(3) VHF and UHF bands. The VHF and UHF bands cover frequencies from 30 to 300 MHz
and from 300 to 3000 MHz respectively. Radio waves in these bands travel in nearly straight lines, and
normally require a line-of-sight path between the transmitting and receiving antennas. Useful
transmissions in these bands are generally limited by the curvature of the earth to 25 to 30 miles (40 to
48 km). However, longer transmissions may be possible under abnormal conditions.
(4) Microwave. Microwave radios operate in the UHF and SHF bands. Microwave
frequencies start at approximately 1000 MHz in the UHF band and range upward into a portion of the
SHF band. The SHF band extends from 3 gigahertz (GHz) to 30 GHz. Radio waves in the SHF band
also travel in nearly straight lines. This generally limits transmissions to line-of-sight paths and
distances under 50 miles (80 km). The exception is when a portion of the SBF band is used for
tropospheric scatter transmissions.
(5) Tropospheric scatter. Tropospheric scatter radio sets operate in the SHF band. Stable
and reliable radio propagation is possible over great distances by forward scatter of SHF waves (figure
13). Tropospheric scatter is one of two types of scattered transmission. Ionospheric scatter is the other,
but it is useful only at frequencies below 60 MHz. In tropospheric scatter, radio waves are reflected off
areas of turbulence in the troposphere 6-10 miles (10-16 km above the earth). The reflected energy is
scattered back to earth beyond the horizon, permitting reliable communications at distances of 100 to
200 miles (160 to 320 km). The results of scatter propagation are generally predictable and useful.
Learning Event 3:
MICROWAVE MULTICHANNEL RADIO ANTENNAS.
1.
transmitter and fed to a transmitting antenna through a transmission line. The antenna radiates this
energy into space at approximately the speed of light. A receiving antenna absorbs part of this energy
and sends it to the receiving equipment through another transmission line.
a. Functions of antennas. The transmitting antenna converts the output power generated by the
radio into an electromagnetic field that is radiated through space. Thus, the transmitting antenna
converts energy from one form to another. The receiving antenna has the opposite function. It converts
the electromagnetic field that sweeps by it into energy that the radio receiver can use. In transmission,
the antenna operates as the load for the transmitter. In reception, it operates as the signal source for the
receiver.
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