Antenna Fundamentals and Radiation Mechanism

Antenna Fundamentals

• An antenna is a transducer that converts electrical power to electromagnetic waves, or vice versa.
• Antennas can be used in transmission or reception.
• The word "antenna" comes from zoology, describing insect feelers.
• Metallic conductors form antennas, wires form aerials.
• Antennas act as a transition between a guiding device (e.g., transmission line) and free space.
• Antennas convert guided waves into free-space waves efficiently and create a desired radiation pattern.
• Antennas intercept electromagnetic wave power to generate a voltage.
• Antennas are composed of conductive elements connected to a receiver or transmitter.

Radiation Mechanism

• Radiation requires a time-varying current or accelerating charge.
• A stationary charge does not create radiation.
• A charge moving with uniform velocity does not radiate if the wire is straight and infinite.
• A curved or discontinuous wire with oscillating charges does radiate.
• Electric and magnetic fields create electromagnetic waves that propagate between conductors.
• Open space creates free-space waves as the electromagnetic waves propagate.

Maxwell's Equations and Field Quantities

• Maxwell's equations describe how electric charges produce electric and magnetic fields.
• Electric field (E) is measured in volts per meter (V/m).
• Magnetic field (H) is measured in amperes per meter (A/m).
• Electric flux density (D) is measured in coulombs per square meter (C/m²).
• Magnetic flux density (B) is measured in webers per square meter (Wb/m²) or teslas (T).
• Free electric charge density (p) is measured in coulombs per cubic meter (C/m³).
• Electric current density (J) is measured in amperes per square meter (A/m²).

Basic Types of Antennas

• Antennas are categorized by physical structure, operating frequency, and application.
• Wire antennas (straight-wire, loops, helices) include simple dipole antennas, usually for low-frequency applications.
• Aperture antennas exploit openings in structures (e.g., horns, waveguides) and are effective for high-power microwave applications.
• Reflector antennas use reflective surfaces to focus radiation (e.g., parabolic reflectors) for radio telescopes, satellite communications, and microwave applications.
• Lens antennas use refractive surfaces to focus radiation (e.g., convex/concave lenses) for very high-frequency applications.
• Antenna arrays combine multiple elements to gain control over the antenna's spatial radiation pattern, this can be used for a variety of applications.

Antenna Basic Parameters

• Antenna parameters include circuit quantities (impedance, radiation resistance, temperature), physical quantities (size, weight), and space quantities (field patterns, power).
• Impedance matching between antennas and transmission lines is critical.
• Voltage standing-wave ratio (VSWR) is a measure of impedance mismatch.
• Ideally, a perfect match has VSWR = 1.

Radiation Pattern

• The radiation pattern of an antenna describes how the antenna radiates power in space.
• A plot is often used to represent the radiation pattern.
• The major lobes are the directions of maximum radiation.
• The minor lobes are other radiation directions.
• Back lobes radiate in the opposite direction of the main beam.