious Emissions - Careful filtering required for clean output
- Power Amplifier - External PA required for higher power levels
- Antenna Matching - Proper impedance matching for efficiency
Software Development
- HAL Abstraction - STM32 HAL required modifications for specific needs
- I2C Communication - OLED display interface debugging
- SPI Flash - Data storage and firmware management
- Real-time Processing - Digital signal processing requirements
Manufacturing Challenges
- Component Sourcing - Specialized RF components and connectors
- Assembly Process - Reflow soldering with proper thermal profiles
- Testing Procedures - RF performance validation and calibration
- Documentation - Comprehensive build instructions and support
Applications and Use Cases
Amateur Radio
- Digital Voice - DSTAR, DMR, and other digital protocols
- Data Communications - Telemetry and general data transmission
- Emergency Communications - Reliable digital voice in emergency situations
- Experimental Use - Testing new protocols and modulation schemes
Professional Applications
- Telemetry Systems - Remote data collection and transmission
- Industrial Communications - Reliable digital voice for industrial use
- Research and Development - Platform for RF and digital communications research
- Educational Use - Learning digital radio and RF design principles
Advanced Features
- TDMA Repeater - Time-division multiple access repeater operation
- GPS Integration - Location-aware communications
- Data Logging - SPI flash storage for transmission logs
- Protocol Development - Platform for experimental protocols
Project Impact
Community Response
The SBHT project gained significant attention in the amateur radio and electronics communities:
- 10.6k Views - High visibility on Hackaday.io
- 2.5k Followers - Strong community interest
- 48 Likes - Positive reception from the community
- Open Source - Gerber files and firmware available on GitHub
Technical Contributions
- Open Source Design - Complete hardware and software available
- Educational Value - Comprehensive documentation and build instructions
- Protocol Support - Multiple digital radio protocols in one device
- Innovation - Novel approach to handheld digital radio design
Lessons Learned
Hardware Design
- RF Layout - Critical importance of proper RF PCB layout
- Component Selection - Choosing appropriate components for power and performance
- Thermal Management - Heat dissipation considerations for high-power operation
- Manufacturing - Design for manufacturability and assembly
Software Development
- HAL Limitations - Abstraction layers often need modification
- Real-time Requirements - Digital radio requires precise timing
- Protocol Complexity - Multiple protocols increase software complexity
- Testing Procedures - Comprehensive testing essential for RF systems
Project Management
- Documentation - Thorough documentation crucial for open source projects
- Community Engagement - Active participation in project discussions
- Iterative Development - Multiple revisions needed for optimization
- Knowledge Sharing - Open source approach benefits entire community
Future Development
Potential Improvements
- UHF Version - Adaptation for UHF amateur bands
- Higher Power - External VCO design for improved phase noise
- Enhanced Protocols - Support for additional digital protocols
- Improved UI - Enhanced user interface and display capabilities
Technical Enhancements
- Better Filtering - Improved RF filtering for cleaner output
- Power Optimization - Further reduction in power consumption
- Size Reduction - Even more compact form factor
- Feature Additions - Additional capabilities and interfaces
Getting Started
For Builders
- Review Documentation - Study the complete project documentation
- Source Components - Obtain all required components and PCBs
- Assembly Process - Follow detailed build instructions
- Testing Procedures - Validate RF performance and functionality
- Firmware Installation - Load and configure the software
Resources
- GitHub Repository - Complete source code and documentation
- Hackaday.io Project - Project updates and community discussion
- Build Instructions - Step-by-step assembly guide
- Community Support - Active community for questions and support
This project represents a significant achievement in amateur radio digital communications, combining modern microcontroller technology with RF design expertise to create a versatile and capable digital radio system.