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Why is the aperture efficiency of an optimum pyramidal horn only about 50 per cent?

  • A.Half the power is reflected back into the waveguide at the throat junction
  • B.The horn radiates half its power backwards from the open end of the guide
  • C.The amplitude taper and the residual phase error both reduce it
  • D.The metal walls of the horn absorb about half of the transmitted power

Answer: C

The dominant waveguide mode supplies a half sine taper in one plane, and the flare leaves a quarter to half wavelength of phase error at the rim, and the two together cost about half the aperture. Neither absorption nor reflection accounts for anything like that much, and a horn radiates forward. Corrugated and dual mode horns improve on the figure by attacking the amplitude and phase distributions directly, which is what makes them worth their extra manufacturing cost.

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