Horn and Reflector Antennas Questions and Answers

Answer: C

The symmetry depends on the two modes arriving at the aperture with a particular relative phase, and since they travel at different velocities that relationship holds over only a modest band. Corrugations impose the boundary condition directly and so work over a wider range. Step reflection is a small effect, the higher mode is above cutoff by design, and aperture size affects gain rather than symmetry. Any technique relying on a phase relationship between two propagating modes is narrowband for the same reason.

Answer: D

Ridges in the throat lower the cutoff frequency and widen the single mode range of the guide, giving decade bandwidths at the cost of gain and power handling. That makes ridged horns the standard antenna for spectrum surveillance and emissions testing. Maximum gain comes from a plain optimum horn, circular polarisation needs a polariser, and the ridges actually reduce the breakdown voltage by concentrating the field. Bandwidth bought with gain and power is the trade throughout this family.

Answer: A

A TEM horn is formed from two flaring conductors, so the guided wave has no cutoff and the structure works from very low frequencies upward, which suits impulse radar and time domain measurement. Its aperture field is neither uniform nor free of dispersion, and no waveguide mode propagates to zero frequency, which is exactly why a two conductor structure is needed. The name refers to the mode it guides and has nothing to do with microscopy.

Answer: B

The defining property is that the total path from a plane across the aperture to the surface and on to the focus is constant, so all contributions arrive in phase. Equal angles of reflection are true of any mirror and explain nothing particular about the paraboloid, the curvature relationship stated is not the criterion, and the centre of curvature of a paraboloid is not its focus. Recognising that focusing is a statement about equal path lengths makes the extension to shaped and dual reflector systems straightforward.

Answer: C

The f over D ratio is the single number that characterises the depth of a reflector, and it decides the feed pattern required, the tolerance to feed displacement and the cross polar performance. A ratio of 0.3 to 0.5 is typical for prime focus. Aperture ratio is the optical term for a related quantity, taper describes illumination, and magnification belongs to dual reflector geometry. It is the first quantity quoted when a reflector design is described.

Answer: D

A deep dish subtends a large angle at the focus, so the feed must radiate broadly to reach its rim, whereas a shallow dish needs a narrow feed pattern. Matching the feed pattern to the subtended angle, with about ten decibels of edge taper, is the central task of feed design. A narrow pattern would underilluminate a deep dish, an on axis null would waste the centre, and no practical feed is uniform to ninety degrees. Feed and reflector must be designed as one system.

Answer: A

A corner reflector places a driven element in front of two flat sheets meeting at ninety or sixty degrees, and the images in the two planes reinforce the direct radiation, giving some ten to twelve decibels of gain from a very simple structure. It is cheap, broadband and easy to build. Faceted approximations, trimmed rims and folded paths describe other arrangements entirely. The image principle explains its behaviour completely, which is why it remains a textbook favourite.

Answer: B

A spacing near half a wavelength puts the images in the right phase to reinforce the direct wave and gives close to the maximum gain, with the impedance still manageable. Moving much closer collapses the radiation resistance and moving much further breaks the beam up into lobes. The figure follows from the same image reasoning that governs a dipole above ground, and recognising the two situations as the same problem saves learning either of them separately.

Answer: C

A single plane gives one image and about three decibels of gain, whereas the two planes of a corner produce three images whose combination gives ten decibels or more. Size, polarisation and spacing are not the distinguishing factors, since a plane reflector must also be a wavelength or two across and preserves polarisation in the same way. Counting images is the quickest route to understanding any reflecting geometry made of flat conducting sheets.

Answer: D

A small disc facing the open end of an axial waveguide turns the radiation around so that it illuminates the reflector from the focus, without the bulk of a horn pointing backwards. It is cheap and compact and is common on offset domestic dishes and small radars. Weather protection, mounting and polarisation conversion are all separate functions handled by other parts. Redirecting radiation with a small reflector rather than a shaped horn is a neat piece of packaging.