极低频（ELF, 3 Hz - 30 Hz）到特低频（ULF, 300 Hz - 3 kHz）频段的电磁波在透地、透水等应用场景中具有不可替代的作用。传统的天线依靠电磁波谐振来产生辐射，因此天线尺寸要和电磁波的波长相当才可以有效辐射，这导致了ELF - ULF频段（波长100 - 10⁵ km）天线尺寸过大的问题。旋转永磁式机械天线提供了一种新的解决方案，它不是通过电磁波谐振来产生电磁波，而是通过永磁体的机械旋转来产生电磁波，其辐射效率可比常规线圈天线高出多个数量级。本文主要针对旋转永磁式机械天线的辐射机理和信息调制方法进行了研究，主要包括以下几个方面：

1、从工作带宽和能量效率两方面比较了旋转永磁式机械天线和常规电小天线的基本特性。介绍了常规电小天线的效率和带宽需要满足Bode-Fano理论限制，从工作机制上说明了旋转永磁式机械天线可以突破该理论限制的原因。另外，虽然旋转磁铁可以无损地产生磁场，但是在机械旋转时会消耗能量，本文对该功耗进行了分析，发现当工作频率高于一个特定的值f_m时，相同磁矩、尺寸和重量条件下，旋转永磁式机械天线的效率反而会低于传统线圈天线，我们从理论上推导了该频率上限f_m的表达式。

2、推导了旋转永磁式机械天线的均匀无限大空间辐射模型。电磁波通常是由发射天线上的时变电流源产生的。永磁体可用磁化电流等效，当永磁体机械旋转时，该磁化电流成为时变激励源。根据这一机理，本文推导了旋转永磁式机械天线在无限大均匀空间的辐射公式。与准静态近似模型相比，所推导模型不仅适用于陆上环境，也同样适用于海水、土壤等有耗传播媒质。介绍了旋转永磁式机械天线的多物理场仿真方法，分别在空气媒质和海水媒质中利用数值仿真结果对理论模型进行了验证。最后，分析了旋转永磁式机械天线的磁场空间分布特性、传输损耗、传播距离和辐射功率等辐射特性。

3、研究了旋转永磁式机械天线的跨介质传输特性。在ELF-ULF频段，平面电磁波在海面的反射系数接近于1。然而，在实验中发现旋转永磁式机械天线产生电磁波在水面处并未出现明显的反射现象。根据传输线阻抗匹配、准静态磁场特性两个方面定性解释了旋转永磁式机械天线在跨介质传输的物理机制。最后，基于索末菲积分方法，给出了任意姿态的旋转永磁式机械天线在平静海面环境中的电磁场分布精确计算方法，分析了从空气到海水、海水到空气的跨介质场分布特性，探讨了准静态磁场近似的适用条件和侧向波的影响。

4、自主设计了实验测试系统并进行了磁场测量和通信实验。制作了基于交流伺服电机驱动的旋转永磁式机械天线样机，以及灵敏磁场信号采集系统。在陆上环境中，我们测量了机械天线样机的磁场随方向和距离的变化曲线，在空气-海水跨介质环境中，测量了机械天线样机的跨介质磁场随接收深度的衰减曲线，磁场测量结果均与理论分析吻合。在利用该样机与水下10 m接收点进行了FSK通信实验，验证了旋转永磁式机械天线在空气-海水跨介质通信中的可行性。另外，分析了FSK调制中由于惯性影响而产生的速度调制失真，分析了该调制失真对误码率的影响。

5、提出了一种面向旋转型机械天线的惯性兼容调制方案。该方案中信号的调制是通过驱动力矩的瞬时变化实现的，当驱动力矩发生变化时旋转角加速度可以瞬间改变。旋转永磁式机械天线中与角加速度直接相关的信号参量为啁啾率，因此我们称这种调制方法为啁啾键控（chirp-rate shift keying, CSK）。为了实现通信中的最优检测，还提出了CSK正交调制方法，包括相干正交调制和非相干正交调制。实验结果表明，与常用的FSK调制方式相比，CSK调制方式需要更少的调制转矩，在相同的误码率水平下能够实现更高的可靠通信速率。该方法是目前已知的第一种在不违反力学定律情况下，理论上可以实现理想正交调制的旋转型机械天线调制方案。

The electromagnetic waves with frequencies from the extremely low frequency (ELF, 3 Hz - 30 Hz) band to the ultra low frequency (ULF, 300Hz-3KHz) band have small path loss in water and soil, therefore, they play irreplaceable roles in the fields of underwater and underground communication. Traditional antennas rely on electromagnetic wave resonance, which leads to antenna sizes that are comparable to the electromagnetic wavelength. Therefore, the traditional ELF-ULF antenna has a very large size due to the very long operating wavelength (100 km-10⁵ km), which has hindered its wide application. A potential solution is the application of a mechanical antenna. The mechanical antenna generates electromagnetic waves by the mechanical rotation of permanent magnets rather than electromagnetic wave resonance. Such mechanical antennas were shown to be orders of magnitude more efficient than conventional antennas in the ELF band. This dissertation focused on the radiation mechanism and information modulation method of the rotating magnet-based mechanical antenna (RMBMA), mainly including the following aspects:

1. The RMBMA and the conventional electric small antenna were compared in terms of bandwidth and energy efficiency. The Bode-Fano limit of conventional electrically small antennas was introduced. Based on the physic mechanism, we explained why the application of RMBMA can break through the Bode-Fano limit. In addition, the rotating magnet can losslessly generate a magnetic field but will consume power caused by mechanical rotation, which is related to the operating frequency. So the rotating magnet is not always more power-efficient than the coil. The operating frequency range applicable to the rotating magnet needs to be investigated. It is found that when the operating frequency is higher than a certain value called f_m, the efficiency of the RMBMA will be lower than that of the traditional coil antenna, and we derive the expression of f_m.

2. We derived in detail the expressions of the electromagnetic field generated by a rotating magnet applicable to the lossy media. The electromagnetic behavior is governed by Maxwell’s equations, which must be solved for a particular time-varying current on the transmitting antenna. The quantum spin of the inner charged particles of the permanent magnet can be equivalent to magnetizing current. This magnetizing current becomes a time-varying excitation source as the permanent magnet rotates mechanically. Compared with the quasi-static approximate model, the derived model is not only suitable for the terrestrial environment but also suitable for lossy propagation media such as seawater and soil. The multiphysics simulation method of RMBMA was introduced, and the theoretical model is verified in air medium and seawater medium respectively. The multiphysics simulation method of RMBMA was introduced, and the theoretical model was verified by numerical simulation results in air medium and seawater medium respectively. Finally, the radiation pattern in near field, pass loss, communications range and radiation power were analyzed.

3. The propagation characteristics of the electromagnetic field generated by the RMBMA from air to seawater and from seawater to air were studied. In the ELF-ULF frequency band, the reflection coefficient of plane waves on the sea surface is close to 1. However, by analyzing the wave impedance characteristics of the electromagnetic wave generated by the RMBMA, the physical mechanism of propagation across the sea surface is explained according to the transmission line theory. The low attenuation of the quasi-static magnetic field generated by the mechanical antenna at the air-seawater interface is verified by experiments and simulations. Finally, based on the Sommerfeld integration, we calculated the magnetic field of the RMBMA with an arbitrary attitude by equivalenting the RMBMA to a three-dimensional orthogonal magnetic flow element. And the performance of the RMBMA in several specific applications in an air-seawater environment was analyzed.

4. Designed and manufactured the experimental test system and carried out magnetic field measurement and communication experiments. A prototype mechanical transmitter using a cylindrical rare-earth magnet (Nd-Fe-B) driven by a servo motor was created. In the terrestrial environment, the magnetic field of the mechanical antenna prototype versus the direction and distance was measured. We also measured the attenuation curve of the magnetic field of the mechanical antenna prototype versus the receiver depth. The magnetic field measurement results are in good agreement with the theoretical analysis. The surface-to-undersea communication using this prototype mechanical transmitter was demonstrated at a distance of 10 m. Due to inertia, mechanically modulated FSK waveform is not an ideal square wave, so we also analyzed the influence of this modulation deviation on signal demodulation.

5. An inertial compatible modulation scheme for rotating mechanical antennas is proposed. The modulation of the signal in this scheme is realized by the instantaneous change of the driving torque, and the rotational angular acceleration can be changed instantaneously when the driving torque changes regardless of inertia. The signal parameter directly related to the angular acceleration is the chirp rate, so we call this modulation method chirp-rate shift keying (CSK). To achieve optimal detection, the orthogonal modulation methods of CSK were also established, including coherent orthogonal modulation and non-coherent orthogonal modulation. To demonstrate the feasibility of CSK modulation, we conducted a wireless communication experiment that used a rotary magnet-based mechanical antenna as the transmitter. The experimental results indicated that CSK modulation requires less torque and can achieve a higher reliable communication rate at the same bit error rate (BER) level compared to conventional frequency-shift keying (FSK) modulation. Notably, to our best knowledge, this is the first orthogonal modulation scheme for the mechanical antenna that can theoretically be perfectly realized without violating Newton’s second law for rotation.