Practical_Antenna_Handbook_0071639586

654 P a r t V I I I : M e c h a n i c a l C o n s t r u c t i o n a n d I n s t a l l a t i o n T e c h n i q u e s
nas tend to work better up in the air than they do nearer the ground. The primary
reasons for this are, of course:
• At VHF and above, there is no ionosphere to help extend our transmission and
reception range through the reflection (or refraction) of radio waves back to
earth. Except for some highly unpredictable propagation modes, most radio
communications at these frequencies are limited to line of sight or slightly
beyond. Thus, raising the antenna high enough to clear nearby obstacles is an
important part of the station design.
• At MF and HF, ground reflection effects can enhance or degrade signal paths by
many decibels. Proper siting and intelligent choice of antenna height can
provide signal enhancements equivalent to a tenfold increase in transmitter
power compared to a station assembled with no forethought! In general, at MF
and HF, the height required is inversely proportional to the frequency for
comparable ground enhancement of the radiation angle.
Because television and FM broadcasting are confined to VHF and above, towers for
those services are often 1000 ft tall or more—especially in areas of the United States
with flat terrain—to extend line-Âof-Âsight reception as far as possible. In metropolitan
areas with hilly or sloping terrain or extremely tall downtown buildings, unique siting
opportunities for the broadcast transmitter and tower may allow coverage of the primary
service area with shorter towers.
In contrast, the AM broadcast band is based on the use of the ground wave, so AM
broadcast towers are the (vertically polarized) radiating elements. Their heights are
more likely to be dictated by the exact wavelength of each station and the extent to
which it needs to suppress the sky wave component of its signal in certain directions at
night, when skip can cause its signal to raise havoc with other stations sharing the same
frequency in other regions of the country. Typical heights for AM stations near the top
of the broadcast band (1500 to 1700 kHz) might be expected to range from only slightly
greater than those of a well-Âoutfitted 160-Âm amateur station—i.e., around 150 ft for a
quarter-Âwave grounded monopole—to slightly more than twice that figure. At the other
end of the AM band (530 to 700 kHz), optimum heights can rise to 400 ft or more.
Because amateurs have multiple bands of frequencies ranging from MF to UHF and
beyond, and multiple modes of propagation, including line of sight and ionospheric skip,
their tower requirements are “all over the map”. Today, small businesses and active
amateurs employ towers as short as 6 or 8 ft (tripod towers designed to straddle peaked
rooftops) and as tall as 300 ft. But probably the bulk of the nonbroadcast tower installations
fall within the 30-Â to 100-Âft range. Given the generalization that “higher is better”,
why are so many users content with relatively modest towers? Here’s a list of possible
reasons, in no particular order:
• Cost (a very rough rule of thumb has cost increasing in proportion to the square
of the height)
• Available space (the taller the tower, the larger the guy wire circle and the safe
fall zone required)
• Terrain (at some frequencies, properly sloping land or nearby saltwater can
provide the same long-Âhaul signal boost as a tall tower on flat land)