# Characterization of a Mechanical Antenna Based on Rotating Permanent Magnets
## 2 Related Works
The used NdFeB magnets are cylindrical with a radius of 5 mm and a height of 3 mm and have a residual flux density of 1.23 T. The magnets are fixed on a disc made of an aluminum alloy with a diameter of 100 mm and a thickness of 2 mm. Reference [9] showed a mechanical antenna based on a vibrating permanent magnet. This mechanical antenna radiates electromagnetic waves in the ELF band by an alternate motion of the permanent magnets and can generate an induced voltage with a peak value of 0.179 mV at 1 m.  
Another type of mechanical antenna with permanent magnets is the resonant Magneto-Mechanical Transmitter (MMT). The MMTs described in [10; 11] use resonant angular oscillatory motion of one or more permanent magnet rotors to produce a ULF signal. This approach was experimentally demonstrated by the authors by using two MMT systems. The first MMT prototype uses a construction with a single rotor that works at 166 Hz and generates a magnetic field of 1.09 fT at 1 km. The second MMT prototype has six identical cylindrical rotors and two cuboidal stators. When they are adequately overlapped, two modules of six rotors can work as a larger MMT system that can produce twice the time-variable field. The magnetic field produced by two modules with six cylindrical rotors has a frequency of 900 Hz and an amplitude of 0.34 fT at 1 km. Based on the large-volume splitting method, [PERSON] et al. [12] used permanent magnets with small volumes to form a mechanical antenna array. The experimental results show that if a time-variable magnetic field sensor with a sensitivity of more than 100 fT is used to receive the generated signal, the developed prototype has a transmission distance of 700 m. Also, the results show that the best structure of mechanical antenna radiation source is the axially magnetized cylindrical permanent magnet. The magnets used for this mechanical antenna array were 30 mm in diameter and 60 mm in height. In [13], a magnetoelectric (ME) mechanical antenna based on the magneto-mechanoelectric (MME) effect was presented. The MME effect results from the combination between the piezo-driven magnets' motion and converse magnetoelectricity (ME). The system consists of a fixed PZT/Metglas structure with a permanent magnet attached to its free end. The oscillating magnet is used as an additional vibrating magnetic dipole that improves the radiation of the mechanical antenna. For one of the devices they made, the authors reported, at a frequency of 11.32 kHz, a magnetic field with a measured value of \(\sim\)0.01 nT at a distance of 5 m.  
Mechanical antennas were also built based on the piezoelectric effect. The piezoelectrictiong effect was also used to build mechanical antennas. Thus, in order to significantly reduce the dimensions of low-frequency antennas, [PERSON] and [PERSON] [14] proposed a pair of low-frequency transmitting and receiving mechanical antennas. An AC voltage applied to the transmitter generates an alternating strain and an electromagnetic wave, while by applying an electromagnetic wave to the receiver, an alternating strain and a voltage are generated. Both antennas are made up of a magnetostrictive Terfenol-D and piezoelectric PZT laminate. In order to improve the performances at small dimensions, in both antennas, small Rb magnets that provide a bias DC magnetic field were added. The pair of miniaturized mechanical antennas work at a mechanical resonance frequency of 37.95 kHz, and a signal amplitude of 14.51 \(\upmu\)V was obtained at a distance of 9 m.  
For the various mechanical antennas involving permanent magnets, Table 1 shows a summary of the measured results extracted from the related literature along with the theoretical model used to calculate the generated magnetic field and the applications these antennas were intended for.