- 2069 Bhadra (old course) · 4+4 marks
Band applications and microwave transmission lines
Short answer
Microwave bands (IEEE): L GPS, S Wi-Fi/oven, C satellite, X radar, Ku DTH, K police radar, Ka broadband, V WiGig, W car radar. Unlike low-frequency lines, microwave lines are distributed, have phase delay and impedance varying as tan beta-l, suffer reflections, use waveguides/microstrip with TE/TM modes, and serve as stubs, filters and couplers.
Classify microwave frequency bands and state their major applications. Describe how microwave transmission lines are different from the conventional low frequency transmission lines.
Microwave frequency bands and their major applications (IEEE letter bands)
| Band | Frequency (GHz) | Major applications |
|---|---|---|
| L | 1–2 | GPS (1.575 GHz), GSM-1800, Inmarsat |
| S | 2–4 | Wi-Fi/Bluetooth 2.4 GHz, ovens, weather radar |
| C | 4–8 | Satellite (6/4 GHz), 5 GHz Wi-Fi, links |
| X | 8–12 | Military, marine and weather radar, deep space |
| Ku | 12–18 | DTH TV, VSAT (14/12 GHz) |
| K | 18–27 | Police radar, short-haul links |
| Ka | 27–40 | Broadband satellites, 5G mm-wave |
| V | 40–75 | 60 GHz WiGig, inter-satellite links |
| W | 75–110 | 77 GHz car radar, imaging |
How microwave transmission lines differ from LF lines
- Electrical length: an LF line is much shorter than λ and acts as a plain connection. A microwave line is several λ long, so it is distributed and has phase delay βl.
- Input impedance: at LF, Zin ≈ ZL. At microwave frequency, Zin = Z0(ZL + jZ0 tan βl)/(Z0 + jZL tan βl), which changes with length. A λ/4 line inverts the impedance.
- Reflections: a mismatch at microwave frequency produces Γ, standing waves and VSWR, so matching is needed.
- Structure: LF lines are two-wire lines or ordinary cables. Microwave lines are coax, rectangular or circular waveguides, microstrip, stripline and coplanar waveguide, with closed or shielded fields.
- Modes: LF lines carry TEM. Microwave waveguides carry TE/TM modes with a cutoff frequency.
- Losses: microwave lines suffer skin-effect loss (∝ √f), dielectric loss and radiation, so low-loss materials are used.
- Use as components: microwave line sections serve as stubs, filters, couplers and resonators.
- Analysis: Smith chart and S-parameters instead of simple circuit laws.
✓ Answer checked by Claude Opus 5.5 (high effort)
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