Antenna Fundamentals and Parameters Questions and Answers

Answer: C

Once the current distribution contains regions of opposite phase, their contributions cancel in some directions and add in others, so the broadside lobe splits and multiple lobes appear. The axial nulls remain, since the axial field is zero for any linear current. The pattern therefore becomes more complicated rather than simpler, and this is the reason long wire antennas have such intricate patterns and why the half wave length is a natural choice.

Answer: D

The short dipole's sine squared pattern gives a directivity of 1.5, or 1.76 dBi, only slightly below the half wave dipole's 1.64 or 2.15 dBi. That the two differ so little is significant: shortening an antenna costs efficiency and bandwidth but almost nothing in pattern directivity. A value of 1.0 would be isotropic, and 2.15 is a decibel figure being offered as a ratio, which is a standard trap.

Answer: A

The second is correct and the first is not. Directivity falls only from 1.64 to 1.5, a difference of less than half a decibel, whereas radiation resistance falls from about 73 ohms to a few ohms or less, which is what destroys the efficiency. The pair is worth holding together because it locates the real penalty of making an antenna small: not the pattern, but the loss and the bandwidth.

Answer: B

A reflecting surface produces an image whose field adds to the direct one, producing lobes and nulls in elevation and often several decibels of gain at particular angles. The height above ground in wavelengths determines where those lobes fall, which is why broadcast antenna heights are chosen so carefully. Ground does change the impedance too, absorbs only partially, and gain is perfectly well defined for an installed antenna provided the environment is stated.

Answer: C

At roughly half a wavelength the direct and reflected waves reinforce at a moderately low angle, which is why that height is a common compromise for long distance work at high frequency. Very close to the ground the reflected wave nearly cancels the direct one for a horizontal antenna, giving a pattern almost entirely upward, which is useful only for short skip communication. Height above ground is one of the most consequential and least appreciated installation choices.

Answer: D

The image in the ground is out of phase for a horizontal element, so at low heights the two nearly cancel at low angles and what remains is directed steeply upward. This near vertical incidence arrangement is used deliberately for regional communication over a few hundred kilometres. It is not a failure to radiate but a redistribution, and it illustrates once again that an antenna's pattern belongs to the whole installation rather than to the element alone.

Answer: A

For a perfectly conducting plane, a vertical element's image reinforces it while a horizontal element's image opposes it, so a vertical antenna works close to the ground and gives good low angle radiation while a horizontal one does not. Real ground losses reduce the advantage, which is why a radial system matters so much. Length, conductivity dependence and radiation resistance are secondary and do not explain the fundamental difference in behaviour.

Answer: B

In a prime focus reflector the feed and its supporting struts sit in the path of the reflected wave, removing power from the aperture and scattering it into the side lobes. It costs both gain and side lobe performance, which is why offset feed geometries are preferred where the extra complexity is justified. A radome adds loss without blockage in this sense, far field obstacles are a propagation matter, and taper is a deliberate illumination choice.

Answer: C

Moving the feed out of the beam path removes blockage and strut scattering, raising efficiency and lowering side lobes, which is why domestic satellite dishes are almost all offset. The price is a more complicated surface to manufacture and a cross polarisation penalty that must be managed. It is not cheaper, does not generate circular polarisation on its own, and bandwidth is set by the feed rather than by the geometry.

Answer: D

A radome keeps weather, ice and birds off the antenna, and in exchange adds transmission loss, a small amount of reflection and some pattern distortion. Its thickness is chosen relative to the wavelength to minimise reflection, and wet or iced radomes degrade sharply. It focuses nothing, is certainly not electrically invisible whatever the material, and does not alter polarisation unless deliberately designed to. Every protective measure in antenna engineering has an electrical price.