Space wave factors; MW and SW

Factors affecting space wave communication

Space wave = direct wave + ground-reflected wave, used at VHF and above.

1. Antenna heights: LOS range d ≈ 4.12(√ht + √hr) km and field E ∝ ht·hr; higher antennas give longer range and stronger signal.
2. Earth curvature: limits range to the radio horizon, reduces effective antenna heights, creates a shadow zone, and spreads (diverges) the reflected wave.
3. Tropospheric refraction: normal decrease of refractive index gives k = 4/3 (horizon extended about 15%); abnormal profiles cause super-refraction, ducting or sub-refraction (fading or loss of signal).
4. Ground reflection: reflection coefficient depends on ground conductivity, permittivity, polarisation and grazing angle; causes interference lobes and multipath fading.
5. Frequency: for fixed antenna gains, a higher frequency means more free-space path loss, more shadowing by obstacles and less diffraction.
6. Obstacles and terrain: hills, buildings and trees block or diffract the wave; first Fresnel-zone clearance (≥ 0.6) is required.
7. Atmospheric absorption and precipitation: rain, fog, O₂ and H₂O absorb above about 10 GHz.
8. Distance: beyond the horizon the field falls rapidly; within LOS it falls as 1/d² at large d.
9. Multipath from buildings and moving objects: causes fast fading (mobile radio).

Characteristics of MW and SW radio propagation

MW (medium wave) propagation, 300 kHz – 3 MHz (AM broadcast 526.5–1606.5 kHz):

• Daytime: mainly ground (surface) wave; the D layer absorbs the sky wave. Range ≈ 100–300 km depending on power and soil (longest over sea).
• Nighttime: D layer disappears, the E layer reflects the sky wave, giving 1000–2000 km range, but also co-channel interference and fading where ground and sky waves mix.
• Vertical polarisation and tall λ/4 mast antennas with radial ground systems are used.
• Ground-wave attenuation increases with frequency, so the lower end of the band covers more area.
• High atmospheric noise; narrow bandwidth (10 kHz channels).

SW (short wave) propagation, 3 – 30 MHz (HF):

• Mainly sky wave via the F2 (and E) layer; ground wave dies within a few tens of km.
• Very long range with modest power: one F2 hop up to ≈ 4000 km, worldwide by multi-hop.
• Governed by critical frequency, MUF and skip distance; a skip zone exists between the end of ground wave and the first sky-wave return.
• Fading due to multipath and changes in the ionosphere; diversity reception is used.
• Frequency must change with time of day, season and sunspot cycle: higher frequencies by day, lower at night; OWF ≈ 0.85 MUF.
• Affected by ionospheric storms and sudden ionospheric disturbances (radio blackouts).
• Horizontal antennas such as dipoles, rhombic and log-periodic arrays are common.