Aperture Antennas Questions and Answers

Answer: C

Effective aperture is defined as the received power divided by the power density of the incident wave, so it has units of area and can be measured without knowing anything about the antenna's construction. It equals the physical area multiplied by the aperture efficiency, which includes far more than radiation efficiency alone. The other options describe a physical dimension, a beam definition and an incomplete product. Defining a quantity by how it is measured rather than by how it is built is what makes it useful across every antenna type.

Answer: D

Because the far field begins at two D squared over lambda, a large aperture at microwave frequencies needs a range of hundreds of metres or kilometres, which is rarely available. Scanning a probe over a plane close to the aperture and transforming the result gives the far field pattern indoors. The method is not intrinsically more accurate, rotation is not the difficulty, and a near field range still needs careful absorber treatment. It is a case of substituting computation for physical distance.

Answer: A

The probe records both amplitude and phase at a grid of points on a plane, cylinder or sphere near the antenna, and a transform yields the far field pattern. Phase is essential, which is why the scanner's mechanical accuracy must be a small fraction of a wavelength. Total power, surface profile and terminal impedance are all measured by other means, though holographic processing of the same near field data can in fact reveal surface errors, which is a valuable by-product of the technique.

Answer: B

Transforming a measured far field pattern back to the aperture plane recovers the phase distribution across the reflector, and departures from a plane wave map directly onto surface errors. Panels can then be adjusted by the amounts the map shows. It is the standard method for setting large radio telescope surfaces. Documentation imaging, polarisation measurement and impedance testing are unrelated tasks. The technique is a neat example of the aperture to far field transform being used in the reverse direction.

Answer: C

Beam efficiency is the power contained in the main lobe divided by the total radiated power, and it matters wherever radiation from outside the beam contaminates a measurement, as in radiometry, remote sensing and radio astronomy. A heavily tapered aperture has excellent beam efficiency and mediocre aperture efficiency, which is why the two specifications pull in opposite directions. The other options define radiation efficiency, aperture efficiency and an illumination measure, and the three efficiencies must never be interchanged.

Answer: D

A radiometer measures the temperature of what its beam sees, so any energy arriving through the side lobes from warm ground is indistinguishable from signal and biases the result. Accepting a wider beam and lower gain in exchange for very low side lobes is therefore the right trade. Receiver saturation is not the issue at these levels, measurement convenience is irrelevant to the design choice, and large apertures are entirely feasible. Matching the antenna specification to what the instrument actually measures is the general point.

Answer: A

Cross polarisation is the unwanted orthogonal component generated by the antenna itself, arising from curvature of the reflector, asymmetry in an offset geometry or imperfection in the feed. It is specified as a level below the co-polar peak, and it limits how well two polarisations can reuse the same frequency. Faraday rotation is a propagation effect, coupling between antennas is isolation, and reflection depolarisation is a scattering phenomenon. The distinction between what the antenna does and what the medium does is worth keeping sharp.

Answer: B

In a symmetric reflector the cross polar contributions from opposite sides cancel, whereas the offset geometry destroys that symmetry and leaves a residual cross polar lobe. Using a matched feed or a dual reflector arrangement recovers much of the performance. Feed tilt is part of the same geometry rather than a separate cause, absence of blockage does not create the lobes, and the path lengths are equalised by the surface shape. It is the price paid for removing blockage, and it is usually worth paying.

Answer: C

The Cassegrain uses a hyperboloidal subreflector to fold the path so the feed can sit at the vertex of the main dish rather than at its focus, which shortens the feed run, keeps the receiver accessible and allows a longer effective focal length. Prime focus is the direct arrangement, the paired paraboloid describes a beam waveguide differently, and the corrector plate belongs to Schmidt optics. Folding an optical path to place equipment conveniently is an idea borrowed directly from telescope design.

Answer: D

Feed line loss ahead of the amplifier adds directly to the system noise temperature, so placing the amplifier at the vertex where the run is a few centimetres rather than at the focus is worth a great deal of sensitivity. The subreflector contributes no gain and in fact adds blockage of its own, and while small beam movements by subreflector tilt are possible they are a secondary benefit. Noise temperature considerations dominate the design of any receiving station.