This page documents digital mode experiments and projects from the WZ4JM shack that don’t fit neatly into a single category — ongoing builds, monitoring setups, and things worth trying.
Raspberry Pi Zero WSPR Beacon — 20m
WSPR (Weak Signal Propagation Reporter) is a digital mode designed specifically for propagation research. Transmissions are extremely low power, highly structured, and decoded by a worldwide network of receiving stations that upload spots to WSPRnet.org in real time. A single WSPR transmission carries your callsign, grid square, and power level — nothing more — but that is enough for receiving stations thousands of kilometers away to decode and report your signal.
The WZ4JM WSPR beacon runs 24/7 on 20m using a Raspberry Pi Zero and WsprryPi software, transmitting directly from the Pi’s GPIO pin 7 without any additional transmitter hardware. It is one of the most elegant low-cost RF projects available in amateur radio — a $15 computer acting as a surprisingly capable HF transmitter.
How WsprryPi Works
WsprryPi drives GPIO pin 7 on the Raspberry Pi as a square wave oscillator at the target frequency. The Pi’s clock hardware is capable of generating signals across the HF spectrum with enough stability and accuracy for WSPR, which has a very narrow bandwidth of around 200Hz. A simple low pass filter on the GPIO output removes harmonics before the signal reaches the antenna, cleaning up what would otherwise be a fairly dirty square wave into a usable HF signal.
Output power is in the milliwatt range — typically 10-20mW depending on the Pi model and frequency. That sounds like nothing, but WSPR is designed to work at these levels. A 20mW signal on 20m with a reasonable antenna routinely produces spots from Europe, South America, and beyond. The mode’s sensitivity is extraordinary — stations running a fraction of a milliwatt have been spotted across the Atlantic.
The WsprryPi software handles all the timing and frequency management. WSPR transmissions occur on a strict two-minute schedule synchronized to GPS or NTP time. WsprryPi uses the system clock via NTP for timing, which is accurate enough for reliable WSPR operation.
Hardware
The setup is minimal:
- Raspberry Pi Zero W — handles both the RF generation and WiFi connectivity for NTP time sync and internet reporting
- A simple low pass filter on the GPIO pin 7 output — a handful of toroids and capacitors wound for 20m, reduces harmonics to legal levels
- BNC connector wired to the LPF output
- Short coax run to the end-fed wire antenna
The entire transmitter fits in a small enclosure and draws under 1W from a USB power supply. It runs headless — no monitor, no keyboard — managed entirely over SSH.
Software Setup
WsprryPi is installed directly on Raspbian and runs as a background process. Configuration is straightforward — set your callsign, grid square, transmit frequency, and power level in the command line arguments, and the software handles the rest. On 20m the transmit frequency is 14.0956 MHz, the standard WSPR dial frequency.
To keep the beacon running continuously across reboots a simple systemd service entry handles automatic startup. NTP is configured and synced before the beacon starts transmitting to ensure timing accuracy.
Spots are automatically uploaded to WSPRnet.org by the receiving stations that decode the signal — no upload configuration needed on the transmitter side. Check WSPRnet.org or PSK Reporter and search for WZ4JM to see where the signal is being heard in real time.
Results and Observations
Running 24/7 on 20m produces a useful picture of propagation across the day. Early morning UTC sees consistent spots from Europe as the grey line crosses the Atlantic. Afternoon UTC brings South American and Caribbean spots. Evening and overnight UTC the band closes to DX and domestic spots from the eastern US dominate.
The spot map on WSPRnet gives a clear visual of which paths are open at any given time. Over weeks of continuous operation patterns emerge — which paths are consistent, which are seasonal, and how solar conditions affect reach. It is one of the most informative passive experiments you can run from a fixed station.
The Pi Zero has proven extremely reliable in continuous operation. Running headless with a minimal Raspbian install and no desktop environment, it sits at around 15% CPU utilization and has been up for extended periods without issues.
Further Reading
WSPRnet.org — spot database and map, search WZ4JM to see live results
WsprryPi GitHub — github.com/JamesP6000/WsprryPi — source code and installation instructions
WSPRnet live map — wsprnet.org/map — real time global WSPR activity