The Ultra-Portable ATS25 max-Decoder Receiver
- by Jan Buiting
- Reading time: 12 min
73, all hams and radio fans! Elektor is thrilled to unveil the ATS25 max-Decoder — a powerful and ultra-compact multimode DSP receiver. Covering LW, SW, MW, FM, and Ham bands, this cutting-edge device is driven by an ESP32 core and features Wi-Fi connectivity for advanced decoding modes. Experience the ultimate blend of embedded and radio technology!
Radio, especially ham radio, has long been a pioneering, challenging, and rewarding field for microcontroller enthusiasts, programmers, and embedded systems fans. This surge in interest is largely due to the recent advent of ICs and plug-and-play integrated modules that handle all high-frequency signal conditioning and provide easily controllable pins for both input and output of digital signals. What more could a programmer ask for?
Embedded — with a Radio Attached
One IC family that has successfully encouraged even the staunchest analog RF technology enthusiasts to embrace digital is the SkyWorks Si473x series. Among them, the Si4735 stands out as a "CMOS 100% AM/FM/SW/LW radio receiver," integrating the entire tuner function from antenna input to audio output. Although the chip gained popularity among hams and radio enthusiasts when Arduino software was developed for it, its full potential wasn't realized until an international group of hams and programmers employed a more powerful microcontroller, the ESP32. They began creating ingenious software, not only for the LW/MW/SW/FM broadcast bands but also for the amateur radio bands from 160 m to 10 m, including associated "ham modes" like CW, RTTY, USB/LSB, FAX, and more.
The Si4735 chip and the ESP32 micro first appeared together in the "ATS25" multimode receiver, which has been available for a few years, excelling as a general-purpose radio for broadcasts and ham communications. Recently, the "Jstvro" group enhanced the standard ATS25 with a range of new features, including advanced digital decoding, Wi-Fi connectivity, and more sophisticated DSP audio processing. This upgrade resulted in a comprehensive overhaul of the control software, giving the ATS25 a significant boost in functionality and performance.
Can’t Believe It’s So Small
The first thing that struck me when unboxing the ATS25 max-Decoder was its compactness, sturdiness, and lightweight design. The front panel features a single control: a 40 mm diameter knob that functions as a rotary encoder with push-button action. On the back of the radio, you'll find two antenna inputs—one SMA socket for the Wi-Fi (2.4 GHz) antenna and one BNC socket for the main antenna. There’s also a USB-C connector, which serves both to charge the internal Li-Ion battery and to communicate with your PC. Completing the user interface on the back panel are a headphone socket and two small slide switches.
I was pleasantly surprised to find a stylus pen, a foldable Wi-Fi antenna, a telescopic rod antenna for FM, a USB-C cable, and even a cloth—presumably for dust protection or cleaning the touchscreen—included in the package. Aside from the radio itself, the highlight of the box was the ATS25 max-Decoder Manual. Although not perfect, it stands head and shoulders above the usual fare that accompanies Chinese electronics. The 24-page manual is properly printed and features clear pictures with legends and captions to explain the most important functions. It's definitely worth browsing through with the radio off! I was informed that Elektor is considering enhancing the manual and printing their own version.
Antenna First — Always
All microcontroller enthusiasts should remember this: it’s not bits and bytes, but a good antenna that’s your radio’s best ally. The included telescopic antenna works well for FM band reception when fully extended, but it falls short for LW/MW/SW or ham bands. Instead, try unrolling at least 10 meters of flexible wire, stringing it up outdoors, and clipping the free end to the telescopic antenna. That’s when the real fun begins—unless, like me, you encounter massive interference from digital sources such as LED lights, computers, smartphones, and my 7.5 kWp solar panel installation, particularly the SolarEdge 3-phase inverter. 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, as I discovered by silencing the antenna input with a 50-ohm terminator. The remaining noise, which is burst-like, comes from operating the rotary encoder and the OLED screen.
Due to radio wave propagation, daytime reception on the MW and LW bands is poor, even with a wire antenna. However, the ATX25 max-Decoder managed to find several stations around 675 kHz and 1000 kHz, performing on par with my Grundig Yacht-Boy and even a 1960s Philips “BX” tube radio, both of which use ferrite rod antennas. I also picked up good old Droitwich on 198 kHz LW, catching a BBC cricket match report. The MW band, in particular, comes alive in the evening hours, making it great fun to tune in and discover pirate and "low-power" stations for private use.
On my ATX25 max-Decoder, the FM band ranges from 64 to 108 MHz, a unique feature I haven't seen before. This range allows me to monitor the 4-meter (70 MHz) band, which isn't listed under the "HAM" bands. Additionally, I picked up some 80 MHz police radio traffic from just across the border in Germany.
I eventually switched to a wire loop antenna, which responds to magnetic fields rather than electric fields like the plain wire antenna. However, the small loop antenna available from Elektor comes with an SMA connector, while the ATS25 receiver uses a BNC socket. To solve this, I improvised by connecting an SMA-to-BNC adapter to a BNC male-to-male adapter. This setup worked well, significantly reducing man-made noise. The loop antenna's directional nature allowed me to tune it to minimize interference from sources like LED lamps and LCD screens.
For optimal performance, hang the loop antenna outdoors, away from noise sources, and run the cable inside to your receiver.