Decode 433.92 MHz weather station data

I bought a simple weather station, with temperature and humidity display for indoor and up to 3 wireless outdoor sensors (sold with only one though). Unfortunately, the one I got, had a non-functional outdoor sensor. I took it back to the store and got a replacement for the entire product. The outdoor sensor can be hanged on a wall with a small screw, but it can easily fall down.

Given the facts I initially got a broken sensor, the weather station can receive data from up to 3 sensors and because sensors are exposed to outdoor conditions, I decided to analyze the wireless protocol and maybe build my own device which will be able to emulate this kind of sensor. There is no information about wireless protocol of this weather station (sold by Lidl in Europe under the Auriol brand), except the frequency: 433.92 MHz. That was all I needed. And a software defined radio (I used RTL2832U dongle).

Receive FT8 with WSJT-X and Gqrx (Linux)

FT8 has become the most popular data mode for ham radio. It is a digital mode created by Joe Taylor, K1JT and Steve Franke, K9AN which uses 8-FSK modulation. A transmission lasts for exactly 12.64 seconds and occupies about 50 Hz bandwidth (8 tones at 6.25 Hz spacing). FT8 works very well even with a lot of noise and it is simple to receive and transmit using the computer soundcard and SSB transceiver.

This post will focus on receiving only. We'll decode FT8 messages using HackRF hardware with software defined radio (SDR) application. However, SDR tools come with support for common analog modulation. For FT8 decoding and generation there is WSJT-X software, but this one accepts only audio input. So, I'll be using a SDR application (Gqrx) set to SSB (upper side band) demodulation and I will pipe its audio output to WSJT-X. The latter can be configured to automatically set the frequency of the SDR software. Besides configuration, a virtual audio cable is needed.

Receive FT8 with WSJT-X and SDRSharp (Windows)

FT8 has become the most popular data mode for ham radio. It is a digital mode created by Joe Taylor, K1JT and Steve Franke, K9AN which uses 8-FSK modulation. A transmission lasts for exactly 12.64 seconds and occupies about 50 Hz bandwidth (8 tones at 6.25 Hz spacing). FT8 works very well even with a lot of noise and it is simple to receive and transmit using the computer soundcard and SSB transceiver.

This post will focus on receiving only. We'll decode FT8 messages using Realtek RTL2832U hardware with software defined radio (SDR) application. However, SDR tools come with support for common analog modulation. For FT8 decoding and generation there is WSJT-X software, but this one accepts only audio input. So, I'll be using a SDR application (SDR# on Windows, Gqrx on Linux) set to SSB (upper side band) demodulation and I will pipe its audio output to WSJT-X. The latter can be configured to automatically set the frequency of the SDR software. Besides configuration, a virtual audio cable is needed, as well as virtual serial ports (on Windows only).

Lightweight ACARS decoders for RTL-SDR

ACARS is a digital data link system for the transmission of messages between aircraft and ground stations. The transmissions usually take place in VHF band, between 130 and 137 MHz. The wide availability of cheap software defined radios makes it easy for anyone to receive and decode ACARS transmissions. Since RTL2832U based USB sticks are the cheapest SDR hardware this is what I will use for reception. There are many tutorials on this subject and most of them involve piping an audio stream from a SDR application to an ACARS decoder application. This is not reliable and takes a lot of resources. More than that, by selecting one frequency, demodulating it and sending the audio stream to ACARS decoder software you are limited to that single frequency.

But there are a few tools that work directly with RTL-SDR hardware, are free, cross-platform and perform ACARS demodulation. More important than that, the software tools you will see here can listen to multiple frequencies at once. The only requirement is that those frequencies should fit in the bandwidth of RTL-SDR (about 2.4 MHz).

Receive DRM Radio on Linux with Gqrx and Dream

DRM (Digital Radio Mondiale) is the universal, openly standardised digital broadcasting system for all broadcasting frequencies, including the AM bands, as well as VHF Bands. The great flexibility of DRM supports all types of coverage needs – from local, regional, nation-wide to international. If you want to listen to DRM broadcasts, you need a dedicated receiver or a software radio. This post will focus on the cheaper method, the software defined radio (SDR).

SDR performs demodulation in software. The RF signal is sampled by a high speed ADC (analog-digital converter) and sent to a computer which does the filtering, amplification, demodulation and decoding of the signal. The cheapest hardware for SDR is the popular RTL2832U based DVB-T stick.

Most DRM broadcast can be found in SW bands. Here comes an important issue. The tuner from the DVB-T stick can't receive this band. One of the solution is to add a frequency downconverter. Or you can simply disable the tuner and feed RF straight to the ADC. I wrote about RTL2832U direct sampling in this post. This method has some limitations, but nevertheless it should provide satisfactory results.

Dream decoding a DRM broadcast

FM Radio Player for RTL2832U sticks

USB TV sticks using RTL2832U chip are well known for their ability to send raw samples to USB host. This makes it possible to process various signals in software (SDR). There are many software tools for this purpose and most of them can demodulate also FM radio, with stereo and RDS support.

The problem is that these applications have full featured SDR abilities, with spectrum analysis and waterfall display. This makes them not at all lightweight. I've been looking quite hard for a simple FM radio player for my RTL-SDR dongle.

Some of the RTL2832U dongles come with FM capable player on the CD. Unfortunately not all. After some searching I was able to find a simple tool that demodulates FM stereo signal and displays RDS data. The application was most probably a test tool designed by Realtek to illustrate features of RTL2832U.

Heatsink for RTL-SDR dongle

Some of the popular RTL2832U based DVB-T dongles that can be used as SDR tend to heat too much. As temperature increases, frequency drifts and sensitivity drops. Some dongles, especially those using R820T may stop working at all at high temperatures.

Marko Cebokli (S57UUU) used a thermal camera to measure the temperature of the dongle. He measured up to 85°C at the tuner chip (R820T). His images prove also that the dongle PCB doesn't do a good job at dissipating heat.

Although I'm using a R820T2 dongle (which is said to heat less than the predecessor R820T), I was surprised to see that after only 10-15 minutes the dongle was hot. Opening it up, I discovered that all ICs were hot (the tuner R820T2, the RTL2832U, the 1117 regulator, even the EEPROM was heated inside the closed case of the dongle).

Turn RTL-SDR dongle into RTL2832U breakout board

The most popular RLT2832U dongles are the ones with E4000 and R820 tuners. This is due to the high frequency range these tuners have. Elonics E4000 covers 52 to 2200 MHz with a gap at about 1100 MHz and Rafael Micro R820T covers 24 to 1766 MHz.

But there are also RTL2832U based dongles with other tuners. One example is FCI FC2580 tuner (found in Trust 16738 dongle) which is capable of receiving 146 - 308 MHz and 438 - 924 MHz, limiting its use to DVB-T only! It can't even receive FM radio 88 – 108 MHz.

If you’re having such a dongle don’t throw it away. There are situations when you don’t need a tuner at all (the direct sampling mode of RTL SDR that allows direct reception of signals on frequencies lower than 28 MHz). There is also the possibility of changing the existent tuner with, for example, a satellite receiver tuner that will receive 950 to 2150 MHz.

So, I wanted a universal RTL2832U breakout board that I could easily connect to with different tuners. This is how the USB dongle board looked like.

RTL2832U dongle