Over on YouTube user k2nccvids has posted a short video showing the Logic Trunked Radio (LTR) Analyzer software working with SDR# and the RTL-SDR. Logic Trunked Radio is a type of radio trunking system that uses distributed control channels modulated into the analogue voice channel instead of using just one signal control channel.
The popular trunking decoding software Unitrunker now supports the RTL2832U R820T RTL-SDR directly in its new version. This means that extra SDR receiver software like SDR# is no longer required to use Unitrunker.
You can download the latest version of Unitrunker here. (NOTE: Unitrunker has recently been updated to V2.1 and so the tutorial below may look a little different now)
In a normal radio system, one company (or talkgroup) might use a single frequency for radio communications. However, this is very inefficient as the frequency may not be in use for the majority of the time. In a trunked radio system, a small set number of frequencies are shared between a large number of talkgroups. Each radio receives a special computer controlled control channel. The control channel determines a vacant frequency that a particular talkgroup should use. This helps to make radio frequency allocations more efficient.
Because a talkgroup might switch between various frequencies often, it can make listening to a conversation difficult for radio scanners. Unitrunker can be used to decode the control channel and follow a voice conversation as it hops across various frequencies. With two RTL-SDR dongles you can set up a trunking receiver station with just Unitrunker. What follows below is a tutorial on how to set this up.
Over on YouTube user MrCircuitMatt has uploaded two videos on how he was able to decode a temperature weather sensor using an RTL-SDR and GNU Radio. His videos go through the GNU Radio technical steps as well as the signal encoding theory he used to decode the temperature.
Using an RTL-SDR Clayton Smith was able to reverse engineer his remote controlled ceiling fan. To do this he first used his BladeRF to determine that the remote control was transmitting a signal at 303.747 MHz. He then used a simple GNU Radio flow graph with the RTL-SDR to plot the amplitude of the signal over time which suggested that the signal was using on-off keying. From the plot he was then able to visually determine the bit pattern sent from each button on the ceiling fan remote.
Next he used his bladeRF and another GNU Radio flowgraph to replicate and transmit the the bit pattern which was able to control the ceiling fan from the PC.
Clayton notes that all this reverse engineering was done in half an hour, demonstrating the power of software defined radio.
Update: There is now a newer driver that allows HF tuning without hardware mods via a different mechanism. The new mod seems to work better than this one. Read about it at https://www.rtl-sdr.com/new-experimental-r820t-rtl-sdr-driver-tunes-13-mhz-lower/.
Over on the Reddit RTL-SDR forums user Jengal has posted a modified RTLSDR.dll dll file for SDR# which allows the direct sampling mode to be used on the R820T without the need for any hardware modifications. The modified dll is compiled from keenerds experimental branch of the RTL-SDR driver, which uses code for the no hardware mod written by tejeez. The no hardware direct sampling code was inspired by Anonofish’s discovery where he found that the E4000 based RTL-SDR could tune to AM radio without the need for the direct sampling mod (though this appears to be now patched in the newer rtlsdr drivers).
The direct sampling mod is a hardware modification to the RTL-SDR which allows it to receive HF frequencies between 0-14.4 MHz without the need for an upconverter. It works by connecting an antenna directly to the RTL2832U chip, thus bypassing the tuner. Teejez’s modification tells the RTL-SDR to bypass the tuner in software, allowing antennas to be connected to the normal antenna port. HF reception with the experimental driver is very poor in comparison to the direct sampling hardware mod or an upconverter, but even so Jengal was able to receive AM Radio, an SSB ham radio signal and an HF weather report with a simple longwire antenna.
To use the modified dll, simply download it from this link, rename it to rtlsdr.dll, and replace the original rtlsdr.dll in the SDR# folder. Then connect an HF antenna to the normal antenna port and in SDR# tune to a frequency between 0-14.4 MHz. Next turn ON the RTL AGC option in the configure menu. Jengal replaced the function of the RTL AGC option with the direct sampling mod. He found that best reception occurred when he set the gain to 48 dB.
Over on YouTube Adam, the creator of the LNA4ALL, LNA4HF and UP100 upconverter has uploaded a video showing that the noise that is produced by the RTL-SDR dongle itself can degrade performance when combined with an LNA and/or upconverter.
Most commonly we’ve seen people mount the RTL-SDR dongle together with an upconverter and/or LNA in the same shielded box right next to each other. However, these results show that the RTL-SDR should be shielded separately from the LNA and upconverter for best performance.
The latest version of SDR# has now been updated to include a noise blanker plugin. A noise blanker can reduce impulsive noises like those created by spark gaps and electric motors. Other SDR programs like Linrad and HDSDR have had noise blanker functions for a while so this is a welcome addition to SDR#. Below is a comparison of the noise blanker turned on and off on a noisy CW signal.
To update SDR# to the latest version simply run install.bat again. Take note that updating will remove any plugins you have added to the SDRSharp.exe.Config file so you may wish to save it first.
Noise Blanker Off
[audio https://www.rtl-sdr.com/wp-content/uploads/2014/03/nb_off.mp3]Noise Blanker On
[audio https://www.rtl-sdr.com/wp-content/uploads/2014/03/nb_on.mp3]YouTube user Tiago Sousa shows how he was able to receive weather Radioteletype (RTTY) data using the RTL-SDR, an upconverter and MultiPSK. Weather RTTY is broadcast by weather services such as the Deutscher Wetterdienst (DWD) which is broadcast from Hamburg, Germany. It is intended for people at sea.
Tiago used an RTL-SDR with upconverter and an 11 meter long wire antenna. For the software he used SDR# and piped the audio to MultiPSK.
Tiago has also uploaded a second video showing him receiving Weatherfax with the same setup. Weatherfax is a signal that transmits a fax image showing weather patterns.
