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Following up on the success of the low-power CW beacon built by Steve G4AQB, Ross G6GVI has been working on another 2m beacon transmitter, now incorporating a satellite receiver to include time and location in each message and using Frequency-Shift Keying to transmit more data at a faster rate.
This beacon operates as G0BWC around 144.021MHz, transmitting just 15 milliwatts three times each minute. First it sends 4FSK "Horus Binary v2" modulation, followed by a message in CW Morse (around 20WPM).
The 4FSK can be decoded using the HorusGUI software, set for 50 baud and 244Hz tone spacing and the program will automatically calculate the distance, bearing (and elevation) from your receiver. If you can't get this to run under Windows11, try the experimental web-based decoder: https://horus.sondehub.org/
The Morse message is simply "G0BWC ALT nnnnn M", where nnnnn is the altitude in metres.
Please use the form below to report your reception of the beacon.
Each telemetry frame (32 bytes) consists of the following elements:
Callsign, frame_number, time, latitude, longitude, altitude, speed, satellites, CPU_temp, CPU_voltage, then four null fields, checksum.
If the "Upload to Sondehub" box is ticked, the decoder will automatically send the received packet to the Sondehub Amateur site, along with your received Signal-to-Noise Ratio and frequency (if you've entered your Radio Dial Freq, in the panel). Please don't enable this feature whilst we're testing on the ground (instead you could Enable Logging in the "Other" tab and save a local .CSV file).
This Horus Binary 4FSK mode has been popular on 70cm for High-Altitude Balloon flights in Australia and around continental Europe over recent years, but we believe that this is the first time it's ever been deployed on 2m.
The frequency reference is just a crystal, so the transmission will drift over hundreds of Hertz as the temperature changes. The radio is designed for use at 433MHz, so a custom low-pass filter (see below) has been fitted to reduce spurious harmonic emissions when operating at 144MHz.
The Mk1 prototype (based on an ESP32 micro) was assembled from left-over bits from other projects plus an atmospheric sensor to measure temperature, pressure and humidity. This version originally transmitted alternating 2FSK (RTTY) and 4FSK (Horus Binary) formats at 100 bauds, but has since swapped from RTTY to CW alongside the 4FSK.
Later, this test-jig was adapted to run with the smaller Arduino Pro Mini, now transmitting just its position (without any atmospheric sensor data) using 50 baud 4FSK, plus Morse. Finally the code was ported onto one of M7TXF's lightweight miniature "TinyThing" boards, for further testing on the ground and ultimately in the air! After rebuilding the low-pass filter on a smaller board (using SMD inductors and an MCX socket), the weight is less than half an ounce (without antenna or battery).
Original Mk1 prototype (ESP32 )
Arduino Pro Mini development platform
TinyThing version
Output spectrum of 433MHz radio operating on 2m
Output spectrum with low-pass filter fitted
After fitting the low-pass filters, the beacons have been tested with several designs of lightweight "halo" antennas (omnidirectional horizontal), which gives distant stations with high-gain beam antennas a better chance of receiving than the vertical dipole used in our earlier flight. These halos were built from either piano wire or copper foil tape on a cardboard support and as well as being lightweight, they are designed to have "low impact" when descending (under a parachute) from a high-altitude flight.
The Mk1 beacon operating into a piano-wire halo
A prototype copper foil tape halo
VSWR measurement of foil tape halo
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