The LibreCellular project, which aims to provide an end-to-end open-source technology stack for cellular networks, has celebrated another major milestone with the development of a new, highly-versatile radio-frequency front-end (RFE) offering 2×2 Multiple-Input Multiple-Output (MIMO) operation.

“This is designed to be connected to an SDR and provides uplink and downlink amplification, thereby enabling greater coverage,” explains Andrew Back of the latest hardware design to come out of the LibreCellular project. “The RFE supports up to 2×2 MIMO (2× transmit plus 2× receive signal paths) and can be used with both FDD [Frequency-Division Duplexing] and TDD [Time-Division Duplexing] operation.

“The selected Qorvo amplifiers have a specified frequency range of 500MHz – 4GHz, with a gain of 19 dB and a P1dB of 21.2 dBm,” Andrew adds. “However, the RFE has yet to be fully characterised to determine the insertion loss and performance across the full frequency range, with reduced performance expected at higher frequencies due to the low cost FR4 PCB substrate used. Given the wide operating frequency range of the RFE and variety of useful features which it integrates, it is likely to also prove useful in many applications outside of building mobile networks. Not to mention as a general RF lab tool also.”

As well as the new RFE, the LibreCellular project has announced an update to its fork of the srsRAN 4G project, which adds native LimeSDR support, and ongoing progress to provide a driver for use with the new Lime Suite NG software stack. “[This] will provide support for recent hardware, such as the LimeSDR XTRX,” Andrew notes, “along with support for numerous new features which owners of LimeSDR Mini v1 & v2, plus LimeSDR USB, will also be able to benefit from.”

More information is available in Andrew’s latest project update.

Pseudonymous MyriadRF community member Tryton77 has released an unofficial work-in-progress port of Lime Suite NG, with SoapySDR, for Google’s Android platform – providing the ability to use the software with an unmodified smartphone or tablet.

“[The] goal of this project is to port [the] Lime Suite NG library with SoapySDR to Android. [The] Lime Suite NG lib can be used standalone if someone doesn’t want to use SoapySDR,” Tryton77 explains. “It works on unrooted devices, but it requires more tests. For now I’ve tested it with simply receiving some samples.

“Lime Suite NG uses libusb to make [a] connection with [a] device, but Android (without root) doesn’t allow us to enumerate devices from libusb level,” Tryton77 explains of how the fork enables unrooted Android support. “However we can establish a connection via Android’s USB subsystem to get [the] device descriptor and share it with libusb.

“I just wanted to have a practical spectrum analyser,” Tryton77 says of the thinking behind the project, “but carrying [a Raspberry] Pi was quite inconvenient. If you are interested please test it on your own and give me some feedback or open an issue on project.”

More information is available in the MyriadRF forum, while source code and build instructions are available on GitLab.

Amateur radio operator Daniel Estévez has been putting a LimeSDR to use in tracking aircraft – by capturing signals between the aircraft and a ground-based Distance Measuring Equipment (DME) station.

“DME works by measuring the two-way time of flight of pulse pairs, which are first transmitted by the aircraft, then retransmitted with a fixed delay by the ground station, which acts as a transponder, and finally received back by the aircraft,” Daniel explains. “DME operates between 960 and 1215MHz. It is channelised in steps of 1MHz, and the air-to-ground and ground-to-air frequencies always differ by 63MHz.

“I had two different ideas about how to use the LimeSDR to record the two DME channels. The first idea consisted in using a 70Msps output sample rate. This approach worked well on my desktop PC, since in 70Msps I had the two DME channels and then I could use GNU Radio to extract each of the two channels (for instance with the Frequency Xlating FIR Filter). However, the laptop I planned to use to record on the field couldn’t keep up with 70Msps.

“The second idea was to use the on-chip DDC [Digital Downconverter] in the LMS7200M to extract the DME channel and deliver a much lower sample rate over the digital interface. The point here is to tune the LO to a frequency between the two DME channels, set the sample rate high enough that both DME channels are present in the ADC [Analogue to Digital Converter] output, and finally to use each of the two RXTSP [datapaths] to extract one of the DME channels, sending it at a low sample rate through the digital interface.”

Using this approach, Daniel was able to capture the signal at 80Msps then use the RXTSPs to downconvert and decimate the two channels to 2.5Msps – a sampling rate well within the reach of a modest laptop.

The full project write-up is available on Daniel’s website, while a two-hour recording has been published on Zenodo.

Radio amateur Ciprian “YO6DXE” Popica has created a revised version of the Pititico, a one-transistor transceiver originally designed by Miguel “PY2OHH” Bartié.

“[The] Pititico CW [Continuous Wave] transceiver has a consumption in Rx of 1.5mA and a power of about 700mW,” Ciprian explains. “With a few modifications and using other transistors, the power can get as high as 1W. I found the original schematic designed by PY2OHH long time ago and I always wanted to try it.

“I built my Pititico CW transceiver in a small wooden box initially using the Manhattan style construction for the circuit. I also made a PCB design that you can use to make PCB boards using the toner transfer method.

“To listen to the audio output,” Ciprian notes, “you should use a 300 Ohm phone speaker as Miguel (PY2OHH) recommended. It didn’t work for me, maybe I used the wrong speaker. Even better is a high gain audio amplifier. I’m using an LM386 amplifier set to the highest gain possible. If you do not want to use an external amplifier, I would recommend you to try building Pititico II, that has the audio amplifier included in the circuit. Is a lot like a Pixie transceiver, more or less.”

Ciprian’s full write-up, with schematic, is available on his website; a demo video is available on YouTube.

Pseudonymous Romanian YouTuber FesZ has created a video walking through how a coax stub filter works – including how to optimise its performance.

“[These are] commonly built from pieces of transmission line which are either open or short-circuit terminated,” FesZ explains. “These are also known as ‘stub filters.’ Now, such filters can come in all shapes and sizes, but building from pieces of coax cable is usually a sensible option.

“Fundamentally, a stub filter relies on a piece of transmission line’s ability to create an impedance transformation. The big limitation in using coax cable is that you only have a few impedances to work with: commonly available are 50 and 75 Ohm impedance cables.

“Now,” FesZ continues, “you can get a few more impedances by putting cables in parallel – so if you take two 50 Ohm cables in parallel, these will behave like a 25 Ohm cable. Two 75 Ohm cables will behave like a 37.5 Ohm cable, and if you try to match a 50 and a 75 Ohm cable you get about 30 Ohms.”

FesZ’ video, part of an ongoing filter-focused series now 11 episodes long, walks through simulation, optimisation, and the creation of a practical implementation. “Coax cable stub filters can be quite a useful construction when a narrowband high-Q filter is needed at relatively low frequencies. [They] offer the performance of a distributed element filter in a frequency range usually occupied by lumped element filters, and at the same time it’s also a fun device to build and fine-tine.”

The full video is available on the FesZ Electronics YouTube channel.

Researchers at Montevideo’s Universidad de la República have taken the concept behind classec TEMPEST image-signal sniffing and applied it to digital signals on an HDMI cable – capturing enough unintended radiation to replicate on-screen text with surprising accuracy.

“Eavesdropping on digital video displays by analysing the electromagnetic waves that unintentionally emanate from the cables and connectors […] is known as TEMPEST,” the team explains. “Compared to the analogue case (VGA), the digital case [HDMI] is harder due to a 10-bit encoding that results in a much larger bandwidth and non-linear mapping between the observed signal and the pixel’s intensity. As a result, eavesdropping systems designed for the analogue case obtain unclear and difficult-to-read images when applied to digital video.

“The proposed solution is to recast the problem as an inverse problem and train a deep learning module to map the observed electromagnetic signal back to the displayed image. The proposed system is based on widely available Software Defined Radio and is fully open-source, seamlessly integrated into the popular GNU Radio framework.”

Dubbed Deep-TEMPEST, the system captures a blurry, hard-to-see ghost of the image being displayed – but then feeds it through a deep-learning system trained primarily on large quantities of synthetically-generated samples. This can then create an interpretation of the original image, admittedly blurry for pictures but sharp and clear for textual content with a character error rate of under 30 per cent.

The team’s work has been released on GitHub under the GNU General Public License 3; additional information is available in a preprint on Cornell’s arXiv server.

Finally, software developer Evan Pratten has gone camping with a radio — and a 100-foot length of wire, trailed around the campsite to act as its antenna.

“Last time I had been camping happened to coincide with the period of time that I was starting to gain curiosity about amateur radio. I vividly recall being out there wishing I had a radio that I could use to communicate from the campsite,” Even explains. “So, to appease my past self, the present-tense radio-license-having version of me took my HF rig along to make some contacts.

“A glaring problem in this plan was that I didn’t actually have an antenna to bring. So a week in advance, I set out to build one for the trip. I opted to build a duplicate of my current fixed-in-place at-home antenna, a slightly more sketchy variant of WB3GCK’s speaker wire end-fed half-wave. Now, I’m not very good at following other people’s antenna instructions. I like to tinker with the specifics. WB3GCK’s design was probably intended to produce two distinct antennas out of a single 50ft spool of speaker wire, but I prefer to use the whole thing as a single unbalanced dipole.

“Having already built one of these antennas before, I just set out to the local Canadian Tire to buy the same components again and repeat my last antenna build. Slight problem, they didn’t have speaker wire. So I began searching for literally anything conductive, and came across a pile of spools of 24-ft ‘lamp wire.'”

That lamp wire, suitably spliced, would become a 100ft antenna with handy insulated paracord hanging-hooks at the tips. Initially tested in a more urban setting, it proved its value in the radio-quiet woods of the campsite: “In terms of operation, this setup was awesome,” Even says. “The worst noise I had to deal with was barely pushing an S2, and I was able to make far more (and better) contacts than I had expected. In terms of contacts, I made a bunch. All over the bands (although mainly 20m), and I even set my new distance record!”

Evan’s full write-up is available on his blog.