Antenna Fundamentals and Parameters Questions and Answers

Answer: B

Aperture efficiency is the ratio of effective to physical area, and it accounts for illumination taper, spillover past the reflector, blockage by the feed and surface errors. A typical prime focus dish reaches fifty to sixty per cent and a well designed Cassegrain rather more. Radiated over accepted power is radiation efficiency, gain over directivity is the same thing, and the last option describes beam efficiency, which matters in radiometry.

Answer: C

Side lobes arise from the sharp truncation of a uniformly illuminated aperture, so reducing the illumination gradually towards the rim removes that discontinuity and the side lobes fall sharply. The price is a wider main beam and a lower aperture efficiency, both of which move the wrong way. It is exactly the same mathematics as windowing a data record before a Fourier transform, and the same trade between resolution and leakage appears in both.

Answer: D

Beamwidth in radians is approximately the wavelength divided by the aperture dimension, so doubling the aperture halves the beamwidth and quadruples the directivity, since the beam is narrowed in both planes. The relation is the practical reason radio telescopes and radars are built large. It is also why resolution at a given aperture improves with frequency, which drives millimetre wave imaging.

Answer: A

Using the usual approximation of about 51 degrees divided by the aperture in wavelengths:

\[ \theta \approx \tfrac{51}{20} \approx 2.6 \]

giving about 3 degrees. The precise coefficient depends on the illumination taper, running from about 51 for uniform to 70 for a heavily tapered aperture, which is one more expression of the trade between beamwidth and side lobe level. An order of magnitude estimate of this kind is usually enough to decide whether a design is feasible.

Answer: B

EIRP combines transmitter power and antenna gain into the single number that determines the field a distant receiver sees in the main beam, so it is the correct basis for comparison and the quantity regulators limit. Radiation resistance is a circuit property, standing wave ratio describes the match rather than the output, and beamwidth describes coverage. Choosing the right figure of merit is half the work in any system level comparison.

Answer: C

A nearby conductor carries induced current and re-radiates, adding vectorially to the dipole's own field and producing a pattern with a preferred direction. Whether the mast acts as a reflector or a director depends on its spacing and length, which is the principle a Yagi exploits deliberately. Resonance does not immunise an antenna against its surroundings, an omnidirectional result would be a coincidence, and no arrangement makes a dipole radiate along its own axis.

Answer: D

Any reflecting surface acts as a second source, so the measured pattern becomes the sum of the direct and reflected contributions and side lobes in particular are badly falsified. An anechoic chamber lined with absorber, or an elevated outdoor range, suppresses those paths. Power levels used for pattern work are small, some antennas do have short far field distances, and outdoor conditions rarely match the installation. Controlling the environment is the whole purpose of a range.

Answer: A

A shaped reflector illuminated by a feed collimates the wave so that a plane wave front exists across a quiet zone only a few metres away, allowing far field measurement in a room. It replaces distance with optics. Measuring at half the far field distance would simply give wrong results, portability is not the definition, and scaling the frequency changes the antenna under test rather than the range. Near field scanning with a mathematical transformation is the other standard answer to the same problem.

Answer: B

The first and third are correct. Gain is directivity reduced by radiation efficiency, and the two decibel references differ by the dipole's own gain over isotropic. The second statement reverses the relation: gain can never exceed directivity, since efficiency cannot exceed one. A published gain larger than the computed directivity is therefore a sign of an error or of a different reference, and it is worth checking whenever a specification looks too good.

Answer: C

Directivity states how far radiation is concentrated, axial ratio how nearly circular the polarisation is, return loss how well the impedance is matched, and beam solid angle the equivalent angular extent of the main beam. Directivity and beam solid angle are two expressions of the same fact, being reciprocally related through four pi, which is why either may be quoted. Sorting the parameters by what they describe is the fastest way to read an unfamiliar data sheet.