HOPE in a Bottle
Jack Kang, James Lukas, Yeonwoo Son
dongwon900@berkeley.edu, jameslukas@berkeley.edu, yson630@berkeley.edu
Overview
HOPE in a Bottle is an amateur radio WSPR (Weak Signal Propagation Reporter) transmitter designed for deployment in a buoy.
What is WSPR?
WSPR is a digital radio communication protocol designed for weak-signal communication for amateur radio. The protocol is designed for exchanging low-power transmissions in order to test propagation on MF and HF amateur bands.
Receiving stations with internet access can upload their reception data to a central database/website called WSPRnet, which can display received stations in a map. We can use this as a tool to track our beacon.
WSPR is a "F1D" type of radio emission, which means that it is a frequency modulated, single-channel (no subcarrier) signal containing data information. This simple encoding scheme allows us to use a simple synthesizer chip and power amplifier to implement a transmitter.
A WSPR transmission is a time-synchronized signal containing a message in the form of <callsign> + <4 maidenhead grid locator> + <dBm transmit power>. For example, the message "K4NGD CM87 20" would mean that the operator K4NGD is transmitting at grid location CM87 with +20dBm of power (100mW).
MCU (ESP32-S2-SOLO)
We are using the ESP32-S2-SOLO as the MCU for this project, and it is the central control unit that handles GPS data, message encoding, and control of the synthesizer chip.
The implementation is a bare-bones setup that features boot and reset buttons, USB receptacle for the onboard USB interface, power LED, and a 3.3V LDO that we also use for other components that require a 3.3V supply.
GPS (Adafruit Ultimate GPS PA1616D)
A GPS receiver is critical in the operation of this device, as WSPR transmission requires accurate time synchronization and location data. WSPR transmissions are just short of two minutes long, and start one second after each even minute of UTC.
We are using the PA1616D, a 33 tracking, 99 acquisition-channel GPS/GLONASS/GALILEO receiver that features an on-board patch antenna. We have included an LED for the GPS FIX signal for ease of use (it blinks at 1Hz until it locks onto a satellite). The GPS receiver is connected to the ESP32 via one of its 2 UART interfaces.
RF Backend (Si5351 + BS170 PA + LPF)
At the core of our RF backend is a versatile Si5351A-B-GT synthesizer chip, which is an i2C clock generator that has 3 independently configurable channels that can generate frequencies up to 200MHz. We are only using CLK0 (first channel) for our application, but the rest of the channels have been made accessible for flexibility. The Si5351 is connected to the ESP32 via I2C.
The power amplifier section has the option to have up to 3x BS170 MOSFETs in cascade for increased power, but the user may choose to include less. A variable resistor allows for adjustable bias, and an RFC (FT37-43 toroid with 25 turns of enameled copper wire) provides the MOSFETs with the supply voltage.
We use the LPF modules from QRP Labs, which makes swapping out filters very easy; all the user needs to do to operate at any frequency band is to modify the code to alter the operating frequency, and replace the filter module with one that is designed for the desired frequency band. We are using a module designed for 20m (14MHz) in our demo.
BOM
Testing
Initial testing of the device was done with an oscilloscope, making sure that the output signal was a clean sine wave at the desired frequency. We also checked the spectrum analyzer to make sure that the LPF was successfully filtering out unwanted emissions.
A steady, clean 14MHz signal output from the transmitter.
We further tested the integrity of the output signal with the FlexRadio Flex-6500, a high-performance SDR HF/50MHz transceiver. For ease of testing, we simply transmitted into a 50-ohm dummy load near the transceiver, which was enough for the signal to couple to the transceiver feedline.
We used WSJT-X, a PC application initially created by Joe Taylor, K1JT (the creator of WSPR), for use in modulation and demodulation of various digital modes including WSPR. We were able to successfully demodulate the signal coming out of our device.
Our signal in the spectrum
WSJT-X successfully receiving our WSPR message
Test antenna consisting of 64:1 un-un transformer and a long (~85ft) wire
The singal output from the Si5351 is inevitably inaccurate to a certain degree, and is subject to frequency errors on the order of a couple 100Hz. To mitigate this, we used the aforementioned test setup (RX-SDR, TX-our device) to check the actual output of the device in the frequency spectrum. We fine-tuned the output frequency so that it sat right in the middle of the USB bandwidth of 2.8KHz at 14.0956MHz (the commonly accepted dial frequency for WSPR on the 20m band).
With a simple wire antenna and ~0.5W of power, we were able to get our signal out to as far as New York and Hawaii, which are both almost 2500 miles away from Berkeley, CA!
Next Steps
With our transmitter working as intended, our next step is to verify that the transmitter is able to operate under all conditions without exhibiting unwanted behaviors--this is a very important step as once the beacon is deployed, we will be unable make modifications to the device in any way.
Another important step is to design an enclosure (vessel) and antenna that is suitable for deployment as a buoy/beacon, as well as figuring out deployment logistics--the vessel must be launched far enough out from the shore such that it is able to wander into the ocean without washing back.