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By E.V. Jull

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

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16 used. Corresponding experimental results have also been reported ( M l , 1962,1963). The patterns show 38 Near-field radiation patterns that with a directive measuring antenna near-field sidelobe levels either above or below the far-field values may be observed. They also show the tendency for a highly directive antenna to record the far-field pattern in the near field. A practical difficulty of measuring far-field patterns at reduced ranges in this way is the interaction which occurs between measured and measuring antennas and which is not accounted for here.

3) If aperture field expressions of the form of eqn. 34 are inserted in eqn. 3, integration over homogeneous and inhomogeneous plane waves results. The integral over kl + k\ > k is related to the reactive energy stored in the evanescent waves and is excluded from this calculation by taking the real part of the integral. For apertures of sufficient size the radiating electric and magnetic fields are related by essentially free space conditions. Then eqn. 4) In the far field of the aperture H = Yor x E and the time-averaged power flux is hY0\E(r,d9

1 Here this assumption is removed and the radiation field in what is known in optics as the Fresnel zone of the aperture is examined. With the condition r > X maintained, the reactive near fields of the aperture are excluded from consideration. Fig. 1 Path-length differences in the near field of an aperture Fig. 1 shows ray paths xp and op in the x—z plane which are not parallel. From the geometry S2 _r2 = (5 - r ) ( 5 + r) = x* - 2rx sin 0. 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.

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