This paper presents a prototype of a lower cost tool that allows to measure the channel occupancy and signal levels on a wide range of the electromagnetic spectrum, this tool scan the radio channels of each radio service to establish if there are available as white spaces for cognitive radio or used by a primary or licensed operator in a certain portion of the spectrum. The architecture of the prototype is presented, the hardware components such as the RTL-SDR, the development of the software based on GNU Radio, and finally some results of measurements for frequencies between 470 to 900 MHz are presented.