Marking tag for animals, such as an ear tag, comprising a radio frequency identification RFID device (100, 200, 300, 400, 600) suitable to be attached to an object and to be tuned at assembly time to the material surrounding the device comprising: - a wireless communication device (101, 201, 301, 401), such as an RFID chip or strap, having at least a pair of contact points (221, 521), and - an antenna structure (102, 202, 302, 402, 602) having a sheet (103, 203, 303, 403, 503, 603) of electrically conductive material being provided with an inner opening (104, 204, 304, 404, 604) so as to provide a hole through the antenna structure (102, 202, 302, 402, 602), the inner opening being configured to facilitate a locking mechanism engaging with the tag through the opening, the sheet (103, 203, 303, 403, 503, 603) further being provided with an elongated slot (105, 205, 305, 405) with a first and a second side, the slot (105, 205, 305, 405) having an inner end (106, 206, 306, 406) at a periphery of the opening (104, 204, 304, 404, 604) and an outer end (107, 207, 307, 407) at a periphery of the sheet (103, 203, 303, 403, 503, 603), the slot (105, 205, 305, 405) being formed so as to provide a spacing between the first side and the second side, where - the conductive material is electrically connected from the first side of the slot (105, 205, 305, 405) to the second side of the slot (105, 205, 305, 405) via the contact points and through the communication device (101, 201, 301, 401) at a position along the length of the slot between the inner end (106, 206, 306, 406) and the outer end (107, 207, 307, 407), the position along the length of the slot being determined so as to tune the RFID device to a pre-determined resonance frequency, allowing the device to be adapted to specific electronic parameters and tuned to different configurations depending on the materials directly surrounding device, whereby the marking tag further comprises an attachment member for piercing a part of the animal, such as an ear, the attachment member comprising engagement means, and wherein the opening (104, 204, 304, 404, 604) of the marking tag comprises locking means, which locking means are adapted for being lockingly engaged with the engagement means of the attachment means. - a wireless communication device (101, 201, 301, 401), such as an RFID chip or strap, having at least a pair of contact points (221, 521), and - an antenna structure (102, 202, 302, 402, 602) having a sheet (103, 203, 303, 403, 503, 603) of electrically conductive material being provided with an inner opening (104, 204, 304, 404, 604) so as to provide a hole through the antenna structure (102, 202, 302, 402, 602), the inner opening being configured to facilitate a locking mechanism engaging with the tag through the opening, the sheet (103, 203, 303, 403, 503, 603) further being provided with an elongated slot (105, 205, 305, 405) with a first and a second side, the slot (105, 205, 305, 405) having an inner end (106, 206, 306, 406) at a periphery of the opening (104, 204, 304, 404, 604) and an outer end (107, 207, 307, 407) at a periphery of the sheet (103, 203, 303, 403, 503, 603), the slot (105, 205, 305, 405) being formed so as to provide a spacing between the first side and the second side, where - the conductive material is electrically connected from the first side of the slot (105, 205, 305, 405) to the second side of the slot (105, 205, 305, 405) via the contact points and through the communication device (101, 201, 301, 401) at a position along the length of the slot between the inner end (106, 206, 306, 406) and the outer end (107, 207, 307, 407), the position along the length of the slot being determined so as to tune the RFID device to a pre-determined resonance frequency, allowing the device to be adapted to specific electronic parameters and tuned to different configurations depending on the materials directly surrounding device, whereby the marking tag further comprises an attachment member for piercing a part of the animal, such as an ear, the attachment member comprising engagement means, and wherein the opening (104, 204, 304, 404, 604) of the marking tag comprises locking means, which locking means are adapted for being lockingly engaged with the engagement means of the attachment means.