Modulation And Demodulation Algorithm And Receiver System Design Based On Permanent Magnet Mechanical Antenna

The traditional radio frequency transmission principle of antenna in very low frequency wireless communication system is that the oscillating current inside the conducting conductor is coupled by an amplifier to generate radio frequency signal for radiation.The size of antenna directly determines the efficiency of radiation.In order to send long-distance signals,the size of antenna often occupies a large area,thus greatly restricting the development of very low frequency wireless communication system.However,the attenuation coefficient of very low frequency electromagnetic wave in underwater and underground media is small,and its advantages in ground communication and submarine communication are obvious.Therefore,the miniaturization of low frequency antenna has become the focus of relevant researchers.The thesis presents a permanent magnet mechanical antenna,which generates very low frequency electromagnetic waves by rotating the permanent magnet.This method does not need impedance matching network.It only needs to change the volume of the permanent magnet to increase the radiation efficiency,which provides reference value for the miniaturization of the low frequency antenna.In this thesis,the basic principle of mechanical antenna is described,and then the magnetic field intensity and electric field intensity radiated by electric dipole and magnetic dipole are theoretically deduced.Combined with the existing material technology,the Nd Fe B permanent magnet is selected as the rotating material of mechanical antenna,and the current loop placed orthogonally is used to simulate the rotating magnetic dipole in the near field The simulation of the far-field magnetic field intensity and the transient direction of radiation shows that the radiation of the rotating magnetic dipole conforms to the rotating radiation characteristics.Because of its portability and low frequency of electromagnetic wave,mechanical antenna is more practical in the field of submarine communication and ground communication.This thesis analyzes the feasibility of mechanical antenna in submarine communication and ground communication through theoretical derivation and experimental measurement.Through theoretical derivation,we can know that the conductivity of underwater medium and the frequency of electromagnetic wave determine the propagation attenuation and skin depth of electromagnetic wave under water.Because of its low frequency band,the attenuation of extremely low frequency electromagnetic wave under water is small,and the skin depth basically meets the distance requirements of submarine communication;In the analysis of ground communication,through the experimental measurement and theoretical calculation of conductivity,permeability and dielectric constant of common underground materials,we know that the attenuation coefficient of extremely low frequency electromagnetic wave in underground medium is small,which verifies the feasibility of mechanical antenna to submarine communication and ground communication.Because of the low frequency band and small data carrying capacity,a modulation method of 3-ary A-F hybrid modulation(3AFK)is proposed based on FSK and ASK to improve the transmission of information.The modulation method can effectively improve the digital rate and increase the transmission of information.Then,the baseband orthogonal digital demodulation method is used to demodulate the modulation signal of the mechanical antenna,and the demodulation process is discussed.At the same time,a receiver system of mechanical antenna excitation signal is designed.The receiver system combines software radio technology and hardware circuit to complete the reception and demodulation of extremely low frequency signal.