Aperture Antennas Questions and Answers

Answer: C

The subreflector redirects the converging wave back towards the vertex, giving a long effective focal length in a short physical structure and putting the feed and receiver where they can be reached. It adds blockage of its own, which is the price. It does not add collecting area, does not create a second beam, and weather protection is a radome's job. Folding a path to place equipment conveniently is the same idea telescope makers adopted centuries earlier.

Answer: D

The hyperboloid magnifies the effective focal length, so a physically compact structure behaves optically like a very long dish, which improves scan performance and cross polarisation. That magnification is the main reason for the arrangement beyond feed accessibility. The path length and the effective focal length are different quantities, which is why the third option is a natural but wrong intuition, and feed position is fixed by the design rather than free.

Answer: A

A subreflector only a few wavelengths across diffracts appreciably at its rim, so it is oversized to capture the feed pattern properly, and the trade against increased blockage sets the optimum. This is why Cassegrain geometry works poorly on small dishes at low frequencies, where the subreflector would be an unacceptable fraction of the aperture. Mechanical margin, deliberate blockage and interchangeability are not the reason. Diffraction becomes a first order effect whenever a component is only a few wavelengths in size.

Answer: B

Blockage loss goes as the square of the blocked diameter fraction, so on a small dish the necessarily oversized subreflector removes an unacceptable share of the aperture. Prime focus or offset feeding is used instead below a diameter of perhaps twenty wavelengths. The feed sits at the vertex by design, tolerance scales with wavelength rather than with dish size, and gain is not the criterion. Knowing where a technique stops working is as useful as knowing how it works.

Answer: C

Rain both attenuates the signal along the path and raises the sky noise temperature, and a film of water on the radome or feed window adds further loss just when the margin is smallest. The two effects coincide, which is why outage statistics are worse than attenuation alone would predict. Mechanical distortion from rain weight is negligible, feed impedance is not humidity sensitive to any useful degree, and the amplifier is temperature stabilised. Correlated failures of this kind deserve particular attention in any availability calculation.

Answer: D

Misalignment is the commonest and cheapest fault to find, and a feed even slightly off focus or off axis costs gain and distorts the pattern. Surface measurement is a much larger undertaking and is done when alignment has been eliminated. Amplifier noise affects the signal to noise ratio rather than the gain, and polarisation error would show as a loss on one polarisation only. Working from the cheapest and likeliest cause outward is what distinguishes fault finding from part swapping.

Answer: A

An efficiency budget sets out taper, spillover, blockage, phase error, surface tolerance and cross polar loss as separate multiplying factors whose product is the aperture efficiency. Listing them separately shows at once where an improvement would be worth making. It is not a power, cost or bandwidth accounting, though those exist too. Breaking a single measured shortfall into its named contributions is the standard engineering approach to any composite quantity.

Answer: B

On a well made dish at moderate frequency the taper against spillover compromise typically costs the most, holding aperture efficiency near sixty per cent even when everything else is good. Surface error dominates only at high frequency, cross polarisation is a small power loss though an important specification, and feed mismatch is normally a few per cent at worst. Shaped dual reflectors exist precisely because they attack this dominant term, and they are worth the complexity for that reason.

Answer: C

Shaping both surfaces lets the illumination be made nearly uniform while the feed pattern still falls away at the rim, breaking the usual taper against spillover trade. That is why large earth station antennas are almost always shaped. Blockage remains and is dealt with by offset geometry, surface error is a manufacturing matter, and the feed size follows from the optics. Escaping a trade by changing the geometry rather than by choosing a point on it is the most satisfying kind of engineering answer.

Answer: D

A parabolic reflector is analysed from the field across its opening and so belongs to the aperture class, along with horns, slots and lenses. Dipoles, loops and whips are wire antennas whose radiation is computed from the current along a conductor. The two classes are not different physics but different bookkeeping, chosen according to which quantity is easier to write down, and recognising which applies is the first step in tackling any antenna problem.