Thank you to Michael B for letting us know about recent fixes to the Raspberry Pi kernel which affect RTL-SDR users. If you've been experiencing error "rtlsdr_read_reg failed with -7" when running RTL-SDR software on Raspberry Pi 4's running a Linux kernel with version 6.1 or higher, a Raspberry Pi kernel fix has been pushed which should fix the problem.
This problem "rtlsdr_read_reg failed with -7" appears to occur after having closed any program that uses an RTL-SDR, and then reopening it.
This doesn't seem to have been an issue for the older 5.12 and 4.19 kernels where this issue was previously fixed, but Raspberry Pi recently moved to the 6.1 kernel in May 2023 where the issue came back. Raspbian releases after this date may have been problematic.
The official Raspbian should eventually update, but if you've been experiencing this issue, you could try update your kernel now using:
sudo apt install rpi-update
sudo rpi-update
Alternatively according to Michael, kernel version 6.6.y should also have this problem fixed:
sudo rpi-update rpi-6.6.y
Note that updating the kernel could break other software, so doing this is at your own risk.
In addition to the last Hydrogen Line radio astronomy post from a few minutes ago, we've also recently seen a post on Hackaday about a research paper (PDF) that describes a Hydrogen Line Radio Telescope made from a cooking Wok, LNA and RTL-SDR dongle.
In the paper Leo W.H. Fung et al of Hong Kong University of Science and Technology uses a 61cm cooking Wok with a custom made dipole feed at the calculated focal point. A filtered LNA sits after the feed, and is connected to an RTL-SDR Blog V3 dongle enclosed within a metal cookie box for additional shielding.
The results show that the Hydrogen Line was indeed detected, and measurements of the galactic rotational velocity were possible.
Again we note that we will soon by crowdfunding for a product called the 'Discovery Dish' that will be fairly similar in size and shape. It is designed for receiving L-band weather satellites, but can also be used as a Hydrogen Line telescope too.
In one of his videos from a few days ago Matt from the Tech Minds YouTube channel tests out OpenWebRX+, an unofficial fork of OpenWebRX. OpenWebRX is open source software which enables users to put software defined radios like RTL-SDRs on the internet, allowing people from all over the world to access the receiver if desired, or just letting yourself access it remotely if you want to keep it private.
OpenWebRX+ adds several additional decoders and features on top of the official version. In the video Matt demonstrates OpenWebRX+ running on a Raspberry Pi 4, with an SDRPlay RSPdx. He demonstrates the web GUI in action and shows decoding examples of the various decoders that OpenWebRX+ comes with.
Back in 2020 we released a tutorial about how to use a 2.4 GHz WiFi Grid Dish antenna as a radio telescope which can detect and measure the Hydrogen line emissions in our Milky Way galaxy.
Recently matt from the TechMinds channel has uploaded a video showing this same project but using the NooElec mesh antenna that has been slightly modified for improved performance on 1.7G and 1.4G.
In his video Matt sets up a drift sky scan, where the rotation of the earth drifts the Milky Way through the beamwidth of the dish. Matt uses Stellarium to virtually visualize the live sky map, SDR# and the IF average plugin to average the spectrum, and an Airspy software defined radio.
Over on his YouTube channel Tall Paul Tech has uploaded a video that demonstrates the FM (frequency modulation) capture effect. Apart from the costs and difficult logistics to change from AM to FM worldwide, the FM capture effect may be one additional reason as to why aircraft still choose to use AM modulation for communications instead of FM.
The FM capture effect is a phenomenon that occurs when two FM transmitters transmit on the same frequency at the same time. What will happen with FM is that the stronger of the two transmissions will be the only one heard, with the weaker one totally muted. This is in contrast to AM where both signals can be heard, albeit garbled like two people talking at the same time.
With aircraft this is important as for example if some aircraft accidentally leaves a blank transmission open, another aircraft can still transmit on top of the blank transmission and still be heard. Or allowing air traffic control to hear if multiple aircraft are trying to transmit at once, and handle communications appropriately based on urgency. The disadvantage is that without the capture effect, AM is more prone to interference from interference and atmospheric noise like lightning.
In his demonstration Paul uses two HackRF's with their clocks linked and an RTL-SDR to simulate two transmitters and a receiver.
Thank you to Chris (NNN0BOC) for writing up a glowing review of the RTL-SDR Blog V4. In the review Chris covers the enhancements that the VB4 brings to the HF bands, the driver replacement, and tests it out on various signals such as STANAG S4285, HF FAX, HFDL, GMDSS, MilSpec 141A ALE and various SWL bands. In his tests he uses SDR-Console V3, of which there is now a beta version that has built in support for the RTL-SDR Blog V4 (scroll down the page to find the V3.3 beta download).
Chris makes note that the RTL-SDR Blog V4 does not have the Nyquist aliasing problem that occurs in the RTL-SDR Blog V3 and other RTL-SDRs that enable HF reception through direct sampling. Nyquist aliasing on the RTL-SDR means that signals will be folded around 14.4 MHz. So for example a real signal at 8 MHz would also show up on 14.4 + (14.8 - 8) = 20.8 MHz and vice versa. The lack of Nyquist aliasing makes for a much cleaner spectrum.
Chris summarizes with the following:
I must admit I spent _a lot_ of time just looking at the beauty of HF with the V4, watching all the crazy goings-on, the weird sweepers and random signals popping up and disapearing.
I didn't run into any dynamic range issues or stability issues stemming from the V4, I wondered if I would have to perhaps place the RTL SDR AMBC Filter in line to suppress the many AMBC signals always present, never had a prob. The 120+ foot horizontal loop antenna feeding the V4 only has a 30MHz lowpass filter in line to hinder any rf that impinges upon it.
For such an inexpensive and tiny device, and free-to-use software, the capabilities are really kinda amazing. I now want an RTL SDR embedded into a cheap phone to use as a spectrum display on non-sdr HF receivers!
The V4 seems to atone for some of the sins of the V3, especially on HF with specific regard to Nyquist aliasing, have no fear tuning above 14.4MHz dear friends, this thing won't be making aliases unless you are overloading it.
At this time I can't think of a better value for the money when shopping for an SDR to use with a pc or phone in the sub - $50 price range.
We note that the RTL-SDR Blog V4 dongle only is currently in stock on our store from our international shipping warehouse, and the V4 dongle + antenna set will be in stock in about a weeks time.
The RTL-SDR Blog V4 dongle (dongle only), is back in stock for international shipping today! The V4 Dongle + Antenna set will be back in stock after about a week. Please note that shipping will be a few days delayed due to a multi-day public holiday in China this week. Amazon will be stocked with V4 dongles in about five weeks time.
Please remember that not all software is compatible with the RTL-SDR Blog V4 yet. The majority of programs on Windows and Linux are already compatible, or just require a simple driver swap, but some programs on MacOS and Android will need more time to update because on these platforms the drivers are bundled with software.
Please consult the V4 users guide for the latest information about software compatibility and how to update the drivers.
We note that there have been some false rumors on various forums that the RTL-SDR Blog V4 is totally out of stock already as it is a limited edition product. We want everyone to know that that while it is a limited edition product, there should be enough stock for about a year.
For the past few years we have been working on finding the best way to help beginners get started with L-band weather satellite reception and basic radio astronomy. We have now come up with a solution that we're calling the 'Discovery Dish' - a lightweight 65 cm diameter dish and active filtered feed set.
Discovery Dish: Simplified system for weather satellite reception and hydrogen line radio astronomy
Discovery Dish is a 65-cm diameter aluminum satellite dish and active filtered feed designed for receiving GOES HRIT, GK-2A LRIT, FengYun LRIT, NOAA HRPT, Metop HRPT, Meteor M2 HRPT and other weather satellites that operate around 1.69 GHz. The dish is designed to weigh under one kilogram, and it splits into three petals, making it easier to ship worldwide. The 1.69 GHz feed contains a built-in LNA right at the feed point, as well as filtering, which means that there is almost no noise figure loss from cables or connectors.
Note that the prototype images show an early non-petalized prototype with rough laser cut wind holes. The production version will obviously be a lot neater looking!
In testing the 65 cm diameter Discovery Dish with it's highly optimized feed has proven effective at receiving the GOES HRIT satellite signal with SatDump. We typically achieve SNR values of 3-4 dB to GOES-18 at 24 deg elevation, and with SatDump an SNR of 1 dB is about the minimum required to receive images so there is plenty of margin. It can also easily receive LRIT from GK-2A and Fengyun, and also when combined with an antenna rotator (or manual hand rotating) can receive HRPT weather satellites too.
The feed on the Discovery Dish consists of a tuned dipole feed with two 5V bias tee powered low noise figure LNAs, and two SAW filters (centered at 1680 MHz with 69 MHz Bandwidth). The feeds are also easily swapped out, and we will also be selling a 1.42 GHz Hydrogen Line feed for those who want to use the dish to get started with radio astronomy. Because the LNA's are right by the feed there is are no losses from feed to LNA, so we can use thinner and easier to handle cabling like RG58 without any loss issues.
In the past we've recommended and relied on 60 x 100 cm WiFi dish antennas for L-Band geosynchronous satellites and Hydrogen Line reception, but at 1.6kg these are too heavy, wide and exert too much torque for light duty antenna rotators to handle. At about half the weight of an equivalent WiFi Dish, the Discovery Dish is much easier to handle.
In the future we hope to be able to provide a low cost light duty antenna rotator that compliments the Discovery Dish. Currently we have tested the Discovery Dish with the AntRunner antenna rotator and found it to be light enough for that rotator to handle, versus a WiFi dish which is far too heavy for it.
Also when compared to a WiFi dish, the Discovery Dish is much easier to optimally set the offset skew as you can simply rotate the feed, versus having to rotate the entire dish at 45 degree increments.
We will also be offering an outdoor electronics enclosure that can be used to house a Raspberry Pi, RTL-SDR and other components like POE splitters. In our tests we have been running an RTL-SDR Blog V4, Orange Pi 5 and POE splitter in the enclosure, and running the SatDump GUI directly on the Orange Pi 5. This results in a neat contained system where only one Ethernet cable needs to be run out to the enclosure.
As we are in pre-launch, pricing is not yet confirmed, but we expect the Discovery Dish to sell for less than US$200 with reasonable worldwide shipping costs. It will be a similar cost to what you would pay if you purchased a WiFi dish, filtered LNA and cabling yourself. Obviously please check what satellites can be seen in your region.
