IEEE microwave band classification and advantages

The IEEE (Standard 521) gives letter names to the radar and microwave bands. Microwaves broadly cover 300 MHz to 300 GHz.

IEEE frequency band classification

Advantages of microwaves over lower frequencies

1. Large bandwidth: a 1% bandwidth at 10 GHz is 100 MHz, which carries many channels or high data rates.
2. High antenna gain from small antennas: gain ≈ 4πA/λ², so a small dish at microwave frequencies gives a narrow, high-gain beam.
3. Line-of-sight propagation: low interference, and the same frequency can be reused in nearby areas.
4. Ionosphere penetration: makes satellite and space communication possible.
5. Radar: target size is comparable to λ, so radar cross-section and resolution are good.
6. Less fading and noise: man-made and atmospheric noise is low.
7. Small components: λ-sized circuits are compact, such as MMICs and patch antennas.
8. Molecular resonances: allow heating, spectroscopy, and remote sensing.

Disadvantages

1. Line of sight only: towers or repeaters are needed every 40–50 km because of earth curvature.
2. Atmospheric attenuation: rain fade above about 10 GHz, and oxygen and water-vapour absorption.
3. Costly components and tight tolerances: precise machining, special substrates, and expensive test equipment.
4. Lumped analysis fails: field and S-parameter analysis is needed, which makes design complex.
5. Low device power and efficiency: ordinary transistors fail because of transit time, so special tubes and diodes are needed.
6. Higher losses: skin effect and dielectric loss rise with frequency.
7. Radiation hazards: high power densities can heat tissue, so safety limits apply.