Horn and Reflector Antennas Questions and Answers

Answer: A

An extended source fills the beam and often the near side lobes too, so the fraction of the pattern's power inside the main lobe determines how much of the measured signal really comes from the region being observed. A poor beam efficiency biases every brightness temperature. Beamwidth follows the aperture, tolerance follows the wavelength, and multiple receivers are a focal plane matter. Which efficiency matters depends entirely on what the instrument is being asked to measure.

Answer: B

Ground pickup and confusion from outside the beam corrupt a radiometric measurement, so a heavier taper is accepted in exchange for lower side lobes and higher beam efficiency, even though it costs gain. A communication antenna is usually more concerned with the signal to noise ratio in its main beam. Reflector size, receiver sensitivity and frequency are not the reason. The design follows from what the instrument's output is going to be used for.

Answer: C

Sunlight on one side of a structure expands it unevenly, deforming the surface and moving the feed relative to the focus, which shows up as a gain and pointing variation through the day. Insulation, ventilation and thermal models are the usual remedies, and some instruments observe only at night at the highest frequencies. Physical temperature affects noise rather than radiated power, feed detuning is negligible, and air heating is a small propagation effect. Thermal deformation is a first order problem at millimetre wavelengths.

Answer: D

Solar heating distorts the surface and the support structure by amounts comparable to the tolerance at millimetre wavelengths, so the surface is at its best in thermal equilibrium after dark. Water vapour does often fall at night, which helps too, but the deformation argument is the one specific to the antenna itself. Interference and receiver temperature are separate considerations. When a measurement is scheduled to avoid an effect, identifying which effect is being avoided explains the rest of the practice.