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:

1. Module Testing - Individual components tested separately
2. PCB Design - Complete integrated design in KiCad
3. Board Assembly - Reflow soldering with custom stencil
4. Firmware Development - STM32 HAL-based software
5. 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

1. Review Documentation - Study the complete project documentation
2. Source Components - Obtain all required components and PCBs
3. Assembly Process - Follow detailed build instructions
4. Testing Procedures - Validate RF performance and functionality
5. 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.