This communication concerns the detection of 13.56 MHz (HF) RFID “small” tags. Herein, the term “small” refer to an effective area below 1 cm2 and the detection principle is in volume, especially inside a tube of 9 cm in diameter and 2m in length. The ability to detect the “small” tags in the tube is achieved by using a coil resonator conformed on the tube surface, following a principle of multiple magnetic coupling, also referred as magnetic field guide. Theoretical considerations on mutual coupling formula and electrical model fit to CST simulations and VNA measurements concerning the evaluation of impedance and coupling factors range. Detection tests with an RFID reader (from IB technology) and NXP SLI-X chip confirm the possibility of detection by providing a first result of 2 cm range. This detection was impossible inside the tube without using the resonator. Perspectives of improvement evocated at the end of the paper are numerous for that structure.