| These pulses are applied
to the transmitter, which converts them to pulses of 600-mc energy.
They are then radiated from the antenna, which is coupled to the transmitter
by means of a coaxial line. When one of these pulses strikes an object
some of the energy is reflected. Measurement of range is facilitated
by injecting into both cathode-ray tubes a series of markers spaced 107
microseconds apart, and also synchronized with the transmitted pulses.
These markers represent 10-mile intervals and provide the scale by means
of which the range is measured.
Antenna and Propagation
The antenna used with the AN/
TPS-3 is a 10-ft parabolic reflector with a radiator at its focus.
This produces a free-space beam which is about 10-deg wide at its half-power
points. Consider such an antenna situated at height h above a plane
earth as shown in Fig. 5. Assume a reflecting target at point p at a great
distance from the antenna. The antenna will appear as a point source
as seen from p. The radiation pattern shows the free-space pattern
of the antenna. It is obvious that energy can reach the point p from
the antenna by traveling two paths, one directly from the antenna and the
second reflected from the ground. These will be called the direct
ray and the reflected ray. The angle made by the direct ray with
the horizontal is almost exactly equal to the angle made by the reflected
ray and the horizontal. Therefore, the amount of energy reaching
point p along each of these two paths will be almost equal. However, because
the distances along the two paths are not equal, the phase of the direct
ray and the reflected ray at point p will in general not be the same. Therefore,
the total energy at point p is the vector sum of the energies reaching
it along the direct path and reflected path.
In order to determine this total
energy it is not necessary to know the length of each path but merely the
difference between the path lengths, which will determine the difference
between the phases of the direct and reflected rays at point p. It
may be assumed that the |
| earth is a perfect reflector.
At the point of reflection there is a 180-deg change of phase. Because
radio energy travels with the speed of light, |
180-deg phase differences occur
every half-wavelength in space. Therefore, in order for the direct ray
and reflected ray to arrive in |
|
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