The Ultra-Portable ATS25 max-Decoder Receiver

    • by Jan Buiting
    • Reading time: 11 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.

    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.

    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.

    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.

    For optimal performance, hang the loop antenna outdoors, away from noise sources, and run the cable inside to your receiver.

    The YouLoop

    The "YouLoop" from the Elektor Store is a great starting point for a proper antenna. Keep in mind, though, it's not designed for permanent outdoor use. Also, make sure there's nothing inside the loop! :-)

    Welcome to Radio

    Once you understand the quirks of radio wave propagation, the tuning and listening experience with the ATS25 max-Decoder rivals that of many “big box” receivers. Operating the ATS25 with the included stylus takes some getting used to. Initially, when I misplaced the stylus, I had to use my fingertip to select the tiny fields and icons on the touch screen, which worked, albeit less precisely. Another minor issue was keeping the radio steady and slightly angled, given the stiff (semi-rigid) coax cable from the loop antenna connected at the back. I resolved this by placing the radio on a smartphone holder. Yes, I’m accustomed to SW receivers weighing 20 kg and more (Collins, Hallicrafters, you know).

    Smartphone holder secures the receiver
    I used an inexpensive smartphone holder to secure the receiver at a comfortable angle for viewing and operation. It even has a 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, although 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?

    I found the AGC (automatic gain control) and RF attenuation functions to be somewhat inconsistent and not quite effective at handling high-power stations within the bands, such as certain Chinese broadcasts.

    Getting Online with Wi-Fi

    Many of the advanced digital decoding functions, such as FT8 and CW, as well as NTP timekeeping, rely partially or entirely on an Internet connection. While the ATS25 max-Decoder is relatively easy to put online, the instructions for doing so could use improvement. The Wi-Fi connectivity, which is essential for the radio's operation, is only vaguely explained on the last page of the manual, and several important steps are not mentioned.

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

    The most reassuring aspect of the ATS25 max-Decoder from Elektor is that it comes with a registered product key, so you don’t have to apply for one separately. The product key is prominently displayed when you switch on the radio, and it's a good idea to write it down and keep it in a safe place. The next challenge is finding the menu to log the radio onto your Wi-Fi network. Duh, that’s on one of the last pages of the SETUP menu!

    After powering on the radio, press NEXT, then SETUP, and then NEXT about 15 times until you reach the "BT/Wi-Fi" menu. There, switch on "Wireless on" and then "Select AP." Select your desired Wi-Fi network and enter the password. I encountered several Wi-Fi scan failures, which caused the software to hang and required me to restart 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. You are now connected and ready to utilize the brilliant online features created by the Jstvro group specifically for the ATS25 max-Decoder.

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

    With your registration key, you're also eligible for software updates. My radio is currently running version "Air 4.17 Beta," and I wasn't able to select "Bluetooth" in the BT/Wi-Fi menu. It seems that this feature might require a future update or an extension of some sort.

    Likes and Likeables

    The "Retro" tuning scale and S-Meter, along with the FT8 mode and CW decoder, quickly became my favorite features of this ESP-powered radio, which boasts an impressive array of options and capabilities.

    Receiver retro mode
    Totally charming: the "retro" mode for the frequency scale!

    The radio's impressive portability and long-lasting rechargeable battery make it an excellent choice for outdoor use, far from man-made noise. Tuning and operating the ATS25-max Decoder is a pleasure once you get the hang of using the supplied stylus to navigate the menus. The internal speaker delivers good sound quality, but I prefer using headphones borrowed from my Samsung smartphone for a better experience without disturbing others.

    For this review, 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 on my computer. What's missing? Not much, and certainly feasible enhancements for the future, considering the radio's robust software capabilities. Here’s what I would have loved to see:

    • VLF band reception, say 50 kHz to 200 kHz for DCF/MSF timekeeping stations, lowfers, etc.
    • 2-m band reception (144-146/148 MHz)
    • VHF Airband reception (not allowed in all countries)
    • NBFM for the CB band (40 channels, European)
    • A little less Italian in the (English) Manual
    • Better guidance for the Wi-Fi setup

    Given that the ATS25 max-Decoder is powered by the ESP32 and Elektor readers are both numerous and skilled in coding for this platform, I'm confident that all of the above enhancements can not only be achieved but surpassed "S9" (with great success).

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