A filtering device intended for being connected to a portable mobile terminal compatible with a PMR network having a wide range of receiving frequencies Bi, includes a fastening system for fastening to the mobile terminal, a first radio-frequency connector intended for engaging with a radio-frequency connector of the mobile terminal, a first channel including a first filter for filtering a first receiving useful band Bui included within the band Bi, and a second radio-frequency connector intended for engaging with a removable RF antenna to transmit or receive radio-frequency signals.