There are radio amateurs who operate. And there are radio amateurs who build, measure, solder, mill, program and repair. Karl from Tretram, OE8JHK, does both – and documents his projects properly on top.
Auf seiner Seite wiki.echtcool.net Karl collects his projects – from a church tower clock to a satellite station, from a chicken coop to an SWR measurement bridge. And anyone who clicks through will quickly notice: this isn’t just tinkering, this is done properly. With test equipment, datasheets and the necessary respect for physics.
He who measures, measures rubbish – as the saying goes. With Karl, that’s exceptionally not the case.
Radio: From repairs to satellites
Karl has been licensed since 1987, was inactive for a long time and started again in 2022 – properly. New antennas on the roof, an end-fed 10-80m with 49:1 ferrite transformer, cable lengths measured via VNA and TDR, 80mm wall feedthrough with PVC conduit. No makeshift solutions, done right.
He also repairs classic radios: an ICOM IC-240, a Yaesu FT-767 (multi-year project), an FT-757 and an ICOM IC-781. Anyone who has ever opened up a 30-year-old radio knows: it’s no walk in the park. But Karl doesn’t just repair his own equipment – if you have a sick transceiver, he’s the right person to talk to. You can learn a lot from him.
SWR measurement bridge with ESP32
Conventional SWR meters are quite inaccurate at low power levels. Karl’s solution: an AD8307 logarithmic amplifier (0.1–500 MHz), a 16-bit ADS1115 ADC, an ESP32 with web server and MQTT – and a Nextion touch display on top. Measurement data is sent via MQTT to a broker, SWR displayed in real time. Really well thought out.
Transmitter location via TDOA
Multiple receiving stations with GPS synchronisation, Raspberry Pi, ESP32 and ICOM IC-9700 receive the FM carrier of a transmitter. The time difference of the signals is evaluated via triangulation – server with SQL database, ASP.NET Core API, REST interface. Locating transmitters the technically clean way.
QO-100 satellite station
A complete QO-100 station: Bullseye LNB with PLL-TCXO for reception, helix antenna with 100W PA for uplink on 2.4 GHz, 95cm dish, CI-V Bluetooth interface for controlling IC-7300 and IC-9700. Plus 3D-printed weather protection parts. From reception to transmission, everything self-built.
Morse key – completely self-built
Because Karl also wanted to build something purely mechanical: a classic Morse key, designed from scratch. Multiple prototypes, documented with photos and videos. Some people buy one on Amazon – Karl builds one in his workshop.
Not just radio: DIY on all channels
What makes the site special: Karl doesn’t just build radio gear. The range is impressive.
Church tower clock
Die örtliche Kirche hatte ein Zifferblatt ohne Zeiger. Karls Reaktion: selber bauen. CNC-gefräste Zahnräder aus Sperrholz, Schrittmotoren für Stunden- und Minutenzeiger, ESP32-Steuerung mit DCF77, GPS und NTP-Synchronisation, Webinterface zur Fernsteuerung. Eine Church tower clock. Selbst gebaut. Respekt.
ChickenGate
An automatic chicken door with ESP32, OLED display, light and temperature sensors, automatic door based on sunrise and sunset, web server for remote control and HomeMatic integration. Version 2.0 is even portable. IoT in the chicken coop – the chickens probably have better WiFi than some neighbours.
LoRa data link (Roofmatic)
Eight DHT22 sensors on an ESP32 with SX1276 LoRa module, binary packet transmission on 868 MHz with spreading factor 12, receiver with MQTT bridge, MySQL database and Pushover notifications. Tested at 7.5 km range. Battery operation for 4–6 months.
And even more…
Solar system with weather data logging, lightning detection station in Köttmannsdorf, six different multicopter projects (2011–2017), a propeller balancing machine, a virtual pinball, a Schnapsomat (indispensable for calibration after measuring), smart home – the list goes on. Everything documented, everything reproducible.
Rohde & Schwarz XK-852: Professional Radio Rediscovered
Karl’s latest major project: a Rohde & Schwarz XK-852-C1, a professional 150-watt HF transceiver built to military specifications. Frequency range 1.5–30 MHz TX and 0.4–30 MHz RX, operating modes CW, AME, USB, LSB, ISB, FSK and Data. 100 programmable channels, built-in speech compressor, BFO, squelch and a complete Built-In Test Equipment (BITE). The unit is microprocessor-controlled – invalid settings are automatically blocked and displayed as “ERROR”.
Particularly interesting: the V.24/RS-232 interface for remote control – via cable, telephone line or even radio link. Karl has documented the complete connector pinouts for all interfaces (power supply, remote, ATU, audio, external) on his wiki page – including all pin assignments and accessory part numbers. A goldmine for anyone working with R&S equipment.
👉 XK-852 documentation on wiki.echtcool.net
ESP32 Remote Control for the R&S Transceiver
Classic professional technology meets modern microcontrollers: Karl has developed an ESP32-based VFO controller that remotely controls the R&S transceiver via its serial V.24 interface. The system features a web portal in the browser for frequency, mode and band selection, a Nextion touchscreen as a local control interface, a rotary encoder with dynamic step width and acceleration (up to 29,999,990 Hz), a real-time clock with backup battery and OTA firmware updates via the web interface.
Control is via ASCII commands to the transceiver – for example *F1420000 for 14.200 MHz or *I2 for USB. Planned extensions include WebSDR integration, remote operation over the internet, digital modes (FT8), an RTTY encoder/decoder and a CW module. A project that shows how to bring historic radio technology back to life with modern tools.
👉 ESP32 Remote Control on wiki.echtcool.net
Worth a look
Besides all these projects, Karl is also regularly QRV on the OE8XNK repeater and happy to help out. If you have a question about one of his projects or just want to talk shop – just call in.
Karl’s wiki is not a polished showcase – it’s an honest workshop documentation. Photos of the build process, schematics, measurements, mistakes and solutions. Exactly what you need when you want to build something yourself. No marketing, no fluff – just projects that work.
73 de Michael, OE8YML
