This presentation details the design, implementation, and testing of a homemade, coherent multi-antenna shortwave software-defined radio (SDR) receiver for direction finding. The project was motivated by scaling a previous QDX-inspired single-channel switching mixer design into a four-channel system. The hardware is built around four stereo ADCs and an FPGA on a repurposed controller board, with a single clock synthesizer ensuring perfect coherence between all channels. During a field day, the system was connected to a four-square vertical antenna array to record 20-meter FT8 signals.
The core of the project lies in its offline signal processing, which uses GNU Octave to perform an eigenvalue decomposition of the 4x4 signal correlation matrix for each detected station. The resulting eigenvector, which represents the phase relationship of the incoming wavefront across the array, is used to compute a beam pattern whose maximum indicates the signal's direction of arrival. Experimental results show high accuracy in determining the direction of European stations and reveal complex, varying propagation paths for a transatlantic signal. Future work includes implementing real-time maximum ratio combining (MRC) to improve SNR and using the derived phase data for transmit beamforming.
A Coherent Multi-Antenna SDR for Shortwave Direction Finding
0:01:17 Project Origin: The project was inspired by a previous QDX-based switching mixer SDR. The goal was to scale the design to a multi-antenna system capable of coherent reception for direction finding and beamforming, similar to a phased array.
0:05:37 Coherent Hardware Design: The receiver uses four separate stereo ADCs (Texas Instruments PCM1820) and four switching mixers. A single clock synthesizer (SI 5351A) drives all channels, ensuring perfect phase coherence for both the local oscillator and the ADC sample clock. An FPGA on a repurposed Bitcoin mining controller board (IBATS 2405) aggregates the four data streams.
0:09:43 Custom Software Development: To interface with the hardware, the presenter developed a custom 32-bit IIO source block for GNU Radio and updated the Linux driver for the SI5351A frequency synthesizer to support quadrature clock generation, which is essential for the switching mixers.
0:15:41 The Field Day Experiment: For real-world testing, a four-square vertical antenna array was deployed at a field day. The receiver was housed in an enclosure and powered remotely using Power over Ethernet (PoE). It was used to record FT8 signals on the 20-meter band for later offline analysis.
0:19:56 Advanced Signal Processing Method: The direction finding algorithm processes the recorded signals in GNU Octave. It calculates the auto- and cross-power spectral densities between all antenna pairs to form a 4x4 correlation matrix for each detected FT8 station.
0:23:17 Eigenvalue Decomposition for Direction Finding: The system performs an eigenvalue decomposition on the correlation matrix. The eigenvector corresponding to the largest eigenvalue represents the phase shifts of the incoming signal across the antenna array. This vector is then used to calculate a beam pattern, with the pattern's maximum pointing to the signal's direction of arrival.
0:31:09 Accurate Directional Results: The system successfully determined the direction of various European stations, with results for signals from Albania, Spain, and Italy showing errors as low as 3-5 degrees compared to the true direction calculated from Maidenhead locators.
0:33:46 Observing Complex Propagation: An FT8 signal from Texas, USA, exhibited a direction of arrival that varied significantly over just a few minutes. This demonstrates the system's ability to visualize the dynamic and complex nature of ionospheric propagation on long-distance paths.
0:35:30 Future Enhancements: Future plans include implementing Maximum Ratio Combining (MRC) in the FPGA to coherently combine signals and improve the signal-to-noise ratio (SNR) in real-time. The derived phase information could also be used for transmit beamforming, directing a transmitted signal back toward the source.
0:42:19 Broadband Operation Limitations: In the Q&A, the presenter clarifies that while the array can work on multiple bands, it is only optimal when the antenna spacing is a quarter-wavelength. On other bands, the beam pattern may become ambiguous or overly broad, reducing gain and directional accuracy.