Ground wave path loss

Last Updated on January 8, 2025 by John Berry

Ground wave path loss and propagation is complex! The ground wave comprises three parts – a direct wave between transmitter and receiver, a reflected wave from the ground between the terminals, and a surface wave. Propagation is partly within the surface of the Earth and hence the received field strength is dependent on the properties of the ground.

Ground wave tends to be relevant in the range from 10kHz to 30MHz and hence is often exploited for local ham nets of up to a few tens of kilometres distance between stations. Ground wave is, as its name suggests, transmitted close to the ground. It does not exploit the ionosphere.

Core model

The ground wave basic path loss, Lb is given by:

Lb (dB) = 142 +20logf -E

Where E is derived by a complex calculation of field strength received that is generally determined by lookup on a series of graphs. The term f is the signal frequency. The graphs are explained further below. Typically, professionals use the GRWAVE computer program harnessing those graphs to determine path loss.

Overall, there are five dependencies: frequency, polarisation, relative ground permittivity (er), ground conductivity (s, in Siemens per metre), and the local environment around the transmitting and receiving antennas.

Variation by ground type

The basic path loss is shown in the figure below.

graph showing ground wave path loss at 30MHz over sea, marsh and land. transmission loss on y axis. Distance on the x axis. (from Roy et al 1987)
Ground wave path loss at 30MHz over sea, marsh and land.
(from Roy et al 1987)

As a reference, the Free Space Loss for a path length of 100km at 30MHz is around 102dB. Using the graph, the ground wave path loss is more like 160db over land and 120dB over sea. Ground wave goes less distance than space wave alone.

There are two parts to the ground wave path loss graphs. Out to 100km, the path loss increases by about 20dB per doubling distance. Beyond 100km, this rises to about 40dB per doubling distance. That’s quite a dramatic drop.

Variation by frequency

The graph below shows how the ground wave path loss varies with frequency. This is for transmission over smooth sea.

Graph showing basic path loss on the y axis for versus distance on the x axis for wave propagation over a smooth sea. σ = 4 S/m; ε = 80. (from Roy et al 1987)
Basic path loss for wave propagation over a smooth sea. σ = 4 S/m; ε = 80.
(from Roy et al 1987)

This illustrates that the useful ground wave range is up to about 50km in the range 3MHz to 30MHz. Above this distance, the path loss rises dramatically, challenging the system value. The path loss rises roughly as 20dB per doubling frequency. So, the lower HF frequencies can exploit ground wave better out to 100km. Above about 20MHz, ground wave is increasingly lossy.

Effects of the ground

A series of curves describe the received field strength term, E, as it varies with relative ground permittivity (er), and ground conductivity (s, in Siemens per metre). The curves are often reported in mS/m (10-3 S/m) for values for ground. The two curves below show the form (for over-sea paths). It’s then a question of selecting the right curves for the mid path conditions. For those interested, the curves can be found in the ITU Handbook of curves for radio-wave propagation over the surface of the Earth.


Ground-wave propagation curves; sea water, low salinity, σ = 1 S/m, εr = 80.
(from Rec. ITU-R P.368-10)

Ground-wave propagation curves; sea water, average salinity, σ = 5 S/m, εr = 70.
(from Rec. ITU-R P.368-10)

The curves have the same form as the path loss curves above, with huge reduction in field strength beyond 100km for HF signals. The graphs suggest a useful ground wave distance of out to 1,000km between 10kHz and 1MHz. Between about 1MHz and 10MHz, the useful range drops to under 100km. And over 10MHz, it’s under about 10km.

It is no surprise therefore that, before satellites, the maritime mobile service used particular frequencies. Around 500kHz was used for long distance CW traffic, and around 2MHz for medium distance voice traffic.

E, the received field strength in dBμV/m, is for 1kW transmit power, and so some scaling (dB for dB) is needed for practical powers. And the two parts of the equation must be considered together – the path loss rises by 6dB per doubling frequency before any consideration for E.

Other effects

The horizontally polarised component from any antenna is almost completely absorbed by the ground. Therefore vertical polarisation predominates.

That’s not to be confused with the electric field and magnetic field which are propagated. The magnetic field is often exploited in the broadcast service by receivers using ferrite rod antennas.

Ground wave propagation over rough terrain must consider propagation by diffraction and additional losses must be added.

There are models to consider mixed paths – part sea, part ground. These models simply make calculations for each part of the path and sum.

Since noise rises as 1/f, and ground wave is a phenomenon of interest at low frequencies, it’s important that station electrical noise is considered. Noise will therefore degrade the receiver threshold quite dramatically at the lower frequencies.