Almost as soon as the Software Defined Radio article was published in the May 2007 issue of Elektor Electronics, we started receiving requests for special-purpose receiver upgrades. The most commonly requested capability was extending the receiver range to include low frequencies. The VFO can be tuned as low as 20 kHz, and the receiver input stage is suitable for very low frequencies, so there are no theoretical obstacles to using the SDR board for the VLF band.