Can a simple medium-wave radio receive shortwave broadcasts? In 2005, Burkhard Kainka answered that question with a creative two-transistor converter that demonstrates the power of clever RF design.
What can you accomplish with two transistors, a handful of inductors, and a solid grasp of RF fundamentals? Quite a lot, as Burkhard Kainka demonstrated with a short-wave converter project in 2005. The circuit is a masterclass in minimalist radio design, using basic frequency conversion techniques to extend the capabilities of an ordinary medium-wave receiver.
The Circuit
At first glance, the circuit looks almost deceptively simple. Built around two BC548C transistors, a few inductors, and some passive components, the converter allows an ordinary medium-wave radio receiver to tune into signals from the 49-m shortwave band. The principle is classic radio engineering: a local oscillator running at approximately 4.4 MHz mixes with incoming shortwave signals near 6 MHz, translating them down to around 1.6 MHz, where a standard MW radio can receive them.
The design features two BC548C transistors, a few inductors, and some passive components.
The entire circuit draws less than 1 mA
"At the upper end of the medium-wave band, select an unoccupied frequency that you want to use for
listening to the converted short-wave signals," Kainka noted. "Good reception performance can
be obtained using a wire antenna with a length of one to two meters."
What makes this design particularly interesting today is not its performance compared to modern SDRs or synthesized receivers. Rather, it is the quality of the solution. Kainka demonstrated how frequency conversion, oscillation, tuning, and mixing can be achieved with minimal hardware and no integrated circuits. The circuit even uses LEDs as voltage-variable capacitors, an unconventional technique that highlights the creative experimentation often found in homebrew radio designs.
Today, projects like this offer a valuable opportunity to explore RF fundamentals that are often hidden inside complex modern devices. Concepts such as heterodyning, tuned circuits, oscillator stability, and impedance effects become tangible when implemented with discrete components.
Most Elektor community members probably would not build such a converter as their primary shortwave receiver. But as an educational project, a historical design study, and a reminder of the ingenuity that characterizes amateur radio experimentation, it remains as fascinating as it was two decades ago.
The Short-Wave Converter Project
The original article, “Short-Wave Converter,” appeared in Elektor July/August 2005. Here is the article.
Editor's Note: The article first appeared in a 2005 edition of Elektor. Since the project is over two decades old, some parts and components might not be available. Still, we think the project will inspire you to start a new design.
Elektor Magazine has been one of the leading sources of information on electronics for engineers, designers, start-ups and companies for 65 years. Our magazine is powered by an active community of electronics engineers – from students to professionals – who are passionate about designing and sharing innovative ideas.
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