Page 223 - Radar Technology Encyclopedia
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213 HOMING, radar HYBRID (junction), microwave
HOMING, radar. Use by a seeker-equipped missile of the optical horizon. This effect is approximated by using, in the
radar energy received from a target for missile guidance is equation, an effective earth’s radius ka equal to 4/3 times the
6
6
called radar homing. The energy source may originate at the actual radius, or 8.5 ´ 10 m instead of 6.5 ´ 10m. The result-
target itself (e.g., from an airborne radar), in which case the ing calculations are adequate for radar and targets at low and
guidance is called passive radar homing. If the missile seeker medium altitudes. For high-altitude radar or targets, more
is the energy source, i.e., an on-board radar, the guidance is accurate results can be obtained using range-height-angle
referred to as active radar homing, and if the energy source is charts plotted according to methods originated by Blake. (See
an external radar, the guidance mode is called semiactive CHART, range-height-angle.)
radar homing. (See GUIDANCE, radar; RADAR, missile To observe the targets lying far behind the radar horizon
guidance; SEEKER, radar.) PCH over-the horizon radars can be used. SAL
Home-on-jam (HOJ) refers to a missile radar homing seeker Ref.: Skolnik (1970), pp. 2.47, 2.48.
guidance mode in which the seeker tracks, and the missile clutter horizon (see CLUTTER).
guides on, an active jammer signal rather than the target
“skin” return. Radar-guided air-defense missiles, both sur- HUYGENS SOURCE. The Huygens source is an elemen-
face-to-air (SAM) and air-to-air (AAM), typically incorporate tary source of electric and magnetic current used to construct
a HOJ mode as a guidance option, which is primarily field distribution in aperture antennas. SAL
intended to defeat self-screening noise jammers (SSJs). HOJ Ref.: Fink (1975), p. 18.5.
may sometimes be effective against poorly implemented The Huygens-Fresnel principle is the set of physical
deceptive SSJs as well. The decision to transition from skin assumptions for the approximate decision for a set of diffrac-
track to HOJ may be made by an external tracking radar, or it tion problems. According to this principle, the wave radiated
may be decided autonomously on board the missile. If the by some source can be represented as an arbitrary point as a
decision is made without target range information, however, superposition of coherent secondary waves of imaginary
the missile is subject to capture by an out-of-range jammer. sources. These sources are considered to be continuously dis-
(see RADAR, missile guidance). PCH tributed along the auxiliary arbitrary surface surrounding the
HOMODYNE (see RECEPTION, homodyne). source. This principle, initially devised in optics theory, is
widely used in the theory of aperture antennas. IAM
HORIZON, radar. The radar horizon is the imaginary line Ref.: Silver (1949), p. 108; Kobak (1975), p. 90.
beyond which targets located below some height h cannot be
t
detected because the radio waves in free space, like optical HYBRID (junction), microwave. A microwave hybrid junc-
tion is “a waveguide or transmission line arrangement with
rays, are straight lines (Fig. H4). From the figure, it is easy to
four ports, which when the ports have reflectionless termina-
compute the distance R from the target to the radar horizon
ht
tions has the property that energy entering at one port is trans-
2 2 ferred (usually equally) to two of the remaining three ports.”
–
R = ( ka + h ) ( ka )» 2kah
h t t
Actually it is a simple power divider (or adder) of one chan-
nel into two others. Hybrids are used in various transmission
where ka is the effective earth radius. From a radar site at alti-
lines: coaxial, waveguide, etc. A coaxial hybrid is usually a
tude h above the earth, there will be an additional range R hr
r
between the radar and the horizon, and the total horizon range U-shaped symmetrical connection of three coaxial lines. If
two arms of this connection are loaded to resistances equal to
will be.
the wave resistance, then the third arm, the supply arm, is
(
R = R + R » 2ka h + h ) mismatched, with a standing-wave ratio of 0.5.
h ht hr t r
Usually the hybrids provide equal power division. Like
any nonuniformity, the branching introduces additional reac-
tivity of a volume or inductance nature depending on the type
of hybrid and its parameters. Compensation of reflections
from hybrids, arising due to the connection of the arms and
due to the nonuniformity, which is the branching, is provided
by four-wave transformers and short-circuited loops for the
coaxial hybrid, and inductive irises and stepped transformers
for the waveguide hybrid.
At microwave frequencies, waveguide hybrids are most
common. They are divided on the basis of shape into T-
shaped (magic tee) or Y-shaped hybrids. The latter is distin-
Figure H4 Radar horizon.
guished by the somewhat greater passband and the capability
of changing the division factor with a replaceable metal insert
Passing through the real atmosphere, in which varying
inside the waveguide.
index of refraction typically decreases with height, radar rays
curve downward and the radar horizon is slightly beyond the