Antennas

Antennas convert guided signals into radiated waves. Learn radiation, near and far fields, dipoles and monopoles, patch, Yagi, horn and parabolic dish antennas, gain, directivity, beamwidth, efficiency, polarisation, input impedance and bandwidth, and arrays with beam steering.

Every wireless device has at least one antenna — phones have many. Antenna design decides range, coverage and interference, and small devices face tough trade-offs between size, bandwidth and efficiency.

An analogy

An antenna is like a loudspeaker for radio waves: an omnidirectional antenna is a bare speaker, and a dish is a megaphone that focuses sound in one direction.

Basic antennas

Half-wave dipole (reference antenna), quarter-wave monopole over a ground plane (car and handheld radios), loops, and PCB/chip antennas in IoT devices. Resonant length depends on wavelength.

Directive antennas

Yagi-Uda (TV), horn (microwave test, feeds), patch/microstrip (GPS, phones, arrays), parabolic dishes (satellite, radio links). Gain comes from concentrating energy — higher gain means narrower beams.

Key parameters

Radiation pattern, gain, beamwidth, front-to-back ratio, polarisation (linear, circular — GPS uses RHCP), impedance bandwidth (VSWR < 2), efficiency.

Arrays and MIMO

Phased arrays steer beams electronically by adjusting element phases (5G mmWave, radar). MIMO uses multiple antennas for spatial multiplexing and diversity. Simulation (HFSS, CST) and anechoic-chamber measurement verify designs.

Formulas

• Half-wave dipole: L ≈ 0.95 × λ/2, G = 2.15 dBi, Z ≈ 73 Ω
• Dish gain: G = η (π D_a/λ)²
• Beamwidth (dish): θ₃dB ≈ 70 λ / D_a (degrees)
• Far-field distance: R_ff = 2 D_a² / λ
• EIRP: EIRP = P_t + G_t − cable loss

Worked example: FM dipole

Given: f = 100 MHz

Find: Dipole length.

• λ = 3 m → λ/2 = 1.5 m
• × 0.95 end effect → 1.43 m

Answer: ≈ 1.43 m

Worked example: Satellite dish

Given: 1 m dish, 10 GHz, η 0.6

Find: Gain, beamwidth, far field.

• G = 0.6 × (π/0.03)² = 6580 → 38.2 dBi
• θ ≈ 70 × 0.03/1 = 2.1°
• R_ff = 2/0.03 = 67 m

Answer: 38.2 dBi, 2.1° beam, far field beyond 67 m

Common mistakes

• Wrong: Placing antennas near metal or batteries. — Right: Follow keep-out zones and tune in the final enclosure.
• Wrong: Measuring gain in the near field. — Right: Measure beyond 2D²/λ.

Applications

• Mobile phones: Multi-band PIFA and MIMO antennas.
• Satellite: Dishes and phased-array terminals.
• 5G mmWave: Beam-steered antenna arrays.

Key takeaways

• Dipole λ/2, 2.15 dBi, 73 Ω.
• G = η(πD/λ)²; θ ≈ 70λ/D.
• Far field beyond 2D²/λ.
• Arrays steer beams electronically.

Going further

• Array factor: Pattern multiplication for arrays.
• Specific absorption rate: SAR limits for phones.
• Metamaterial antennas: Compact and reconfigurable designs.