Ground Wave Propagation

Ground Wave propagation is where radio waves travel parallel to to the Earths surface, and tend to follow the curvature of the Earth.
Range is affected by frequency and ground conductivity, with range increasing with lower frequencies and higher ground conductivity.
At VHF frequencies (30MHz +) and above, surface wave contributes less and less, and the line of sight propagation known as Space Wave (sum of of Direct and Reflected Waves) is dominant.

Travels directly from one antenna to the next. Range is mainly affected by the curvature of the Earth and obstructions such as hills, mountains and buildings in built up areas. At VHF frequencies and above, Direct and Reflected Waves are the most dominant form of propagation.

Travels from one antenna to the next, but is first reflected from the Earth. Is phase shifted through 180° upon reflection and has a longer path to travel, so may or may not arrive somewhat out of phase with the direct wave and thus cause some, or total cancellation (often called multipath).
The sum of Direct Wave and Reflected Wave arriving, along with any cancellation is known as Space Wave.

Travels in contact with the Earth and is limited to fairly low frequencies between 300kHz & 30MHz, becoming less and less effective as frequencies increase. Surface Wave is capable of covering long distances by diffracting around obstacles and the Earth's curvature. 

Surface waves are not readily created nor coupled to the ground when the antenna is many wavelengths above the ground. When the transmitting antenna is close to the ground, Surface waves are created and coupled to the ground, and they exponentially decay with height.
When the transmit and receive antennas are both close to the ground, the Direct and Reflected waves can act in opposition and cancel out. Because of the long wavelengths involved at lower frequencies, most practical antennas tend to be "close" (with respect to the wavelength) to the ground, and this generally causes the surface wave to be dominant over Direct and Reflected waves (again, only at lower frequencies).

Surface Wave propagation works best with vertically polarized antennas, with heavy attenuation occuring with horizontal propagation. This is the inverse of NVIS (Near Vertical Incidence Skywave) where horizontal antennas are used to deliberately reject Surface Wave signals and instead more readily accept skywave signals from directly above.

Surface Wave range varies according to a number of factors:

• Frequency
  Lower frequencies work better than higher frequencies.
  300kHz to 3Mhz is best.
  3Mhz to 30Mhz works but performs less impressively.
  At 100 kHz the range might exceed 500 km, at 1 MHz it might be no more than 150 km, and at 10 MHz it might only be around 15 km.

• Ground Conductivity (σ)

The earth's surface electrical parameters are important in reaching longer ranges. Sea surface is a good conductor, but ground is a poor conductor at the above frequencies. For example, with the same transmitter and receiver characteristics, a 5 MHz signal, which reaches 400-km range over the sea, can only reach up to 40 - 50-km range over the poor ground.

Examples of conductivity across different ground surfaces:

• Atmospheric Refractivity (N)

Atmospheric refractivity describes the way that the Earths atmosphere bends (refracts) radio waves. 

Ground-wave signals travel along Earths surface, and their range and attenuation depend heavily on how the atmosphere bends the wave. Refractivity affects how radio waves curve downward or upward relative to Earth.

Higher refractivity (moist air, low altitude) → stronger bending toward Earth →
longer ground-wave range.

Lower refractivity (dry air, high temperature, high altitude) → weaker bending → shorter range. 

Atmospheric refractivity determines whether the ground wave stays “trapped” near the surface or diverges upward.

Layers of the Atmosphere: