2m FSK beacon

Following up on the success of the low-power CW beacon built by Steve G4AQB, Ross G6GVI has been working on miniature 2m beacon transmitters, 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.

As well as the well-known "RTTY" (2FSK), there is a more advanced "Horus Binary" (4FSK) modulation, which can be decoded using the HorusGUI software: this program will automatically calculate the distance, bearing (and elevation) from your receiver. If you can't install the program, try the new browser-based decoder: https://horus.sondehub.org/

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).

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 it has not yet been deployed from the air on 2m.

If the frequency reference is just a crystal, the transmission will drift over hundreds of Hertz as the temperature changes, but the newer boards use a TCXO for better stability. The Ra-02 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 prototype (based on an ESP32 and Ra-02) was assembled from left-over bits from other projects, plus a BME280 atmospheric sensor to measure temperature, pressure and humidity.

Output spectrum of 433MHz radio operating on 2m

Output spectrum with low-pass filter fitted

This hardware is too heavy to fly under a little balloon, but I've also assembled a lightweight version, using a TinyThing board (Arduino Pro Mini), which operates as G0BWC, sending Horus V2 at 50 baud, plus a Morse message giving its callsign, locator and altitude.

A new version is being developed, using an ESP32-S3-Tiny and EByte VHF (170MHz) module to transmit 0.5W in the 2m band. For initial ground-based tests this will operate in the "personal beacons" allocation around 144.492MHz, sending 100 baud RTTY (7N2, with 240Hz shift), followed by a CW message.

This still comes in under 15g including the output filter (see below).

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