We report on the development of a frequency converter frontend that enables 2.4 GHz IEEE 802.11 WLAN devices to operate in unused parts of the UHF spectrum, the so-called TV whitespaces. The new frontend features separate TX and RX paths with automatic directionality switching and in-hardware energy detection. Equipped with a standard antenna port at the input side, and a fully configurable transmit chain controlled through a user-accessible microcontroller, this whitespace converter (WSC) with commercial off-the-shelf components is particularly suitable for cognitive radio (CR) research. We discuss the specific challenges in the design of a WSC for 802.11 WLAN routers. The complete architecture is presented, and preliminary performance results from a level plan optimization are discussed.