What is the relationship between antenna size and beamwidth at millimeter wave frequencies?
mmWave Antenna Beamwidth
The relationship θ3dB ≈ k × λ/D (where k is a constant depending on the aperture illumination, typically 58-70) is a universal principle of diffraction-limited apertures. It applies equally to optical telescopes, radar antennas, and mmWave communications antennas. At mmWave frequencies, the small wavelength means that physically small apertures achieve the same beamwidth and gain that would require much larger apertures at microwave frequencies.
This property is both an advantage and a challenge for mmWave systems. The advantage: high gain (and therefore long range) from compact antennas. A 100 mm lens antenna at 77 GHz provides the same gain as a 1-meter dish at 7.7 GHz. The challenge: the narrow beam must be pointed accurately. A 1° beamwidth antenna must be pointed within ±0.3° for less than 3 dB pointing loss, which requires precision mechanical or electronic beam steering.
Phased arrays at mmWave leverage the small element size (half-wavelength spacing = 2.5 mm at 60 GHz) to pack many elements into a small area. A 64-element array at 60 GHz fits in a 20mm × 20mm area and produces a 10° beamwidth that can be electronically steered. This is the standard approach for 5G mmWave base stations and handsets.
Frequently Asked Questions
How does this affect 5G mmWave?
5G mmWave systems at 28 and 39 GHz use antenna arrays with 64-256 elements to achieve both high gain (beam reach) and electronic beam steering (track moving users). The array aperture is 50-100 mm, producing beamwidths of 5-15°. Multiple beams are formed simultaneously to serve multiple users.
What pointing accuracy is needed?
For reliable communication: pointing error should be less than θ3dB/3 ≈ 0.2× beamwidth. For a 3° beam: pointing accuracy must be better than ±1°. For a 1° beam (large dish): pointing accuracy must be better than ±0.3°. This requires precision positioners or electronic steering.
Can I use a wider beam at mmWave?
Yes, by using a smaller antenna. But a wider beam has lower gain, reducing the link range. The design trades off coverage area (beam width) against link distance (gain). Sector antennas with 30-90° beamwidth are used for short-range mmWave coverage with moderate gain.