Space wave and sky wave characteristics

Space wave

A space wave (tropospheric wave) consists of the direct wave from the transmitting to the receiving antenna and the ground-reflected wave. It is the main mode at VHF, UHF and microwaves.

  Tx |\---------- direct ------------>| Rx
  ht |  \                          /  | hr
     |    \____ ground reflected _/   |
  ===|================================|===

Characteristics

1. Line-of-sight: range limited by the radio horizon, d ≈ 4.12(√ht + √hr) km (h in m), typically 40–100 km; higher antennas give longer range.
2. Field strength at large distance: E = 88√P·ht·hr/(λd²), so it falls as 1/d² and grows with antenna heights.
3. Interference between direct and reflected waves gives lobes, multipath and fading.
4. Affected by tropospheric refraction (4/3 earth, super-refraction, ducts), terrain, buildings and earth curvature (shadow zone).
5. Above ≈ 10 GHz, rain and atmospheric gases absorb the signal.
6. Not affected by the ionosphere; same performance day and night.
7. Wide bandwidth, low noise; small high-gain antennas.
8. Uses: FM, TV, mobile, microwave links, radar, satellite.

Sky wave

A sky wave goes upward and is refracted back to earth by the ionosphere; it is the main mode for HF (3–30 MHz).

  ~~~~~~~~~~~~~~/\~~~~~~~~~~~~~ ionosphere
               /  \
              /    \
  ========Tx=/======\=Rx======== earth
           |<-skip->|

Characteristics

1. Long range: one hop up to ≈ 4000 km (F2), worldwide by multi-hop, with low power.
2. Works only below the MUF (= fc sec θi); frequencies above it pass through. Critical frequency fc = 9√Nmax.
3. Skip distance and skip zone: no reception between the end of ground wave and the first return.
4. Fading due to multipath and changing ionisation; diversity reception needed.
5. Depends strongly on time of day, season, latitude and sunspot cycle; frequency must be changed (OWF ≈ 0.85 MUF).
6. D-layer absorption in daytime affects low frequencies.
7. Disturbed by ionospheric storms and sudden ionospheric disturbances.
8. Uses: SW broadcasting, maritime, military and amateur long-distance links.