Practical_Antenna_Handbook_0071639586

24 p a r t I I : F u n d a m e n t a l s
medium varying with frequency. We will have occasion to discuss dispersion further
when we discuss modes of fading over propagation paths.
Diffraction Phenomena
Electromagnetic waves diffract when they encounter a radio-opaque object. The degree
of diffraction and the resulting harm or benefit are frequency related. Above 3 GHz,
wavelengths are so small (approximately 10 cm) compared to object sizes that large attenuation
of the signal occurs. In addition, beamwidths (a function of antenna size compared
with wavelength) tend to be small enough above 3 GHz that blockage of
propagation by obstacles is much more effective.
Earlier in this chapter, large-scale diffraction around structures (such as buildings)
was discussed in conjunction with Fig. 2.8B, a top-down view of diffraction effects in
the horizontal plane. But there is also a diffraction phenomenon in the vertical plane.
Terrain or man-made objects intervening in the path between UHF microwave stations
(Fig. 2.9A) cause diffraction, along with the concomitant signal attenuation. There is a
minimum clearance required to prevent severe attenuation (more than 20 to 30 dB, say)
from diffraction. Calculation of the required clearance comes from Huygens-Fresnel
wave theory.
Consider Fig. 2.9B. A wave source A, which might be a transmitter antenna, transmits
a wavefront to a destination C (receiver antenna). At any point along path A-C, you
can look at the wavefront as a partial spherical surface (B l -B 2 ) on which all wave rays
have the same phase. This plane can be called an isophase plane. You can assume that the
d n /d h refraction gradient over the height extent of the wavefront is small enough to be
considered negligible.
Figure 2.9A Terrain masking of VHF and higher-frequency signals.