Aperture antennas and diffraction theory by E.V. Jull

By E.V. Jull

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

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23 depends on the size of the aperture. Only supergain antennas have coefficients an significantly different from zero for n > kb/2. Hence complex amplitudes an for n running from —kb/2 to kb/2, or kb + 1 in all, are required. The same minimum number of equally spaced observation points about the aperture are also needed. The measuring antenna will have some directivity. e. 27) 40 Near-field radiation patterns If the pattern of the receiving antenna pattern is assumed to be of the form of eqn. 28) where K is given by eqn.

05 r/2a2/X Fig. 2 for a rectangular aperture with uniform and cosinusoidal distributions simplify considerably on the beam axis where kx = k2 = 0. 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).

3 might appear in the aperture of a parabolic reflector antenna. Then the aperture edge illumination can be altered through the feed directivity to provide the secondary pattern desired. Fig. 4 shows the pattern halfpower beamwidth for the aperture distribution given by eqn. 15 as well as for other distributions. 24 Fourier transform representation of aperture patterns The effect of aperture blockage by a reflector feed or subreflector also can be investigated by the present theory. Fig. 5a shows a typical field distribution in the aperture of an antenna reflector of width a with a feed of width 5 <^ a in the centre of the aperture plane.

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