Antenna Characteristics & TypesModule 5

Gain measures how effectively an antenna converts input power into radio waves in a specific direction, expressed in dBi (relative to an isotropic radiator) or dBd (relative to a half-wave dipole). A half-wave dipole has 2.15 dBi gain, so dBi = dBd + 2.15 (a 3 dBd antenna is 5.15 dBi).

Directivity is the ratio of the radiation intensity in the maximum direction to the average radiation intensity. Gain = Directivity × Efficiency, so a lossy antenna has less gain than directivity; a lossless one has equal.

Reciprocity: An antenna's transmit and receive patterns are identical — a theorem from electromagnetic theory (Lorentz reciprocity). Gain, pattern and polarisation measured in transmit therefore apply in receive too.

A radiation pattern is a 3D representation of the antenna's radiated energy as a function of angle. The half-power beamwidth (HPBW) is the angle between the -3 dB points of the main lobe. Patterns are measured in the E-plane (containing the electric field vector) and H-plane (containing the magnetic field vector). Gain and beamwidth trade off: for a pencil beam, G ≈ 41,253 / (θ_E · θ_H) with the beamwidths in degrees, so 10° × 10° is about 26 dBi and halving both beamwidths adds 6 dB. Patterns are only valid in the far field, beyond roughly 2D²/λ from an antenna of largest dimension D (a 1 m dish at 10 GHz, λ = 3 cm: about 67 m). Closer in, the near field has a different, unreliable pattern.

VSWR (Voltage Standing Wave Ratio) quantifies impedance mismatch between the antenna and feedline. VSWR = 1:1 is perfect match; VSWR < 2:1 is generally acceptable. Return loss (dB) = -20 log₁₀((VSWR-1)/(VSWR+1)).

Bandwidth is the frequency range over which the antenna maintains acceptable performance (typically VSWR < 2:1). Narrowband antennas (e.g., patch) have 1-5% bandwidth; wideband (e.g., horn) can exceed 2:1 frequency ratio.

Polarisation describes the orientation of the electric field vector. Linear polarisation (horizontal or vertical) is most common. Circular polarisation (RHCP/LHCP) is used for satellite links to avoid Faraday rotation effects. Elliptical is the general case. A linear antenna receiving a circularly polarised wave loses 3 dB (at any rotation angle), while crossed linear antennas (horizontal vs vertical) typically lose 20+ dB, and RHCP against LHCP is a similar deep null.

Antenna dimensions scale directly with wavelength (λ = c/f). A half-wave dipole at 100 MHz is 1.5 m long; at 2.4 GHz it's only 6.25 cm (real wire dipoles are cut about 5% shorter than the free-space λ/2). This fundamental relationship governs all antenna design — lower frequencies require physically larger structures.