Antenna Arrays Questions and Answers

Answer: C

A smart antenna forms beams towards wanted users and nulls towards interferers, raising capacity by allowing the same channel to serve several users in different directions. It is an adaptive array applied to cellular service. Frequency selection, power saving and self diagnosis are all features some equipment has, but none of them is what the term denotes. Spatial reuse is the commercial reason the technology was adopted.

Answer: D

MIMO treats the scattered multipath channel as a set of parallel paths and sends different data on each, raising throughput without extra bandwidth or power. Beamforming instead concentrates one signal in a direction. The two are complementary and are often combined. Element count is not the distinguishing feature, MIMO does not form a narrower beam, and its elements are usually spaced widely to decorrelate the paths rather than coupled. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

MIMO capacity depends on the paths being as independent as possible, and closely spaced elements see nearly the same field, so half a wavelength or more, and in some designs many wavelengths, is used. Polarisation diversity achieves the same end in a small space. Grating lobes matter for beamforming rather than for spatial multiplexing, coupling does not endanger amplifiers, and raw gain is not the objective.

Answer: B

A collinear array places several elements along the same line, all fed in phase, which narrows the elevation pattern of a vertical antenna while leaving it omnidirectional in azimuth. It is the standard base station and repeater antenna for that reason. Side by side arrangements are broadside arrays, circles are ring arrays, and stacking along the beam is end fire. Compressing the pattern towards the horizon is the whole purpose.

Answer: C

Users are spread around the site but almost all near ground level, so squeezing the elevation pattern while keeping the azimuth pattern circular puts the gain exactly where it is wanted. A collinear array of four elements gives about six decibels over a single dipole. Azimuth steering, polarisation and multiband operation are separate design questions. Matching the pattern to the distribution of users is the general principle at work. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: D

The Yagi is an end fire array in which only one element is fed and the others are excited by coupling, a longer reflector behind and several shorter directors in front. Its simplicity and cheapness made it the standard television aerial. A corporate fed array, a collinear stack and a reflector antenna are all different arrangements, though a Yagi is sometimes used as the feed for a reflector.

Answer: A

Each director adds less than the one before, so gain rises roughly with the logarithm of the boom length, and the bandwidth narrows as more elements must be simultaneously near resonance. Beyond fifteen or twenty elements the return hardly justifies the length. Front to back ratio is chiefly the reflector's business, and the directors re-radiate rather than absorb. The diminishing return with aperture is a general feature of all antennas. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

In a log periodic array element lengths and spacings form a geometric progression, so the structure repeats itself logarithmically in frequency and the active region moves along the boom as the frequency changes. Bandwidths of ten to one are routine. It looks like a Yagi but is fed differently, with a transposed feeder connecting every element. Diameter taper and feed network details are secondary to the scaling principle.

Answer: C

At each frequency a small group of elements close to a half wavelength carries most of the current and the rest contribute little, so as the frequency changes the active region simply slides along the structure. The antenna therefore looks much the same electrically across a wide band. Element thickness helps a little, all its elements are driven rather than parasitic, and no compensating network is involved. Geometry defined by ratios rather than by lengths is the key idea.

Answer: D

Since only a few elements radiate at any one frequency, the effective aperture is a fraction of the boom length and the gain is several decibels below a Yagi occupying the same space. That is the price of the bandwidth. Driving every element does not help because most carry little current. The trade between bandwidth and gain for a given size is one more instance of the general rule that nothing in antenna design comes free.