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Radio-Frequency
System
As stated previously, the AN/
TPS-3 employs a 10-ft parabolic reflector. It is also provided with two
different antenna feeds that can be used interchangeably. One is a simple
halfwave dipole at the focus of the parabolic reflector, with a parastically
excited halfwave reflector a quarter wave in front of it. This combination
produces a single free-space lobe such as that shown in Fig. 7A. The purpose
of the reflector is to prevent direct radiation from the dipole, and results
in an increase in gain.
The second type of antenna feed is the one used to produce
phase and antiphase patterns. This array consists of three dipoles spaced
vertically a quarter wave apart in the plane of the focus, with center
dipole at the focal point. Each one of these dipoles has its associated
half-wave parasitic reflector a quarter wave in front of it. These three
dipoles are so arranged that either the center dipole alone or the two
outside dipoles may be driven. When the center dipole alone is driven the
result is a single free
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space lobe such as shown in Fig.
7A. The outside dipoles are so con-nected that they are fed 180 deg out
of phase, which is very simply arranged by connecting the left side of
the upper dipole to the center conductor and the right side of the lower
dipole to the center conductor and then feeding the pair in parallel from
a common point. When these two dipoles are driven the result is a split
pattern such as that shown in Fig. 7B, with the upper lobe 180 deg out
of phase with the lower lobe.
The switching between
the phase dipole and the antiphase dipoles is done by means of a solenoid-operated
plunger which is controlled from a switch on the panel of the radar set.
The plunger merely connects the center conductor of the transmission line
to either the center dipole or the outside dipoles as shown in Fig. 8.
The antenna radiators are connected
to the transmission line by means of a 50-ohm rigid coaxial line.
In order that this line be flat, or without appreciable standing wave,
every precaution is taken to match the antenna radiator to this line. This
is done by means of quar-terwave transformers consisting of sections of
inner conductor of different diameter from the diameter of the inner conductor
of the transmission line itself. By properly choosing the diameter
and position of such a quarter-wave transformer, the antenna radiator can
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matched to the transmission line.
The requirement of a flat line
precludes the use of insulating beads as supports for the inner conductor,
since many beads in the line would produce reflections and, therefore,
standing waves. Instead of beads, quarter-wave stubs are used to
support the inner conductor. A section of transmission line, short-circuited
at one end and a quarter-wave long, has an extremely high impedance looking
into the open end. When such a section is shunted across the line
it does not produce any appreciable reflection. Such quarter-wave
stubs, spaced at intervals along the transmission lines, can be used to
support the inner conductor. However, since a stub is a sharply tuned
resonant circuit it can be used only at one frequency. The AN/TPS-3
operates over a band from 590 to 610 mc, so that provision must be made
for allowing the transmission line to pass this band of frequencies without
appreciable reflections. This is done by making the characteristic
impedance of the support stubs 100 ohms as compared to 50 ohms for the
transmission line proper.
Another interesting feature
of this transmision line is the rotary joint. The antenna must be
able to rotate continuously, so the transmission line must be broken at
some point and means provided to pass energy from the stationary to the
rotating side. This breaking is done |
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