# Characterization of a Mechanical Antenna Based on Rotating Permanent Magnets
## 2 Related Works  
Regarding the structure, antennas with permanent magnets were made in various configurations with one or more parallelepiped or cylindrical permanent magnets with different dimensions, axially or radially magnetized, that have rotating or vibration motion or are fixed with a rotating shutter that enables the alternative passing of the static dipole magnetic field. Thus, in 2018, [PERSON] et al. [1] showed a mechanical antenna with a rotating neodymium permanent magnet that had a remanence of 1.2 T and a rectangular shape with a size of 10 cm \(\times\) 5 cm \(\times\) 2 cm (100 cm\({}^{3}\) volume). In [4], a cylindrical NdFeB permanent magnet with a diameter of 5 cm and a height of 3 cm was used to generate a low-frequency magnetic signal. The magnet was axially magnetized, and its nominal remanence was 1.2 T. The generated signal had a frequency of 35.6 Hz, and the magnetic field measured at a distance of 2 m was 1500 nT and at 8.4 m was 17 nT. [PERSON] et al. [5] proposed a type of mechanical antenna that is based on dual permanent magnets and consists of an actuation module and two neodymium cylindrical permanent magnets radially magnetized with a remanence of 1.25 T and a volume of 3.93 cm\({}^{3}\). The experimental results showed that the magnetic flux density reached 2.35 nT at 1 m in air. An application approached by the authors was positioning of and communication with pipeline robots. Thus, the antenna was placed inside a steel pipeline with an outer radius of 180 mm, length of 1200 mm, thickness of 8 mm, and conductivity of 4.22 MS/m. For the Bx component of the magnetic field measured at a distance of 3 m from the pipeline, a value of 40.57 nT without pipeline and a value of 6.59 nT with pipeline were recorded. [PERSON] et al. [2] presented a mechanical antenna with rotating shutters and fixed permanent magnets. By rotating a shutter with multiple apertures made of a soft magnetic material in front of fixed magnetic dipoles, the static magnetic dipole field is alternately passed or distorted by the shutter. The shutter's number of apertures, \(N\), reduces the rotating speed required to obtain the desired radiation frequency, \(\omega\), to \(\omega/N\). Two versions of mechanical antennas were presented with four and eight rectangular neodymium permanent magnets placed around the DC motor that rotates two aligned steel shutters with eight apertures separated by the same distance as the permanent magnets. In [6], an electromechanical system called a Magnetic Pendulum Array (MPA) was proposed. The array is self-biased; the alignment of each magnet in the array to the array axis occurs by itself due to the field of the adjacent magnets. Around the housing in the plane of the magnets, an external coil is placed to excite the magnets to create a time-varying magnetic field that is perpendicular to the pendulum array plane. The prototype of the magnetic pendulum array consists of 28 pendulum elements suspended by stainless steel ball bearings in a plastic housing. Each pendulum element is made of a diametrically magnetized NdFeB cylindrical magnet with a length of 40 mm and a radius of 4 mm and is supported by aluminum bearing adapter sleeves on each end. The excitation field is provided by a coil that is wrapped around the magnets. A simulated model of mechanical antenna with rotating magnet was shown in [7]. Two kinds of mechanical motions were studied here: a uniform circular motion around the rotating shaft and a shifting motion. The model used a NdFeB cuboid permanent magnet with a volume of 6 mm\({}^{3}\), relative permeability of 1.099, and magnetic coercivity of 8900 kA/m. The simulation showed that very-low-frequency signals below 1000 Hz over a distance of 20 m can be obtained. [PERSON] et al. [8] reported a mechanical antenna based on a triangular array of rotating permanent magnets.