Horn and Reflector Antennas Questions and Answers

Answer: A

An E-plane sectoral horn opens out in the direction of the electric field and keeps the guide dimension in the other plane, so it narrows the beam in the E-plane only. The H-plane version does the opposite, and flaring both gives the pyramidal horn. An unflared guide is simply an open ended waveguide. Sectoral horns are used where a fan beam is wanted, narrow in one plane and broad in the other, which suits many radar and landing aid applications directly.

Answer: B

The pyramidal horn flares in both the E and H planes, giving a rectangular mouth and a pencil beam. It is the commonest laboratory and reference horn and the usual gain standard. Circular cross section gives the conical horn, corrugations give the hybrid mode horn, and dual polarised feeds are a separate refinement that may be applied to several of these shapes. The geometry is simple enough that its gain can be computed accurately from its dimensions, which is why it became the standard.

Answer: C

A conical horn is the circular guide equivalent of the pyramidal horn, flaring a round guide out to a round mouth, and it is the natural feed for a circular aperture such as a paraboloid. Its pattern is still not perfectly symmetric in the plain form, which is why corrugated and dual mode versions were developed. A horn that tapers inward would be a transition rather than a radiator, and a continuously curved flare describes some specialised profiles rather than the basic conical shape.

Answer: D

Opening the flare enlarges the aperture, which raises gain, but the path from the apex to the rim grows longer than to the centre, and that phase error eventually costs more than the extra area gains. The optimum sits where the edge phase error is about a quarter to half a wavelength. Wall loss is negligible, the guide supplies whatever power is fed to it, and higher order modes are a secondary effect. Balancing an area gain against a phase loss is the whole of horn proportioning.

Answer: A

An optimum horn has the flare and length chosen so that the gain from the larger aperture just balances the loss from the quadratic phase error, which fixes the aperture size for a given length. Making it longer for the same mouth would reduce the phase error but is often impractical. Perfect matching, equal beamwidths and zero phase error are all desirable and are pursued by other means. Optimality here is a property of proportions rather than of performance in any absolute sense.

Answer: B

Applying the aperture gain relation with the area expressed in square wavelengths:

\[ G = 4\pi \times 70 \times 0.51 \approx 449 \]

which is about 26.5 dBi. The efficiency of about half is characteristic of an optimum pyramidal horn and comes from the combination of the guide's amplitude taper and the residual phase error. Knowing that figure lets a horn's gain be estimated from its mouth dimensions alone, which is often all the information available.

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.

Answer: D

Quarter wave grooves in the walls present a high impedance to the axial current, allowing a balanced hybrid mode whose aperture field is nearly rotationally symmetric and falls to zero at the rim. The result is a pattern almost identical in every plane, very low cross polarisation and very low side lobes. Stiffening ribs, impedance steps and absorber linings are all real constructions but none produces the mode that gives the corrugated horn its properties.

Answer: A

Because the pattern is the same in every plane and the field falls smoothly to zero at the rim, the reflector is illuminated evenly around its circumference and very little cross polar energy is generated. That is what earth stations and radio telescopes need. Its gain is comparable to a plain horn of the same size, dual band operation requires further design, and it is usually longer rather than shorter. Matching the symmetry of the feed to the symmetry of the reflector is the point.

Answer: B

A Potter horn uses a step or taper to excite a controlled amount of a higher order circular mode, which combines with the dominant one to give a nearly symmetric aperture field. It achieves much of what a corrugated horn does with simpler machining, but over a narrower band, since the two modes must stay in the right phase relationship. Dual polarisation and dual band operation are separate capabilities, and the step is the means rather than the definition.