Antenna

A metal structure used to convert high frequency current into electromagnetic waves, and vice versa.

For wireless systems, an antenna is crucial to reduce the complexity of the rest of the circuit (imagine a well tuned pair of spectacles reducing your cognitive load when trying to process the world).

The basic equation of radiation is:

$$\dot{I}L = Q\dot{v}$$

The rate of change of current through a length of wire is equal to the acceleration of charge. The radiation is perpendicular to the direction of acceleration.

Basic antenna properties

1. Radiation pattern: A mathematical or graphical representation of how the antenna radiates in space.
2. Radiation power density: The instantaneous Poynting vector $\vec{S}$
3. Radiation intensity
4. Directivity: The radiation intensity in one direction over the average radiation intensity in all directions.
5. Gain: Ratio of the intensity in a given direction to teh intensity in that direction if it had had been isotropically raidated.
6. Antenna efficiency:
7. Beam width
8. Beam efficiency
9. Bandwidth: Range of frequencies for which the antenna performance confirms to a certain standard.
10. Polarization

Types of Antennae

1. Wire antennae: Dipole, loop or helical
2. Aperture antennae: For aircraft so they can be mounted flush and shielded with some dielectric.
3. Microstrip antennae: Confirmable to planar or non-planar surfaces. Patch on a grounded substrate. PCB.
4. Array antennae: For radiation patterns not achievable with a single element, we group a bunch in specific patterns.
5. Reflector antennae: Parabolic reflector to communicate very wide distances.
6. Lens antennae: Collimate incident divergent energy