Propagation

Ground Waves

In commercial LF systems, ground-wave propagation is normally the dominant mode. At frequencies as low as 73 kHz the field strength of the ground wave depends solely on the ERP and the distance from the transmitter. Variations in the ground conductivity only become significant after considerable distances.

Field strengths of the ground wave can, therefore, be predicted with some confidence if the ERP is known. Standard curves showing these field strengths have been published by the ITU's International Radio Consultative Committee (CCIR), in CCIR Recommendation 368.

The graph below, which has been derived from the CCIR curves, shows the field stength (in dB relative to 1 µV/metre) that can be expected over:

• Perfectly conducting earth (blue);
• Typical UK ground of conductivity 10 mS/m (green);
• Areas of poor conductivity of 1 mS/m, such as can be found in the Scottish Highlands and in parts of West Wales, Devon and Cornwall (red).

This graph assumes a transmitter power of 1 Watt effective monopole radiated power (EMRP).

As you can see, the ground conductivity only affects the field strength for paths over about 100 km long.