For antennas located above a perfectly conducting plane, image theory can be used to find the total field intensity at any location as that of the original antenna in free space plus the image contribution. For a lossy ground medium, the image term is multiplied by a reflection coefficient (see Chapter 7), but an additional term must also be included to satisfy the boundary conditions. This term is called the “groundwave” because it is strongest near the ground and decreases with distance from the ground. For transmitting and receiving antennas at heights above the Earth surface that are very small relative to the wavelength (as will be quantified later), direct and Earth reflected signals cancel (since the path lengths become equal and the reflection coefficient approaches −1), leaving only the groundwave component of the received field. Attenuation of the groundwave along the interface is partly due to power loss through the conductivity of the Earth's surface, so more poorly conducting grounds lead to more rapidly attenuating groundwaves.
Because of these properties, groundwave contributions to received field intensities are important when both transmitting and receiving antennas are sufficiently low (so that the direct and reflected rays cancel) and when the imaginary part of the ground dielectric constant is large. These conditions are typically met at lower frequencies (around a few MHz or less) since raising antennas above the hundreds ...