Back in March Vi Vitaliy wrote in to let us know about an RTL-SDR compatible SDR program called UnoSDR that he had been working on. Vi now writes that the software has been updated, and the name has been changed to QuestaSDR to avoid confusion with SDRUno – SDRplays official software package. Vi writes:
In new version I added support AirSpy r2, AirSpy mini, Perfomance Window, Record AF, record and play iq raw data, frequency bookmarks, waterfall color scheme, change fft size, fps … calibrate options, support if converter.
The software is still in beta, and Vi would be interested in hearing any feedback or bug reports if you have any.
Recently Luigi Freitas wrote in to us and wanted to share his fairly unique Outernet setup which is based on a Grid dish antenna, low cost SPF-5189 LNA, C.H.I.P mini single board computer generic RTL-SDR, and the open source LeanDVB decoder software.
Last month we made a post about LeanDVB, a lightweight DVB-S decoder, which with a few configuration changes can be used to also demodulate the Outernet signal. Luigi places his 2.4 GHz WiFi grid antenna (which still works for the 1.5 GHz Outernet signal) on a tripod and points it towards the Outernet satellite in his area. He connects the antenna up to a SPF-5189 based LNA, which is a 50 – 4000 MHz LNA that is very cheaply found on eBay for about $7 USD. Then a cheap generic no-TCXO $8 RTL-SDR is used together with the LeanDVB software.
In his post Luigi shows how to set up the LeanDVB software for decoding the Outernet signal by piping the output of rtl_sdr into it, and getting all the settings correct. To get the final files he then shows how to pipe the decoded packets in the Skylark decoder, and then the files can be accessed from the regular Outernet web GUI.
Qrp Gaijin’s post goes into some detail about his circuit and shows the schematic as well. He also shows the results with an active loop antenna, RTL-SDR and the upconverter in some videos. In the email to us he also notes that his upconverter is still a work in progress as the LO is quite noisy and he suspects that it may be too weak to drive the 1N4148 based diode ring mixer. There is also no filtering on the circuit yet, so there is some broadcast FM breakthrough.
Another project he worked on was attempting the direct sampling mod on a standard RTL-SDR. However, Qrp Gaijin’s method is slightly different to most attempts as instead of soldering the wires into the Q-branch holes he simply uses hot glue to hold them mechanically in place. This may be an idea to consider for those who want to attempt the mod on a standard RTL-SDR, but don’t have any soldering tools or experience.
Homebrew RTL-SDR upconverter: physical construction
During July 20 – 24, 2017 the ISS (International Space Station) was transmitting SSTV (Slow Scan Television) images down to earth in celebration of the ARISS (Amateur Radio on the ISS) 20th Anniversary. The ISS transmits SSTV images on celebratory occasions several times a year. More information about upcoming ARISS events can be found on their website ariss.org.
Despite Laptop and PC troubles, he was able to capture several images. He also notes that he was able to use a Baofeng and Yagi antenna to receive the signal indoors.
Note that Tysonpower’s YouTube video is narrated in German, but there are English subtitles available if you turn on YouTube’s closed captions which should be on by default on this video.
[EN subs] ISS SSTV Event Juli 2017 - Empfang von drinnen und V-Dipole
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.
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.
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