VHF Propagation: General Principles

Very High Frequency (VHF)

- VHF Band ranges from 30 - 300 MHz
- Line of sight propagation
- No ground waves
- Usually not reflected by the ionosphere

The aeronautical mobile service uses the frequency range from 118.0 - 136.975 MHz

Lower airspace (GND-FL245) has a channel spacing of 25kHz
Upper airspace (FL246 and up) has a channel spacing of 8.33kHz

Range and transmission quality

- Radio waves generally expand via a ground and a sky wave
- Low frequency waves contain a large share of ground waves (follow curvature of the earth)
- High frequency waves contain a large share of sky waves (not reflected by ionosphere)

Factors that can influence the propagation of the waves;

- profile of the earth's surface
- reflectivity of the ionosphere
- weather (fog, precipitation)
- atmospheric influences (severe turbulence, inversions, ...)

Waves in the VHF range propagate along a straight line (line of sight). They do not reflect by the ionosphere and have practically no share of ground waves.

They are greatly influenced by the height of the receiver and transmitter

Range (NM) = 1.23 x (SQRT H1 + SQRT H2)

H1 = height of transmitter (feet)
H2 = height of receiver (feet)

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Summary of the frequency bands;