Category: HF

Using an RTL-SDR Blog V3 in Direct Sampling Mode to Receive HF DRM on an Android Phone

Over on the SWLing blog contributor Dan Van Hoy wrote in and shared a report on how he's successfully been able to receive HF DRM 30 digital audio with an RTL-SDR Blog V3 dongle running in direct sampling mode on an Android phone.

To do this he used an Android app called "DRM+SDR Receiver" which is available for US$4.99 on the Play store. The app supports RTL-SDR and HackRF devices. So all you need to do is set the RTL-SDR Android driver to run in Q-branch direct sampling mode, then tune to a DRM signal for it to begin decoding.

A demonstration video uploaded to his Google drive account shows clean decoding of the DRM AAC audio, as well as the app displaying Journaline and live metadata. He notes that his signal was very strong, so he only required a short wire, but DXers would need an appropriate antenna.

DRM Received on an Android phone with an RTL-SDR Blog V3 running in direct sampling mode.
DRM Received on an Android phone with an RTL-SDR Blog V3 running in direct sampling mode.

KiwiSDR Now Supports DRM Decoding

KiwiSDR have recently implemented DRM decoding into their OpenWebRX implementation. Digital Radio Mondiale (DRM) is a type of digital shortwave radio signal that is used by some international shortwave radio broadcasters. It provides superior audio quality compared to AM stations thanks to digital audio encoding.

The KiwiSDR is a US$299 HF SDR that can monitor the entire 0 - 30 MHz band at once. It is designed to be web-based and shared, meaning that the KiwiSDR owner, or anyone that they've given access to can tune and listen to it via a web browser over the internet. Many public KiwiSDRs can be found and browsed from the list at sdr.hu.

The new DRM implementation is based on DREAM 2.1.1 which is an opensource DRM decoder that can be used with any HF capable SDR. Due to computational limits of the BeagleBone singleboard computer which the KiwiSDR runs on, only one DRM channel can be decoded at any one time, restricting this capability to only one user at a time. However, if the KiwiSDR is running on the newer BeagleBone AI, it can support up to four DRM channels. KiwiSDR write that work is still ongoing to improve the code, so this situation may improve in the future.

KiwiSDR Decoding DRM
KiwiSDR Decoding DRM

Decoding Differential GPS (DGPS) with an RSPdx and MultiPSK

Over on YouTube the TechMinds channel has uploaded a new video about decoding Differential GPS (DGPS) using an SDRplay RSPdx SDR. DGPS is a terrestrially transmitted long wave signal that is used to help correct and improve GPS position data calculations which may have timing errors due to atmospheric propagation delays. It works by broadcasting correction data calculated by the difference in received GPS location and the known location of the DGPS transmission site. DGPS is typically transmitted on longwave between 285 kHz and 315 kHz, but in Argentina there are two stations at 2570 and 2950 kHz.

In the video TechMinds explains how DGPS works, and some location around the world from where it is transmitted from. Later in the video he shows a DGPS signal being received by a SDRplay RSPdx SDR, and then show a demo of how it can be decoded with MultiPSK.

We note that there also various other DGPS decoders available including decoders for Android and iOS. A list of decoders can be found on the DGPS sigidwiki page.

DGPS Differential GPS Decoding With RSPdx And MultiPSK

Comparing Four Wideband Magnetic Loop Antennas on HF with an SDRplay RSPduo

Over on YouTube the Scanner and Sdr Radio channel has uploaded a video comparing four different brands of HF wideband loop antennas using an SDRplay RSPduo. The loops he tested include the cheap Chinese MLA-30 (~$40), the Cross Country Wireless (CCW) loop ($70), Bonito ML200 (~$442) and the Wellbrook 1530LN (~$305).

The MLA-30 was slightly modified with the cheap coax removed and a BNC connector added. Each of the antennas used a wire loop with diameter of approximately 1.6m, except for the Wellbrook which has a fixed size solid loop of 1m.

The tests compare each loop against the Wellbrook which is used as the reference antenna. In each test he checks each HF band with real signals on the RSPduo and compares SNR between the two antennas.

The results show that the two expensive antennas, the Bonito and Wellbrook, do generally perform the best with the lowest noise floors, but surprisingly the MLA-30 actually performs very well for it's price point, even outperforming the Wellbrook reference on SNR in some bands. We note that some of the improvement may be due to the larger 1.6m loop size used on the MLA-30, compared to the 1m loop on the Wellbrook.

Also we note that it can be hard to compare antennas in single tests, because the differences in antenna radiation patterns could be favorable for some signals, and less so for others, depending on the location.

Comparing 4 magnetic loops for hf

The Malachite-DSP: A $195 Russian Made Portable Wideband SDR Receiver with Touch Screen

Over on the SWLing.com blog we've seen news about the release of a new Russian designed and made portable software defined radio called the "Malachite-DSP". The Malachite-DSP is an "all-in-one" portable SDR that is controlled via a touch screen and two control knobs. It covers 0.1 MHz to 1000 MHz with a bandwidth of up to 160 kHz, and the custom software supports all common modulation types. The whole device consumes 300mA and is powered by a Li-ion cell. It's marketed as a modern DEGEN and TECSUN replacement, so it appears to be targeting the HF short wave listening (SWL) customer.

Production appears to be small, with purchasing currently done by contacting RX9CIM, one of the project creators, directly at his email address (details on this forum post). The cost for a fully assembled unit is 12500 Russian Rubles which is 195 USD (not including international shipping). You can also purchase just the PCB without components for 1100 Rubles (17 USD). Importantly the forum post notes to watch out for scammers, who appear to be trying to take fake preorders for the device.

From the components list we can see that this SDR runs on the MSI001 tuner chip, which is the same tuner chip used in the SDRplay line of units. However, unlike the SDRplay units which use a wideband MSi2500 ADC, the Malachite-DSP uses an audio chip as the RF ADC. This provides a 16-bit ADC, resulting in high dynamic range, but at the expense of the available bandwidth which is only 160 kHz. A STM32H743VIT6 with ARM Cortex A7 processor runs what appears to be custom DSP and GUI software. The software doesn't seem to support DRM, but AM, WFM, NFM, LSB, USB are all supported.

The main place for news and discussion on the Malachite-DSP appears to be on a Russian ham radio forum thread. Judging by the fact that the schematic, software and BOM is all freely released, the project appears to be open source. There is also a group on the Russian Facebook clone vk.com where some discussion is occurring.

The YouTube videos below are by a Russian reviewers. Be sure to turn on the YouTube closed captioning and auto translation feature if you want to follow along in English.

😲ПРИЕМНИК КОТОРЫЙ ЛОВИТ ВСЁ!!!💥🔝 ЭТО ВАМ НЕ Degen и Tecsun ВСТРЕЧАЙТЕ НОВЫЙ МАЛАХИТ DSP V2💯🆕

SDR приемник МАЛАХИТ DSP

The Malachite-DSP reminds us a bit of the unreleased PantronX Titus II SDR, which is supposed to be a low cost (aiming for less than $100 USD) 100 kHz - 2 GHz tablet screen based SDR that was supposed to make DRM reception more popular. However the Titus II hardware has never eventuated since it's initial news in 2016, and at this time appears to be a dead project.

An Active Low Cost HF Loop Antenna Made in the UK

Cross Country Wireless is a UK based company that has created an active HF loop antenna for only $70 USD including international shipping. The loop appears to have already been for sale for a while now, but recently they've created a new version that can be easily powered by a 5V bias tee with at least a 67 mA current capacity. This makes it very easy to use with radios that have built in bias tee's such as our RTL-SDR Blog V3 and SDRplay and Airspy units. The page reads:

The Loop Antenna Amplifier contains all the electronics needed for home DIY construction of an active loop (magnetic loop) low noise receiving antenna.

The amplifier consists of two units, a weatherproofed outdoor unit for connection to a suitable loop and a base unit to further amplify the signal and to provide DC power up the coaxial cable to the outdoor unit.

The outdoor unit is housed in a polycarbonate box with stainless steel antenna connections and a BNC socket. The indoor unit is a PCB with two BNC connectors and a USB socket to take 5V from a USB socket on a PC or phone charger.

Like our other active antenna products it has RF overload protection to allow it to be used very close to transmit antennas without damaging the amplifier or the attached receiver.

The loop depends on what the user has available. We have tested it with simple wire loops or deltas, coax loops and an alloy loop made from a bicycle wheel rim. We supply a 3m (10 ft) length of wire as a simple loop to make a first loop for testing.

The photograph on the right shows the prototype with a 1m diameter loop of LDF4-50 coax cable as a test loop.

With a simple wire loop or delta and a small USB powerbank it makes a very compact and portable receiving antenna for holiday listening or covert use.

The latest version can now have the head unit powered directly from receivers with a 5V bias-tee such as the SDRplay receivers or some RTL-SDR dongle receivers with a bias-tee option.

Specifications:

  • Frequency range: 10 kHz to 30 MHz
  • Loop amplifier input impedance: 0.3 ohms
  • Output impedance: 50 ohms
  • Supply voltage: 5 V from USB socket or charger
  • Supply current (head and base unit): 112 mA
  • Supply current (head unit fed with 5V bias-tee): 67 mA
  • Loop antenna outdoor unit connectors: Two M6 stainless steel threaded studs and BNC female (RF out 50 ohms)

There is no comparison yet that we've seen on how this loop compares against the cheaper US$45 Chinese made MLA-30 loop. In a previous post Martin (G8JNJ) reviewed the MLA-30 and noted several design flaws after reverse engineering the circuit. He has let us know that he will also be reviewing the Cross Country Wireless Active Loop and will let us know his thoughts in the future.

Cross Country Wireless Loop
Cross Country Wireless Loop

Cross Country Wireless Loop Antenna Amplifier VLF test with 1m diameter coax loop

Using a PlutoSDR to compare FreeDV Digital Voice with other Modes

Over on YouTube user Adrian M has uploaded a video where he compares the HF amateur radio digital voice mode known as FreeDV against other common voice modes such as USB, AM, FM and QPSK. To perform the test he uses a PlutoSDR, a GNU Radio program and a GUI called qradiolink.

FreeDV is an open source amateur radio digital voice mode that uses Codec2 compression. It's designed to compress human voice and works with narrow bandwidths and with weak signal power.

In the demonstration Adrian reduces the TX power slowly for each mode, so you can see what the voice sounds like at high and low signal power. The FreeDV mode is not high fidelity in terms of audio quality, but the voice remains able to be copied at low power when the other modes could not.

Transmit and receive FreeDV 1600 and 700C with SDR hardware

G8JNJ Reverse Engineers and Reviews the MLA-30 HF Loop Antenna

Last month we posted a collection of reviews about the MLA-30 which is a budget magnetic loop antenna designed for receiving HF signals. The overall consensus from the reviews was that it worked decently for the price, but of course could never live up to the high end loops that cost hundreds of dollars.

Recently Martin (G8JNJ) reverse engineered the active circuit used on the loop from photos taken by M0LMK and has made some observations on it's performance, noting that it's design isn't very good. First he notes that the amplifier chip is a Texas TL592B two stage video amplifier which isn't that great for this application. His measurements show an OIP3 of 20dBm, a P1 saturation of -3dBm and a noise figure of 12dB.

Of interest, he explains that the creator of this loop has designed it poorly as the impedance match of the loop to low pass filter is very wrong, resulting in a very poor amplitude/frequency response. He shows how the response can be improved with a few termination resistors, but is still not great.

MLA-30 Frequency Response. Ideally should be flat.
MLA-30 Frequency Response. Ideally should be flat.

If you're interested in a cheap magnetic loop antenna, Martin suggests DIYing the M0AYF design which he says works a lot better.

We note that the "YouLoop" design is also in the works as a product that will apparently sell at close to manufacturing cost. The YouLoop is a passive loop idea by the creator of the Airspy that consists only of a simple 1:1 transformer and coax cable as the loop. It works best with high sensitivity radios like the HF+ Discovery.