In his last video YouTube user GusGorman402 showed us how to install OpenWRT and the RTL-SDR drivers on a cheap used $20 router. The idea is that the router with custom third party Linux firmware can be used as a remote device for streaming raw data from an RTL-SDR over a network connection. Normally something like a $35 Raspberry Pi is used for something like this, but an old router could be cheaper and should have even better network performance as it is designed for high data rates (assuming the CPU on your router is powerful enough to run the RTL-SDR).
In his new video Gus shows how to properly configure OpenWRT and RTL_TCP for WiFi streaming of radio data. This includes things like setting up port forwarding and determining network performance.
We’ve also seen this post by GoJimmyPi which was inspired by Gus’s original video. This is a text and screenshot based tutorial which goes through the same process.
Over on his blog John Hagensieker has uploaded a tutorial that shows how to set up SDRTrunk with RTL-SDR dongles. SDRTrunk is an application that allows you to follow trunked radio conversations, and decode some digital voice protocols such as P25 Phase 1. It is similar to Unitrunker and DSDPlus combined into one program. It is also Java based so it is cross platform and so can be used on Linux and MacOS systems as well.
John’s tutorial contains many useful screenshots, so it should be great for a beginner. He starts from the beginning, with finding trunking frequencies over on radioreference.com, then goes on to the installation and use on Linux. He also later explains how the Airspy can be used instead of multiple RTL-SDR to cover 10 MHz of bandwidth so that multiple systems can be monitored.
SDRTrunk Running and decoding a P25 Phase 1 System
Back in 2016 KD0CQ discovered that a $5 DirectTV device with model name SUP-2400 could be used as a downconverter with an RTL-SDR for receiving frequencies of up to 4.5 GHz. A downconverter is a device that converts high frequencies such as 4.5 GHz down into a frequency actually receivable by the RTL-SDR. The SUP-2400 is able to be modified into a useful downconverter by opening it up and manually removing some mixing harmonic filters.
In his latest post KD0CQ shows how you could combine four modified SUP-2400 downconverters to create a quarduple conversion mixer which allows you to receive up to 10 GHz. The post goes into the mixing math and practicalities of this idea. KD0CQ writes that some amplification will most likely be required to push the signal through, and in the past he’s managed to receive up to 7.5 GHz.
The SUP-2400 DirecTV device that can be converted into a downconverter.
Over on his YouTube channel Adam 9A4QV has uploaded a video that compares a coax notch filter and lumped elements filter band stop for the FM band. Bandstop filters are useful as they can be used to block out extremely strong signals that can overload an SDR dongle (or any radio).
A coax notch is a very simple band stop filter that is made from a length of coax cable at 1/4 wavelength of the frequency that you want to block. Just connect the 1/4 wavelength coax with a T-junction connector and you’ll get a notch at the frequency you want to block. A lumped elements filter is one made out of inductors and capacitors. Designing this type of filter generally requires a few more calculations, and ideally simulation. Then building it is a bit more difficult as you either need to buy or make the inductors, and then solder them together.
But as Adam shows in the video coax notch filters have a problem in that the notch is not only at the exact frequency that you want to block. Instead there will be multiple odd spaced harmonics of the blocking frequency as well. For example if your desired notch is at 100 MHz, you’ll also get notches at 300 MHz, 500 MHz, 700 MHz and so on. So a coax notch filter still needs to be carefully designed to not block out your frequency of interest.
Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will only very rarely emit a photon, but since space and the galaxy is filled with many hydrogen atoms the average effect is an observable RF power spike at 1420.4058 MHz. By pointing a radio telescope at the night sky and integrating the RF power over time, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot. This can be used for some interesting experiments, for example you could measure the size and shape of our galaxy. Thicker areas of the galaxy will have more hydrogen and thus a larger spike.
In his tutorial Adam discusses important technical points such as noise figure and filtering. Essentially, when trying to receive the hydrogen line you need a system with a low noise figure and good filtering. The RTL-SDR has a fairly poor noise figure of about 6dB at 1420MHz. But it turns out that the first amplifier element in the receive chain is the one that dominates the noise figure value. So by placing an LNA with a low noise figure right by the antenna, the system noise figure can be brought down to about 1dB, and losses in coax and filters become negligible as well. At the end of the tutorial he also discusses some supplementary points such as ESD protection, bias tees and IP3.
One note from us is that Adam writes that the RTL-SDR V3 bias tee can only provide 50mA, but it can actually provide up to 200mA continuously assuming the host can provide it (keep the dongle in a cool shaded area though). Most modern USB 2.0 and USB3.0 ports on PCs should have no problem providing up to 1A or more. We’ve also tested the LP5907 based Airspy bias tee at up to 150mA without trouble, so the 50mA rating is probably quite conservative. So these bias tee options should be okay for powering 2xLNA4ALL.
Finally Adam writes that in the future he will write a paper discussing homebrew hydrogen line antennas which should complete the tutorial allowing anyone to build a cheap hydrogen line radio telescope.
One configuration with 2xLNA4ALL, 1x interstage filter, and 1x recceiver side filter with bias tee.
Over on YouTube user Keld Norman has uploaded a video showing how he uses an RTL-SDR with gr-gsm and a Python script to create a simple IMSI catcher. IMSI stands for International mobile subscriber identity and is a unique number that identifies a cell phone SIM card in GSM (2G) mobile phone systems. For security IMSI numbers are usually only transmitted when a connection to a new cell tower is made. More advanced IMSI-catchers used by governmental agencies use a fake cell tower signal to force the IMSI to always be revealed. This way they can track the location of mobile phones as well as other data like who or when you are calling.
In the video Keld uses a Python script called IMSI-Catcher. This script displays the detected IMSI numbers, country, and mobile carrier on a text display. The video description shows how to install GR-GSM and the IMSI-Catcher script on Ubuntu.
Leandvb is command line based lightweight DVB-S decoder designed for receiving Digital Amateur TV, including signals like HamTV from the International Space Station. The RTL-SDR can be used together with leandvb and it turns out that leandvb can also be used to decode the Outernet signal. If you were unaware, Outernet is a free L-band based satellite service that provides content such as news, weather data, APRS repeats and more. Currently you can get about 20MB of data a day. Outernet receivers are also all based around the RTL-SDR, allowing for very cheap receivers to be built. At the moment you’ll need a C.H.I.P or their specialized Dreamcatcher hardware to run their special Skylark OS with software decoder, but a general Armbian decoder is in the works.
Alternatively leandvb can be used, and over on their website the folks behind the leandvb software have uploaded a tutorial showing how to use leandvb to decode Outernet. Thanks to some reverse engineering attempts by Daniel Estévez, it was discovered that the Outernet modulation is very similar to DVB-S so the standard decoder can be used with some custom flags. Leandvb only outputs raw frames, not decoded data. They haven’t tested it, but it may be possible to feed the frames into Daniel Estevez’s free-outernet project for obtaining the final files.
During the testing they also discovered some interesting notes about the E4000 and R820T RTL-SDRs. For example by patching the R820T2 drivers to add some additional VGA gain they were able to make the R820T2 chips more sensitive at the Outernet frequency compared to the E4000 chip by bringing the signal further out of the quantization noise. They also tested a 60cm dish vs a patch antenna and found that the dish works significantly better.
HamRadio360 is a bi-weekly podcast all about ham radio and related topics. On their June 13 podcast Nick, KK6LHR came on to discuss his experiences with decoding ADS-B with cheap SDR radio like the RTL-SDR. In the podcast they talk about the history of ADS-B, what it is, the difference between the 1090 MHz and 978 MHz frequencies, what all of the terms and acronyms mean, feeding sites like flightaware and flightradar24 and of course how to decode it with various forms of software packages.
