1. Reflection and Transmission
* Perfect Conductor Cavity: If the cavity is formed by perfectly conducting walls, the TEM wave will be completely reflected. This is because the electric field of the wave cannot penetrate a perfect conductor, forcing the wave to reverse direction. No transmission occurs.
* Partially Conducting Cavity: For a cavity with partially conducting walls, the wave will be partially reflected and partially transmitted. The amount of reflection and transmission depends on the conductivity of the walls and the frequency of the wave. Higher conductivity leads to more reflection, and higher frequencies tend to penetrate less.
2. Resonance
* Cavity Resonance: If the dimensions of the cavity are comparable to the wavelength of the TEM wave, the cavity can act as a resonant cavity. This means that certain frequencies of the wave will be preferentially absorbed and stored within the cavity, leading to a buildup of energy inside. The resonant frequencies are determined by the cavity's size and shape.
* Modes: Resonant cavities can support different resonant modes, each with its own unique field distribution within the cavity.
* Q Factor: The quality factor (Q) of the cavity measures how efficiently it stores energy. A high Q factor indicates that the cavity stores energy for a longer time, while a low Q factor indicates that the energy is quickly dissipated.
3. Waveguide Propagation
* Waveguide Modes: If the cavity is a waveguide (a hollow conductor with a specific cross-section), the TEM wave can propagate through the waveguide in specific modes. These modes are determined by the waveguide's geometry and the frequency of the wave.
* Cut-off Frequency: For each mode, there exists a minimum frequency (cut-off frequency) below which the mode cannot propagate. This means that only certain frequencies can propagate within a waveguide.
4. Energy Dissipation
* Lossy Walls: In real-world cavities, the walls are not perfect conductors and have some finite conductivity. This results in some of the energy of the TEM wave being dissipated as heat within the walls.
5. Applications:
* Microwave Circuits: Resonant cavities are widely used in microwave circuits, such as filters, oscillators, and amplifiers.
* Particle Accelerators: Cavities are essential components of particle accelerators, where they are used to accelerate charged particles using electromagnetic fields.
* Medical Imaging: Magnetic resonance imaging (MRI) uses strong magnetic fields and radio waves to create images of the human body.
In Summary:
The behavior of a TEM wave encountering a cavity is complex and depends on many factors. However, the key concepts include reflection, transmission, resonance, waveguide propagation, and energy dissipation. Understanding these concepts is crucial for designing and analyzing various electromagnetic devices and systems.
