RTL-SDR for WSJT-X FT8

Introduction

In the previous post I looked at receiving WSJT-X WSPR using the RTL-SDR (Ref.1). Normally I place my HF antennas outside on my balcony, but due to renovation construction this is not available to me. So I used an Alex Magnetic Loop antenna inside pointing out of the balcony window and even with this impediment, I was able to receive signals on 30m & 40m with a typical SNR of -25dB! In this post I will examine receiving the extremely popular FT8 protocol in the field using the RTL-SDR and a simple 5m wire antenna. I described and simulated this protocol several years ago (Ref.2).

FT8 Block Diagram

Fig.1 FT8 Block Diagram

Figure 1 shows a block diagram of FT8. The user message consists of two call sign fields 2x28bit, 1 grid field 15bit, 1 flag bit and 5 bits special messages = 77bits. A 14bit CRC is added to make 91bits. Parity bits 83 are added to form an LDPC code (174,91). Three Costas sync arrays are added at the beginning, middle and end. Total frame=237bits. Modulation is 8 level 8FSK, so 79x3bit symbols. Tone spacing is 6.25baud for total transmit BW=50Hz in the USB (300,2700)Hz bandwidth (Ref.3).

FT8 Rx Equipment Setup

Fig.2 FT8 Receive Equipment Setup
Fig.3 FT8 Receive Equipment
Fig.4 SDR# Setup FT8 30m
Fig.5 WSJT-X FT8 Decode 30m

Figure 2/3 shows the receive equipment setup. A simple 5m wire antenna of crocodile clips was used, which is connected to a HamItUp 125MHz converter and RTL-SDR. Reception is by SDR# and output is sent via VB cable to WSJT-X FT8 software. Figure 4 shows the SDR# screen for 30m and Figure 5 shows received copy on 30m.

Scicos Simulation

Fig.6 Scicos FT8-8GFSK Modulator
Fig.7 Scicos FT8 Modulator 8 Level Data & Spectrum

Figure 6 shows the Scicos model used to simulate the FT8 modulator. Figure 7 shows the input 8 level data and output spectrum of 50Hz bandwidth about USB audio Fc=1500Hz. The simulation was previously discussed in detail (Ref.2).

Fig.8 YouTube Video RTL-SDR for WSJT-X FT8

Please send your comments, questions and suggestions to:
contact:

YouTube Channel
YouTube Channel

References

#1. – “RTL-SDR for WSJT-X WSPR”
https://jeremyclark.ca/wp/telecom/rtl-sdr-for-wsjt-x-wspr/

#2. – “WSJT-FT8 Modulator – Scicos Simulation”
https://jeremyclark.ca/wp/telecom/wsjt-x-ft8-modulator-scicos-simulation/

#3. – “WSJT-X User Guide”
https://wsjt.sourceforge.io/wsjtx-doc/wsjtx-main-2.6.0-rc5.html

By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.