What is an antenna used for?

Antennas are bidirectional transducers between guided (wire) and unguided (free space) electromagnetic waves.

Receiving: incoming radio waves induce voltages in the antenna; amplifier extracts and processes the signal.

Transmitting: oscillating current in the antenna radiates electromagnetic waves into space. Antenna types: (1) DIPOLE — simplest, two conductive elements; length typically 1/2 wavelength of operating frequency; common in radio; (2) MONOPOLE — single vertical element above ground plane (which acts as the second element via reflection); typical 1/4 wavelength; whip antennas on vehicles, AM tower antennas; (3) YAGI-UDA — directional antenna with driven element and parasitic reflectors/directors; high gain in one direction; classic TV antennas, ham radio; (4) PARABOLIC DISH — focuses signals from a focal point reflector; very high gain in one direction; satellite TV, radio telescopes, deep space communications; (5) LOOP — circular conductor; small loops are magnetic antennas; (6) PATCH/MICROSTRIP — flat antennas printed on circuit boards; common in GPS, WiFi, cell phones; (7) HELICAL — wound in helix shape; satellite communications; (8) PHASED ARRAY — multiple antenna elements with controlled phasing; can electronically steer beam direction; radar, modern 5G base stations. Antenna characteristics: (1) GAIN — directional concentration of energy compared to omnidirectional reference; (2) POLARIZATION — direction of the electric field (vertical, horizontal, circular); transmit and receive antennas should match; (3) BEAMWIDTH — angular width of main lobe; (4) IMPEDANCE — typically 50 or 75 ohms; must match feed line for efficient transfer; (5) BANDWIDTH — range of frequencies the antenna efficiently handles. Antenna length: roughly proportional to wavelength; lower frequencies need bigger antennas (AM radio antennas are huge, GHz cell phone antennas are tiny). Cell phones have multiple antennas: cellular, WiFi, Bluetooth, GPS, NFC. Modern smartphones cleverly use the device frame and internal structures as antennas. Antenna theory: an electron oscillating produces electromagnetic waves at the oscillation frequency; the wire 'pumps' electrons; the changing electric/magnetic fields propagate outward at light speed.