Figure 152.
Parabolic Reflector Applications.
f. Another type of antenna you will work with has a metal lens instead
of a reflector. This antenna is used for search and tracking systems.
31. Metallic lens antennas.
a. Some of the newer radar equipment uses the metallic lens antenna. A
metallic lens antenna has even more gain and directivity than a reflector
type antenna. These antennas do not use reflectors to direct the RF energy.
Instead, the energy is directed through an array of parallel metal plates
that acts like a lens.
The mechanical construction of a metallic lens
consists of the following three parts:
(1) A waveguide which brings the energy from the transmitter.
(2) A horn which matches the impedance of the waveguide to air.
(3) A metallic lens which focuses the radiated energy into a highly
directional beam.
b. The theory of lens antennas is based on the similarity of radar
waves to light waves.
You learned how a parabolic reflector focuses RF
energy into a highly directive radar beam.
The action is like that of a
reflector in a flashlight.
The action of a metallic lens, on the other
hand, is similar to the action of light waves passing through a glass lens
such as used in eyeglasses and cameras. Figure 153 compares the glass lens
with the metallic lens.
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