Antenna Fundamentals and Parameters Questions and Answers

Answer: C

The assertion is true and the reason is false, option (C). The radiation resistance of an antenna is defined as the value of a hypothetical resistance that would dissipate the same power as the antenna radiates when it carries the same current, so the assertion is correct. For a short Hertzian dipole, the radiation resistance is \(R_{r} = 80\pi^{2}\left(\dfrac{dl}{\lambda}\right)^{2}\) ohms, so the figure of 40 in the reason is wrong; 40 gives half of the true value. Options (A) and (B) accept the reason and option (D) rejects the assertion, which is the standard definition.

Answer: A

Radiation requires time varying current, which is to say accelerated charge: a straight wire carrying charge at constant velocity radiates nothing, and a steady direct current produces only static fields. Acceleration occurs wherever the current changes with time, and also at a bend, a discontinuity or an open end, which is why those points on a structure radiate most strongly. Length matters for efficiency and for pattern but is not the fundamental condition, and no antenna needs to be a wavelength in every direction.

Answer: B

Very close to the structure the fields are predominantly reactive, storing energy and returning it to the antenna each cycle rather than carrying it away. Beyond it lies the radiating near field or Fresnel region, where the pattern varies with distance, and beyond that the far field or Fraunhofer region, where the pattern shape becomes independent of distance. The Fraunhofer and far field names refer to the same region, so two of the options describe the same thing and neither can be the answer.

Answer: C

At 3 GHz the wavelength is 0.1 m, so:

\[ \tfrac{2 \times 1^{2}}{0.1} = 20 \]

metres, giving 20 m as the start of the far field. Pattern measurements made closer than this will not show the true far field pattern, which is why antenna ranges are long and why compact and near field ranges exist at all. The distance grows as the square of the aperture, so a large reflector at high frequency can need kilometres of range.

Answer: D

A pattern plots the relative strength of the radiated field or power against angle, measured in the far field where the shape no longer depends on range. It is normally drawn in two principal planes and normalised to its maximum. Impedance against frequency is a separate characteristic, distance dependence follows the inverse square law in every direction, and the current distribution is what produces the pattern rather than what the pattern displays. Almost every antenna parameter is read off the pattern or computed from it.

Answer: A

Half power beamwidth is the angular width of the main lobe measured where the power has fallen to half its peak, that is by three decibels. The angle between first nulls is the first null beamwidth, roughly twice as wide for a uniform aperture. Side lobe level is a separate specification, and radiation is detectable over essentially all angles, so the last option describes nothing measurable. Beamwidth and directivity are inversely related, which is the quantitative content of the idea of a beam.

Answer: B

Front to back ratio compares the radiation in the main beam direction with that in the opposite direction, and is quoted in decibels. A Yagi may reach twenty decibels and a reflector far more. Comparison with the largest side lobe is the side lobe level, forward gain referred to input power is simply gain, and the ratio of beamwidths describes the shape of the beam rather than any rejection. A high front to back ratio matters where interference arrives from behind the antenna.

Answer: C

Side lobe level is the ratio, in decibels, of the peak of the largest side lobe to the peak of the main lobe, and a uniformly illuminated aperture gives about minus thirteen decibels. Tapering the illumination lowers the side lobes at the cost of a wider main beam and a lower aperture efficiency. The count and position of the lobes follow from the aperture size, and the total side lobe power relates to beam efficiency, which is a different specification used in radiometry.

Answer: D

Radiation intensity is watts per steradian, and being a per solid angle quantity it does not fall off with distance, which makes it the natural variable for describing a pattern. Power per unit area is power density, which does obey the inverse square law, accepted power is an input quantity, and field strength at a reference distance is a related but different measure. Directivity is defined as the ratio of radiation intensity in a direction to its average over all directions.

Answer: A

Directivity compares the intensity in the strongest direction with the intensity an isotropic radiator of the same total power would give in every direction. It is a property of the pattern alone and takes no account of losses. Radiated over accepted power is the radiation efficiency, comparison with a dipole gives gain in dBd, and the aperture relation is a formula for directivity rather than its definition. Gain is directivity multiplied by efficiency.