Long exposure to high radon levels leads to an increase of developing lung cancer risk, due to irradiation of lung tissue by the α particles emitted by radon and its decay products. Currently radon is considered to be the second cause of lung cancer, after only cigarettes smoke. Another very interesting aspect related to the radon concentration is its potential use as a seismic events precursor, according to some geophysical models that suggest the release of radon from the underground as a result of tectonic deformation responsible for the earthquake. The detector evaluated in this study is an ionizing chamber, operating in free air at atmospheric pressure. It consists of a metal cylinder used as a cathode and a wire anode placed along the axis. It provides radon concentration measurements with a fast time response, useful for sudden changes in radon gas emission. The prototype tested and discussed in this paper is extremely compact, economical, with a long autonomy of operation, easy to install and use. A new version of the detector has been developed and characterized, with a cylindrical steel cathode, having a mesh structure. The detection performances are similar to the previous prototype with direct operation into the atmosphere and reduced weight, size and energy consumption. The detector presented could be used in a widespread network on the territory, in order to evaluate the radon gas concentration in living environments and/or to detect radon emission before a seismic event.