Practical_Antenna_Handbook_0071639586

28 p a r t I I : F u n d a m e n t a l s
course, above the VHF region, weather conditions add attenuation not found in outer
space.
The surface wave is subject to the same attenuation factors as the space wave, but it
also suffers ground losses. These losses are caused by ohmic resistive losses in the conductive
earth. Bluntly stated, the signal heats up the ground! AM band broadcast stations
utilize ground-mounted vertically polarized antennas to radiate a surface wave
that provides good signal strength throughout their local coverage area but which relies
on these ground losses to avoid interference to other broadcast stations on the same
frequency in communities farther away.
Surface-wave attenuation increases rapidly as frequency increases. For both forms
of ground wave, reception is affected by the following factors:
• Wavelength
• Height of both the receiving and the transmitting antennas
• Distance between antennas
• Terrain along the transmission path
• Weather along the transmission path
• Ground losses (surface wave only)
Figure 2.11 is a nomograph that can be used to calculate the line-of-sight distances
in miles over a spherical earth, given a knowledge of both receiving and transmitting
antenna heights. Similarly, Figs. 2.12A and 2.12B show power attenuation with frequency
and distance (Fig. 2.12A), and power attenuation in terms of field intensity (Fig.
2.12B).
Ground-wave communication also suffers another difficulty, especially at VHF,
UHF, and microwave frequencies. The space wave is like a surface wave, but it is radiated
from an antenna at least a wavelength above the earth’s surface. It consists of two
components (see Fig. 2.10 again): the direct and reflected waves. If both of these components
arrive at the receiving antenna, they will add algebraically (more accurately, vectorially)
to either increase or decrease signal strength. There is always a phase shift
between the two components because the two signal paths have different lengths (i.e.,
D 1 is less than D 2 + D 3 ). In addition, there may possibly be a 180-degree (π radians)
phase reversal at the point of reflection (especially if the incident signal is horizontally
polarized). If the overall phase shift experienced by the reflected wave as a result of
path length differences and polarization shifts is an odd multiple of 180 degrees at the
receiving antenna, as shown in Fig. 2.13, the net signal strength there will be reduced
(destructive interference). Conversely, if the direct and reflected signals arrive in phase,
the resulting signal strength is increased (constructive interference). The degree of cancellation
or enhancement will, of course, depend on the relative amplitudes of the two
signal components as they arrive at the receive antenna.
The loss of signal over path D l can be characterized with a parametric term n that is
defined as follows:
n
S
S
= r f
(2.13)