Slot Antenna
Understanding Slot Antennas
Slot antennas are among the most versatile planar radiators. They can be fed by microstrip crossing the slot, by coplanar waveguide, or directly from waveguide/SIW. Their complementary nature (slot in ground vs strip above ground) provides unique impedance and coupling properties.
Slot Antenna Properties
- Babinet's principle: Z_slot x Z_dipole = eta^2/4 (where eta = 377 ohms). A resonant slot has Z = ~450 ohms (complement of 73-ohm dipole).
- Radiation: Bidirectional (both sides of ground plane). Unidirectional with cavity backing or reflector.
- Bandwidth: Similar to complementary dipole (~5-15%). Wider with thick slots or cavity backing.
Slot Antenna Types
- Half-wave slot: Resonant at lambda/2. 5-6 dBi gain with ground plane.
- Vivaldi: Tapered slot. Ultra-wideband (10:1 bandwidth). Endfire radiation.
- Annular slot: Ring-shaped slot. Compact, moderate bandwidth.
Frequently Asked Questions
What is a slot antenna?
A slot antenna radiates from an opening in a conducting surface. It is the electromagnetic complement of a dipole. Slot antennas are bidirectional, integrate with waveguide and SIW, and are widely used in arrays, aircraft, and planar circuits.
How is a slot antenna fed?
Common feed methods: microstrip line crossing the slot (simple, common), coplanar waveguide direct feed, waveguide slot (in the waveguide wall), and probe feed. The feed position along the slot controls the impedance match.
What is a cavity-backed slot?
A cavity behind the slot eliminates backward radiation (making it unidirectional), increases gain, and improves impedance bandwidth. Cavity-backed slots are common in aircraft and vehicle installations where radiation behind the ground plane is undesirable.