Over on YouTube "Gadget Talk" has uploaded a useful video showing how he set up an RTL-SDR V3 based panadapter system to use with his traditional amateur radio. The setup involves utilizing an antenna switcher which allows him to transmit with the RTL-SDR connected to the same antenna. The switch grounds the SDR during transmission, ensuring that the RTL-SDR is not overloaded with the transmit signal.
In the video he also shows how to set up the HDSDR receiver software and the HRD Rig Control software for controlling the hardware radio through the software and vice versa.
Over on his YouTube channel TechMinds has uploaded a new video showing how to decode signals from Orbcomm satellites. Orbcomm run a global network of low earth orbit satellites that perform services such as Internet of Things (IoT), Machine 2 Machine (M2M) communications, asset tracking, utilities telemetry, government communications and much more. The signals can be received at around 137 MHz.
In the video he explains how the private client data is encrypted, however it is possible to at least see the encrypted data coming down, and decode some of the data management information such as the transmitted uplink frequencies using a program called Orbcomm Plotter. Ultimately, the data available is quite boring to monitor, however decoding these satellites is still an interesting exercise.
Decoding Orbcomm Satellite Transmissions Using Software Defined Radio
Paolo Romani IZ1MLL has recently created a SDR# users guide document which comprehensively explains all the features and settings available in the program. SDR# (aka SDRSharp) from Airpsy.com is designed for Airspy SDRs, however it is one of the most popular SDR receiver programs used with RTL-SDRs as well.
Paolo's guide appears to build on our own guide at www.rtl-sdr.com/sdrsharp, providing new information and updates since many changes and new features have been released in SDR# since we wrote that guide a few years ago.
The guide can be found on the airspy.com/download page and is available in English, Italian and Spanish.
Cheaper RTL-SDR and SDRs use a low quality crystal oscillator which usually has a large offset from the ideal frequency. Furthermore, that frequency offset will change as the dongle warms up or as the ambient temperature changes. The end result is that any signals received will not be at the correct frequency, and they will drift as the temperature changes. Higher end SDRs and improved RTL-SDRs like our RTL-SDR Blog V3 use a temperature compensated oscillator (TCXO) which has a very small frequency offset and very little temperature drift.
CalibrateSDR can be used with almost any SDR to determine the frequency offset. Andreas notes that CalibrateSDR uses the synchronization channel symbols from DAB+ digital audio stations to determine the offset. His post contains a great explanation of how this works. If you don't have DAB+ in your area, an alternative is Kalibrate-RTL which uses GSM cellphone signals to calibrate.
His results were as expected, showing that the generic RTL-SDRs have large frequency offsets, and his RTL-SDR Blog V3 and LimeSDR have much better precision.
The null symbol (lower amplitude portion) and phase reference (Orange) in a DAB+ signal
Back in 2017 we posted about Adam 9A4QV's simple V-Dipole antenna design which works very well for receiving NOAA and Meteor weather satellites at 137 MHz. This type of antenna is a lot easier to build compared to a QFH or turnstile, and it results in good performance if built and set up correctly. Over the years he notes that he's received a number of questions asking to clarify the design and so he's uploaded a YouTube video which explains the built and dimensions of the antenna clearly.
Over on YouTube Rob from Frugal Radio has uploaded his latest video which explains RF filters in a concise and easy to understand way. RF filters are often used together with SDRs to block out strong interfering signals which can degrade reception at other frequencies.
In the video Rob goes over the different types of RF filters such as high pass, low pass, band pass and band stop, and shows a few SDRs that have filter banks built in.
Filters 101 : Improving reception with your SDR or radio - a look at RF filters
A while back we heard a suggestion that it may be possible to use the 5th harmonic on R820T2 RTL-SDRs to receive frequencies much higher than the normal 1.766 GHz limit. After mentioning this to to Hayati Augen he has recently managed to implement this driver hack, and now R820T2 based RTL-SDRs can tune all the way up to 6 GHz if Hayati's experimental driver branch is used.
Before everyone gets too excited, we need to temper expectations as sensitivity reduces the higher the frequency so an LNA may be required, imaging/aliasing is a major problem, and you will absolutely require a ~1.7 GHz high pass filter to be able to actually see any signals as otherwise the lower bands drown everything out. So when combined with the relatively small bandwidth of the RTL-SDR, the overall usefulness of this feature may be very limited, however it is great that this option at least now exists.
In order to actually receive anything, you will need to filter out all signals below about 1.7 GHz, otherwise they will alias on top of your desired frequency. We used the VHF-1760+ from Mini-Circuits. There are other options available as well, however this filter seems to work well as long as the signals below 1.7 GHz are not too strong.
Testing Harmonic Reception with 700 MHz to 26 GHz Antenna, 1760 MHz High Pass Filter and an RTL-SDR Blog V3
The latest branch with the harmonic hack can be found in the librtlsdr Github on the development branch. The binary releases can be found under Github releases. The easiest way to get started is to download the static zip, and run rtl_tcp. Then we recommend connecting to the rtl_tcp server with HDSDR, and using this custom ExtIO available on the extio_rtl_tcp Github releases. Newer version of SDR# currently seem locked to 2 GHz maximum, so this is why we recommend HDSDR at the moment.
When tuning above 1.766 GHz 5th harmonic tuning will automatically be activated. However, you can also use the -harm flags on the command line tools to choose the 3rd harmonic too if you wish to test that.
In testing we were able to receive signals generated by a HackRF and signal generator up to 4.2 GHz. Sensitivity appears to be decent until around 2.7 GHz, then it begins to lower significantly. There are also a few bugs in the tuning code, as at some frequencies the actual signal will be offset by a few MHz from where we would expect. It appears to be a scaling issue. Also we've also only been able to get up to a maximum of 4.2 GHz on HDSDR, but this may only be due to a HDSDR 32-bit limitation.
We've tested this hack on several RTL-SDRs, and found that signals are overall the strongest on our RTL-SDR Blog V3. This is most likely due to the fact that the V3 has a front end matching circuit that does not attenuate L-Band or higher frequencies, unlike models based on the original TV dongle design.
If anyone has success with this hack for any application, please let us know down below in the comments.
Test signal from a HackRF at 2.45 GHzBaby monitor signal
DragonOS is a ready to use Ubuntu Linux image that comes preinstalled with multiple SDR program. The creator of DragonOS, Aaron, uploads various YouTube tutorials. In his latest tutorial he shows how to install the latest version of the "Boatbod" OP25 development code for receiving P25 Phase 1 on Linux with the latest GNU Radio 3.8. In the video Aaron uses an RTL-SDR, but notes it could also work with other SDRs like the HackRF.
