Aperture antennas and diffraction theory by E.V. Jull

By E.V. Jull

Replacement tools of aperture antenna research are defined during this book.

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Sample text

If A is the path difference at p of radiation from o and x in the aperture (s — r) = — A and, since 5 + r — 2r, —x 2r 4- x sin 6. 1 which vanishes as r -+ °°. The usual requirement for the far-field patterns is that the 32 Near-field radiation patterns maximum error in path length due to finite range is less than X/16 for radiation from all parts of the aperture. 2) r>% This is the rather arbitrary division between the near field or Fresnel zone of an aperture and the far field. The effect of the finite range on the radiation pattern will depend also on the aperture distribution.

In eqn. 7, u2 = —ux = a/\/2r\ and, since C{— u) = — C(u) and S(— u) = —S(u), eqn. 19) For a uniformly illuminated rectangular aperture D2(0) is given by eqn. 19 with v = b/\/2r\ replacing u. The on-axis near field of the aperture of Fig. 1 is, from eqns. 21) Gn = -^-RE(u)RE(v) where with u = a/y/lrX and C(w) and S(u) defined by eqn. 8. The gain reduction factor RE(u) for a uniform line source was first tabulated by Polk (1956). Its behaviour is illustrated in Fig. 1, as functions of the parameter 2d21(U) where d is the aperture dimension a and 91 is the range r.

5 8 " (b) Typical field distributions (a) Blocked aperture distribution (b) Effect on the radiation pattern. Unblocked pattern pattern F'(kx)IF(0). F(k1)/(F[0); blocked the pattern is broad so that over the central portion of the antenna radiation pattern F'ik^^Fik^-S. The patterns F(kx)IF(Q) and F'tf^/FiO) are shown in Fig. , where it is apparent that aperture blockage narrows the main beamwidth. It also raises the level of the first sidelobe. If F(0) = 1 andp is the normalised level of the first sidelobe in the unblocked pattern, then F'(0)/F(0)= 1 - 5 / F ( 0 ) and F'/F(0) =p + S/F(0) at the first sidelobe in the blocked pattern.

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