Electromagnetic Radiation Lecture Notes

Introduction

• Humans are surrounded by electromagnetic (EM) radiation.
• Heinrich Hertz was the first to transmit and detect EM waves.
• Hertz's experiment involved:
  • Applying high-voltage current to metal wires.
  • Creating a spark that generated EM waves.
  • Demonstrated propagation and detection of EM waves.

Historical Background

• James Clerk Maxwell: Formulated four mathematical equations laying the foundation for electromagnetic radiation before Hertz’s experiment.
• Key Question: How do electromagnetic fields detach from wires and propagate through space?

Understanding Electromagnetic Waves

• Moving Electric Charge: When a charge moves at constant speed, it has an electric field around it.
• Effect of Acceleration:
  • When the charge accelerates, the change in the electric field propagates at the speed of light.
  • Regions of the electric field transition, creating a "kink" that radiates outward.
• Electromagnetic Disturbance: The acceleration of the charge causes this disturbance, leading to radiation.

Oscillating Electric Dipole

• An oscillating dipole produces EM radiation sinusoidally.
• Velocity and Acceleration:
  • At both ends of the dipole, velocities are zero; maximum in the middle.
  • Continuous acceleration/deceleration leads to deformation of the electric field lines.
• Radiation Propagation: When charges meet at the center, they detach, and the radiation travels at light speed.

Application to Antennas

• Dipole Antenna:
  • When voltage is applied, electrons move, creating a dipole effect.
  • It can function as both a transmitter (sending signals) and a receiver (detecting signals).
• Design Criteria for Antennas:
  • Length: Should be half of the wavelength for optimal performance.
  • Impedance Matching:
    • Ensures efficient wave radiation.
    • Impedance is the combined effect of resistance, inductance, and capacitance in a circuit.
    • Maximum Power Transfer Theorem: Load impedance must match source impedance.

Example of Impedance Matching

• In a circuit with an alternator as the source and a motor/bulb as a load:
  • Load impedance must match alternator impedance for maximum power transfer.
• Antenna Systems: High-frequency signals require impedance matching between source, transmission line, and antenna.
• Free Space Impedance: Has a value of 377 ohms.
• Parabolic Antennas: Use waveguides with different impedance; feed horns help match impedance with free space.

Conclusion

• Understanding the principles of electromagnetic radiation and antenna design is crucial in engineering.
• Importance of supporting educational content.