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
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