Antenna
Understanding Antennas
Antennas are among the most critical components in any wireless system. They determine how effectively a system radiates and receives energy, in which directions, and at which frequencies. An antenna does not amplify signals; rather, it focuses available energy in preferred directions at the expense of other directions.
Antenna Parameters
- Gain (dBi): Measure of directional focusing relative to an isotropic radiator. Higher gain = narrower beam.
- Radiation pattern: 3D map of radiated field intensity vs. angle. Key features: main lobe, sidelobes, nulls, back lobe.
- Polarization: Orientation of the electric field vector: linear (vertical/horizontal), circular (RHCP/LHCP), or elliptical.
- Bandwidth: Frequency range over which specifications (VSWR, gain, pattern) are met.
- Impedance: Complex impedance at the feed point, ideally matched to the transmission line (50 or 75 ohms).
Common Antenna Types
- Dipole: Simplest antenna, 2.15 dBi gain, omnidirectional in the H-plane.
- Horn: Flared waveguide, 10-25 dBi, wideband, used as feeds and gain standards.
- Parabolic dish: High gain (30-60+ dBi), narrow beam, used for satellites and point-to-point.
- Patch (microstrip): Low profile, 5-9 dBi, used in arrays for radar and 5G.
- Phased array: Electronically steerable, gain scales with number of elements.
G (dBi) = 10 log10(4 pi A_eff / lambda^2)
Effective aperture:
A_eff = G lambda^2 / (4 pi)
Friis transmission:
P_rx = P_tx G_tx G_rx (lambda / 4 pi d)^2
Frequently Asked Questions
What is an antenna in RF?
An antenna converts guided electromagnetic waves (in cables or waveguide) into radiated waves in free space, and vice versa. It is the essential interface between transmitter/receiver electronics and the wireless propagation channel.
What determines antenna gain?
Antenna gain is determined by the electrical size of the antenna (aperture area relative to wavelength) and its radiation efficiency. Larger antennas and higher frequencies produce higher gain. Gain increases as the beam narrows.
What is antenna polarization?
Polarization is the orientation of the radiated electric field vector. Linear polarization (vertical or horizontal) aligns the field along one axis. Circular polarization rotates the field vector, reducing sensitivity to antenna orientation. Polarization mismatch between transmit and receive antennas causes signal loss.