This paper focuses on bats detection, tagged at low frequency (134.2 kHz) with glasstags (typically : length 15 mm and radius 1 mm). The physical link is magnetic coupling. We propose the design of a reader antenna based on a multi-coil structure. The goal is to obtain the highest surface of detection, at a distance of 10-15 cm, i.e. defining a so called “volume of detection”. The cylindrical volume of the glasstags involves a high sensitivity to the magnetic field orientation. As the bats are flying, the tags equivalent surfaces are considered with a random orientation. Using complementary coils principle enables to detect fruitfully the glasstags in two perpendicular orientations named HM (horizontal mode) and VM (vertical mode). We propose the design of a flexible reader antenna structure by means of an adhesive copper tape fixed on a cloth. The prototype is designed after CST simulations and empirical formula evaluations. Measurements show a good agreement with the modelling and tests of detection are performed with the proposed reader antenna. By comparison with a commercial antenna, the prototype reaches the highest volume of detection in both HM and VM modes and fulfills the targeted distance of detection.