- Asked 2/20
- 2070 Bhadra · 4+6 marks
- 2069 Bhadra · 4+8 marks
Radio wave types and band characteristics
Short answer
Radio waves follow direct/ground-reflected (space wave), surface wave, or sky wave paths. VLF to MF rely on surface wave for long range; HF uses sky wave with skip zones; VHF and above use space wave limited to line-of-sight. Lower frequencies propagate farther, while higher frequencies offer wider bandwidth.
With a neat diagram, explain the designation of radio waves according to the path they follow during propagation (surface, ground reflected, direct and sky waves). Also, compare the propagation characteristics for different radio bands.
Designation of radio waves by path
A radio wave from a transmitting antenna can reach the receiver by different paths. Waves are named after the path they follow.
ionosphere
~~~~~~~~~~~~~~~~~~~/\~~~~~~~~~~~~~~~~~~~
/ \ (4) sky wave
/ \
Tx |------(1) direct wave--------->| Rx
| \ / |
| \ (2) ground-reflected / |
| \_________________/ |
===|==(3) surface wave along earth=|===
|<------ space wave = 1 + 2 ->|
ground wave = space + surface
- Direct wave: travels in a straight line (line of sight) from the transmitting to the receiving antenna through the troposphere.
- Ground-reflected wave: reaches the receiver after one reflection from the earth's surface. Direct + ground-reflected waves together form the space wave (tropospheric wave); dominant at VHF and above.
- Surface wave: a vertically polarised wave guided along the earth's surface, following its curvature; it induces currents in the ground and is attenuated. Dominant at VLF, LF and MF.
- Sky wave: goes upward and is bent back to earth by the ionosphere; used for HF long-distance links.
Surface wave and space wave together are called the ground wave. Other modes are tropospheric scatter and duct propagation.
Propagation characteristics of radio bands
| Band | Range | Main mode | Characteristics |
|---|---|---|---|
| VLF | 3–30 kHz | Surface wave; earth–ionosphere waveguide | Very stable, worldwide; huge antennas; tiny bandwidth; penetrates sea water |
| LF | 30–300 kHz | Surface wave | Long range (≈ 1000 km), stable; low attenuation |
| MF | 0.3–3 MHz | Surface wave (day), E-layer sky wave (night) | Day range ≈ 100–300 km; night range long but with fading and interference |
| HF | 3–30 MHz | Sky wave (F2, E) | Worldwide with multi-hop; skip zone, fading; depends on time, season, sunspots; ground wave only tens of km |
| VHF | 30–300 MHz | Space wave (LOS) | Range ≈ horizon (≈ 50–100 km); passes through ionosphere; occasional sporadic-E and ducting |
| UHF | 0.3–3 GHz | Space wave, troposcatter | LOS, multipath, building shadowing; small high-gain antennas |
| SHF | 3–30 GHz | LOS, satellite | Highly directive dishes; rain attenuation above ≈ 10 GHz |
| EHF | 30–300 GHz | LOS | Strong rain and gaseous (O₂, H₂O) absorption; short range; very wide bandwidth |
General rules: ground-wave attenuation rises with frequency; the ionosphere reflects only below about 30 MHz; above that, range is limited by the horizon, while bandwidth and antenna directivity increase.
✓ Answer checked by Claude Opus 5.5 (high effort)
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