FT8 on Dhiru's Notebook

Pocket Chaos: Coding (and Decoding) on the R36S

Tue, 05 May 2026 00:00:00 +0000

Intro

There's something slightly absurd about turning a tiny retro handheld into both an app development platform and a weak-signal radio decoder. The R36S - cheap, hackable, and rough around the edges - wasn't built for any of this.

That's exactly why it works.

The Constraint Advantage

Modern development hides inefficiency. The R36S exposes it.

With limited CPU, RAM, and screen space, you're forced to:

Write tighter code
Design simpler interfaces
Think in constraints, not abstractions

That same constraint-driven thinking applies perfectly to signal processing tasks like FT8.

Headless WSPR Monitoring System (2026)

Wed, 22 Oct 2025 00:00:00 +0000

Setup OS and SDR

Please use the latest Raspberry Pi OS 64-bit on RPi 4 or later.

Using a Raspberry Pi (<=3) is NOT recommended as it has quite a bit of power supply noise on the USB ports, which reduces the performance of the connected SDR.

user@rpi:/etc$ cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 13 (trixie)"
NAME="Debian GNU/Linux"
VERSION_ID="13"
...

Install dependencies:

sudo apt-get update

sudo apt-get install airspyhf \
 soapysdr-tools sox libsox-fmt-all wsjtx \
 pulseaudio-utils git vim fluxbox xterm tightvncserver \
 libqt5websockets5 cmake build-essential

sudo apt-get install cmake pkg-config \
 libusb-1.0-0-dev \
 libasound2-dev \
 libairspy-dev \
 libairspyhf-dev \
 librtlsdr-dev \
 libsndfile1-dev \
 portaudio19-dev \
 libvolk-dev

Build airspyhf-fmradion SDR software:

Inbuilt band-hopping in WSJT-X 3.x

Sat, 18 Oct 2025 00:00:00 +0000

WSJT-X 3.x has inbuilt support for band hopping!

Results

RX results from an experimental version of DDX:

This is super useful for monitoring FT8 traffic on multiple bands.

References

Emulating a full FT8 band for 'real'

Fri, 17 Oct 2025 00:00:00 +0000

This code generates a .wav file which emulates a fully-populated FT8 band!

Usage

This .wav file can then be played with rpitx on actual air (well on a dummy load) for receiver testing purposes.

rm mixed.wav; sox -m *.wav mixed.wav

sox mixed.wav -r 48000 sampleaudio.wav

Now play sampleaudio.wav using rpitx.

Testing

I have tested rpitx on Raspberry Pi Zero 2W TX'ing @ 14.074 MHz.

FT8 Decoding Challenges on Android

Mon, 29 Sep 2025 00:00:00 +0000

Here is the FT8 WAV test file.

On an AMD64 machine:

1 second using the fast decode settings.

We were able to get the MSHV's FT8 engine to work on Android, thanks to the work done by https://github.com/sannysanoff/SDRPlusPlusBrown folks.

Around ~4.5 seconds to decode the same file.

This is why we can't really have full-blown realtime FT8 decoder(s) on Android just yet!

Idea: But we can surely use this full-blown FT8 decoder in our FT8 Decoder app which has no realtime requirements!

Black Box output characterization of CD2003 DCR receiver

Sun, 21 Sep 2025 00:00:00 +0000

Circuit: DDX-Commercial-7

Antenna: Small jumper cable attached

Initial Results

10m: 36 dB on WSJT-X audio meter, on quiet and also with nearby beacon on !!!

12m: 36 dB on audio meter, on quiet and also with nearby beacon on !!!

15m: 46 dB on audio meter on quiet, 65 dB with nearby beacon on - OK

20m: 44 dB on quiet, 65 dB with beacon on - OK

40m: 40 dB on quiet, 60 dB with beacon on - OK

Running Vivado 2025.1 on Ubuntu 25.04 (plucky)

Sun, 21 Sep 2025 00:00:00 +0000

Notes to get Vivado 2025.1 running on Ubuntu 25.04 (plucky) Linux distribution.

Original reference: QMTech XC7Z020 Notes

Setup

Install dependencies:

sudo apt-get update

sudo apt-get --no-install-recommends install \
 bc binfmt-support bison build-essential ca-certificates curl \
 debootstrap device-tree-compiler dosfstools flex fontconfig git \
 libncurses-dev libssl-dev libtinfo6 parted qemu-user-static \
 squashfs-tools u-boot-tools x11-utils xvfb zerofree zip

Hack deps a bit:

sudo ln -s /lib/x86_64-linux-gnu/libtinfo.so.6 /lib/x86_64-linux-gnu/libtinfo.so.5

You can now go ahead and run the Vivado installer.

Initial success with cost-effective 6m WSPR + 2m FT8 beacons

Sat, 16 Aug 2025 00:00:00 +0000

Our 6m WSPR beacon design works fine now. The key is to use a 25 MHz high-quality TCXO, like the 25 MHz HCI 0.5ppm TCXO!

2m WSPR is still out of reach for this VFO design - see the following drifty screenshots:

While WSPR is no good, 2m FT8 works pretty fine!

WiFi VFO, beacon and signal generator

Sat, 05 Jul 2025 00:00:00 +0000

We make heavy use of Easy-Digital-Beacons-v1 for testing RF amplifiers.

This single board is a versatile WiFi VFO and FT8 / FT4 / WSPR beacon.

You can also use our CW-SigGen project to generate a suitable test signal.

Ideas for a 2m (144 MHz) WSPR / FT8 beacon

Tue, 15 Apr 2025 00:00:00 +0000

This is an early design sketch of a 2m WSPR beacon.

Key Points

25 MHz HCI 0.5ppm TCXO powering the Si5351 module instead of the 26 MHz TCXO. If this fails, we can use a 10 MHz OCXO instead (a bit out of spec but works fine).
Tokmas CID10N65F GaN FET might work @ 144 MHz as the 'final'. If not, use Mitsubishi RD15HVF1-501 MOSFET as backup.
2SK3475 as the driver with variable DC bias at the gate
The whole VCC to this amplifier will be PTT switched
Ideally, we would want to keep the driver stage linear but the final stage as switched (Class C/D) Alternate: Perhaps by DC coupling the Si5351 input to 2SK3475, we can avoid the need for DC gate bias
The "RFC" (bifilar coil still needed?) part will need to be figured out for VHF!

Status

Results: To be built and tested soon!

100mW WSPR Beacon (2025)

Fri, 14 Feb 2025 00:00:00 +0000

I have built what is probably the easiest, reasonably rugged, and most cost-effective 100mW WSPR (or FT8) beacon.

The Build

Here is my quick and dirty build with whatever I had lying around:

We replaced the traditional expensive Mini-Circuits transformer with a cost-effective Bourns coupled inductor.

Analysis

Power consumption: Before RF generation starts ➔ 20mA @ 5V. After RF generation starts ➔ 52mA @ 5V. So the input power consumed in the RF generation process is more than 150mW (which is good).

Easy FT8 Encoder

Thu, 13 Feb 2025 00:00:00 +0000

The Easy FT8 Encoder can be accessed here.

In the near future, we want to add the K1JT's original FT8 encoder functionality as well.

Update: FT8Play is crazy good!

My PA0FRI Active Antenna

Sat, 08 Feb 2025 00:00:00 +0000

My homebrewed PA0FRI Active Antenna is working well from a very physically constrained location.

Design

3D render:

Schematic:

Results

The antenna is connected to an RSP1 Clone SDR running custom FT8 band-hopping software. I am using a SOLID 4-Way 5-2400 MHz RF splitter to connect multiple SDRs to the same active antenna. NB: Plug in 75-ohm dummy loads (terminators) on the unused splitter ports; MX-282 / MX-280 75-ohm loads work fine.