Introducing the ATS25 Max: The Ultimate Ultra-Portable Decoder Receiver

by Jan Buiting
Reading time: 11 min

Elektor proudly presents the ATS25 max-Decoder, a powerful yet ultra-compact multimode DSP receiver. This cutting-edge device offers comprehensive coverage of LW, SW, MW, FM, and Ham bands. Powered by an ESP32 core and equipped with Wi-Fi connectivity, it enables advanced decoding modes. Experience the perfect synergy of embedded systems and radio technology with the ATS25 max-Decoder!

For microcontroller enthusiasts, programmers, and embedded systems fans, ham radio has always been a fascinating and rewarding field. The recent surge in interest is driven by the development of new ICs and plug-and-play integrated modules. These advancements handle high-frequency signal conditioning and provide simple control of digital signal input and output. It's an exciting time for programmers and radio hobbyists alike!

Embedded Systems Meet Radio Technology

One integrated circuit (IC) family that has successfully bridged the gap between analog RF technology and digital innovation is the SkyWorks Si473x series. The standout in this series, the Si4735 is renowned as a "CMOS 100% AM/FM/SW/LW radio receiver." This chip seamlessly integrates the entire tuner function, from antenna input to audio output. Initially, it gained traction among hams and radio enthusiasts through Arduino software, but its true capabilities were unlocked when an international group of hams and programmers utilized the more powerful ESP32 microcontroller. This led to the creation of innovative software for not only the LW/MW/SW/FM broadcast bands but also for amateur radio bands ranging from 160 m to 10 m, including modes like CW, RTTY, USB/LSB, FAX, and more.

The combination of the Si4735 chip and the ESP32 microcontroller first emerged in the "ATS25" multimode receiver. This device has been popular for several years, serving as an excellent general-purpose radio for both broadcasts and ham communications. Recently, the "Jstvro" group significantly enhanced the standard ATS25 by adding advanced features such as digital decoding, Wi-Fi connectivity, and sophisticated DSP audio processing. These upgrades led to a comprehensive overhaul of the control software, substantially boosting the functionality and performance of the ATS25 receiver.

Unboxing the ATS25 max-Decoder: Small but Mighty

he ATS25 max-Decoder immediately impressed me with its compact, sturdy, and lightweight design upon unboxing. The front panel is minimalist, featuring a single 40 mm diameter knob that serves as a rotary encoder with push-button functionality. The back of the device houses two antenna inputs: an SMA socket for the 2.4 GHz Wi-Fi antenna and a BNC socket for the main antenna. Additionally, there is a USB-C connector for charging the internal Li-Ion battery and for PC communication. The back panel also includes a headphone socket and two small slide switches.

I was delighted to discover several thoughtful accessories included with the radio: a stylus pen, a foldable Wi-Fi antenna, a telescopic rod antenna for FM, a USB-C cable, and even a cloth for dust protection or cleaning the touchscreen. The standout item, however, was the ATS25 max-Decoder Manual. Unlike the typical manuals that come with many Chinese electronics, this 24-page manual is well-printed and features clear pictures with legends and captions that explain the most important functions. It's definitely worth reading through before powering up the radio! I also learned that Elektor is considering enhancing and printing their own version of the manual.

Prioritizing the Antenna

For microcontroller enthusiasts diving into radio technology, the importance of a good antenna cannot be overstated. The ATS25 max-Decoder comes with a telescopic antenna that works well for FM reception, but for LW, MW, SW, or ham bands, you’ll need something more robust. A simple solution is to extend at least 10 meters of flexible wire outdoors and connect it to the telescopic antenna. This can dramatically improve your reception, unless you encounter interference from digital devices like LED lights, computers, smartphones, or solar panel installations.

My 7.5 kWp solar panel installation, particularly the SolarEdge 3-phase inverter, caused significant interference. Turning off the inverter eliminated 180-kHz spaced harmonics and spurious signals extending well beyond 60 MHz. While the ATS25 does have some internal noise, it’s manageable. Using a 50-ohm terminator to silence the antenna input revealed that the remaining noise came from the rotary encoder and the OLED screen.

Radio wave propagation varies greatly between day and night. Daytime reception on the MW and LW bands is often poor, even with a wire antenna. However, the ATS25 max-Decoder found several stations around 675 kHz and 1000 kHz, performing as well as my Grundig Yacht-Boy and a 1960s Philips “BX” tube radio. I also picked up Droitwich on 198 kHz LW, catching a BBC cricket match report. The MW band becomes much more active in the evening, perfect for discovering pirate and low-power stations.

Ultra-Portable ATS25 max-Decoder Receiver — Scanning the MW band for activity using the built-in spectrum analyzer revealed little during the daytime. However, return at dusk, and the party starts.

The ATS25 max-Decoder features a unique FM band range from 64 to 108 MHz, allowing monitoring of the 4-meter (70 MHz) band, which isn’t listed under the "HAM" bands. This range also let me pick up 80 MHz police radio traffic from Germany. Scanning the MW band during the day showed little activity, but it came alive at dusk.

Switching to a wire loop antenna, which responds to magnetic fields, significantly reduced man-made noise. However, the small loop antenna available from Elektor has an SMA connector, while the ATS25 uses a BNC socket. I improvised with an SMA-to-BNC adapter connected to a BNC male-to-male adapter, maintaining 50 ohms throughout. This setup worked well, allowing me to minimize interference from LED lamps and LCD screens by tuning the loop antenna's direction.

ATS25 max-Decoder manual — My makeshift SMA-to-BNC adapter (crafted in two steps but maintaining 50 ohms throughout), the indispensable stylus (left) and the printed manual.

It’s best to hang up the loop antenna outdoors, as far away from noise sources, and feed the cable inside to your receiver.

The YouLoop — The "YouLoop" from the Elektor Store is a good starting point for a proper antenna. Remember though that it's not really suitable for permanent outdoor use. Also, there should be nothing inside the loop :-)

Welcome to Radio

Once you understand the quirks of radio wave propagation enhances the tuning and listening experience with the ATS25 max-Decoder, making it comparable to many “big box” receivers. Using the included stylus to operate the touch screen can take some getting used to. When I misplaced the stylus, I had to use my fingertip to select the tiny fields and icons, which was less precise. Another challenge was keeping the radio steady and slightly angled due to the stiff coax cable from the loop antenna connected at the back. I solved this by placing the radio on a smartphone holder, which provided stability and a comfortable viewing angle. Yes, I’m accustomed to SW receivers weighing 20 kg and more (Collins, Hallicrafters, you know).

Smartphone holder secures the receiver — An inexpensive smartphone holder proved to be an excellent solution for securing the receiver at a comfortable angle for both viewing and operation. The holder even had a convenient spot for the stylus.

I tuned into numerous local QSOs on the 80-meter SSB band (3.5 MHz), a "chat" band with daytime communication distances of up to 300 km between stations. On the 20-meter band (14 MHz), I received old-school RTTY and even some SSTV signals, though I didn't attempt to decode the SSTV on my PC. The 40-meter band (7 MHz) is excellent for testing the automatic CW decoding capabilities of the ATS25 max-Decoder, provided the radio is connected to your local Wi-Fi network.

Receiver bands list — Jstvro team (and Elektor readers), can we have VLF, 4 meters (70 MHz) and 2 meters (144 MHz) as well, please?

While the ATS25 max-Decoder offers an impressive range of features, I found myself wishing for additional bands such as VLF, 4 meters (70 MHz), and 2 meters (144 MHz). The AGC (automatic gain control) and RF attenuation functions were somewhat inconsistent, struggling to handle high-power stations within the bands, like certain Chinese broadcasts.

Getting Connected with Wi-Fi

The advanced digital decoding functions of the ATS25 max-Decoder, such as FT8, CW, and NTP timekeeping, rely heavily on an Internet connection. While connecting the device to Wi-Fi is relatively straightforward, the instructions provided could be clearer. The Wi-Fi connectivity process, crucial for the radio's operation, is only vaguely explained on the last page of the manual, omitting several important steps.

ATS25 max-Decoder pages of menus — Menus... the ATS25 max-Decoder has pages and pages of them.

The ATS25 max-Decoder is packed with extensive menu options. Upon switching on the radio, you'll notice a registered product key displayed prominently. It's advisable to write this key down and keep it safe. To connect to Wi-Fi, navigate through the menu system. After powering on the radio, press NEXT, then SETUP, and continue pressing NEXT approximately 15 times until you reach the "BT/Wi-Fi" menu. Here, toggle "Wireless on" and then select "Select AP." Choose your Wi-Fi network and enter the password. Be aware that you may encounter Wi-Fi scan failures, which could cause the software to hang, necessitating a restart of the radio. Once connected, exit the menu and save your settings. In radio mode, touch the tiny Wi-Fi symbol on the screen until it lights up green, indicating a successful connection.

With the radio connected to Wi-Fi, you can take full advantage of the brilliant online features developed by the Jstvro group specifically for the ATS25 max-Decoder. These features enhance the overall functionality and user experience of the device.

Receiver developers — Credit where credit is due. 73 to the developers!

Owning an ATS25 max-Decoder with a registered product key means you are eligible for software updates. Currently, my radio is running version "Air 4.17 Beta," which does not support the "Bluetooth" option in the BT/Wi-Fi menu. It appears this feature might be enabled in a future update or extension.

Favorite Features and Desirable Additions

The ATS25 max-Decoder, powered by ESP, boasts an impressive array of features and capabilities, with the "Retro" tuning scale, S-Meter, FT8 mode, and CW decoder quickly becoming my favorites. These elements add a charming and functional touch to this versatile radio.

The ATS25 max-Decoder's portability and long-lasting rechargeable battery make it perfect for outdoor use, away from man-made noise. Once you get accustomed to using the supplied stylus to navigate the menus, tuning and operating the radio becomes a pleasure. The internal speaker provides good sound quality, but I prefer using headphones for a more immersive experience without disturbing others. Although I didn't have time to set up an IDE on my PC, run PUTTY at 115,200 baud, and read decoded messages and station information, the radio's robust software capabilities leave room for future enhancements.

Here are a few features I would love to see added to the ATS25 max-Decoder:

VLF Band Reception: A range of 50 kHz to 200 kHz for DCF/MSF timekeeping stations and lowfers.
2-Meter Band Reception: Covering the 144-146/148 MHz range.
VHF Airband Reception: Note that this isn't allowed in all countries.
NBFM for the CB Band: Supporting 40 channels, particularly the European.
Manual Improvements: Less Italian in the English manual and better guidance for Wi-Fi setup.

Given that the ATS25 max-Decoder is powered by the ESP32 and considering the skilled Elektor readers who are proficient in coding for this platform, I am confident that these enhancements are not only achievable but can be surpassed with great success.