This project adapts a KKmoon digital frequency counter module for use with a classic Grundig GDO 701. The module is specified for 1–500 MHz, but it can be used up to about 1.1 GHz. Accuracy is high below 500 MHz; above 500 MHz, readings should be considered approximate. The unit runs from 9 V and is powered by a standard 6LR61 alkaline battery.

The enclosure can be printed in PLA or ABS using typical settings. Supports are required. The back plate is secured with four small screws, and the housing also includes four holes for Ø6×3 mm neodymium magnets, allowing magnetic attachment to the GDO.

Update: A coil/bobbin is included to add an SMA connector input. Wind a few turns of thin copper wire on the bobbin and connect it to the SMA connector. Using loose inductive coupling, you can measure the frequency of a GDO or other oscillator without making a direct electrical connection.