Microstrip Antennas Questions and Answers

Answer: C

At handset frequencies the board is a substantial fraction of a wavelength and the currents the antenna drives onto it radiate as much as the antenna element itself, which is why performance changes when components or the case are altered. Designers speak of the antenna and the chassis as one radiating system. The board is not the antenna's substrate, shielding and heat spreading are separate functions. A small antenna on a large conductor is really a way of exciting that conductor.

Answer: D

A corporate feed splits the power in a binary tree so every patch is reached by the same electrical length, which keeps the array broadside and free of squint. It occupies board area and its lines radiate a little. A series feed taps power along one line and is more compact but squints with frequency, separate cables describe a different architecture, and parasitic coupling is used in some designs but is not a corporate network. The choice mirrors the same one made in any array.

Answer: A

Microstrip lines on the same layer as the patches radiate at their bends and junctions, and that stray radiation adds to the side lobes and the cross polar level. Aperture coupling moves the network behind the ground plane and cures it. Equal lengths are perfectly achievable with care, the substrate choice is a design decision rather than a requirement, and a symmetric divider splits power evenly. Unwanted radiation from a feed network is the characteristic penalty of printed arrays.

Answer: B

A series feed runs one line past the elements, coupling a portion of the power into each, which is compact and low loss but makes the beam squint as the frequency changes. Some radars use that squint deliberately to scan. Direct end to end connection would not work, feeding from both ends is a centre fed variant, and separate amplifiers describe an active array. Compactness traded against frequency stability is the essential comparison with a corporate feed.

Answer: C

The progressive phase is set by the electrical length of line between elements, and that length in wavelengths changes with frequency, so the beam direction moves. A corporate feed avoids it by making every path the same length. Element resonance, coupling variation and loss all change with frequency too but none produces a systematic beam shift. Distinguishing a fixed phase from a fixed delay explains this and several other frequency dependent effects in arrays.

Answer: D

A patch is a few millimetres thick and can be bonded to or recessed into a skin, adding essentially no drag and surviving the mechanical environment. That is why printed antennas displaced protruding structures on airframes. Gain is that of any aperture of the same size, temperature tolerance is a substrate limitation rather than an advantage, and a feed network is certainly required. Choosing an antenna for how it fits the vehicle, not only for its pattern, is often the deciding consideration.

Answer: A

A conformal array bends the substrate around a cylinder or a fuselage, so the elements point in different directions and the excitation must compensate for both position and orientation. Flexible substrates make it practical. Shaped beam synthesis, dimensional standardisation and flush mounting on a flat surface are all different ideas. Curvature turns a straightforward array problem into a considerably harder one, which is why conformal arrays are used where the mechanical benefit justifies it.

Answer: B

A lossy dielectric converts part of the stored energy into heat, so less is radiated and the efficiency falls, which matters most on high permittivity substrates where more of the field is in the material. It does broaden the measured bandwidth, but only by lowering the quality factor through loss rather than radiation, which is a bad bargain. Resonant frequency depends chiefly on permittivity, and surface waves depend on thickness and permittivity. A wider bandwidth bought with loss is not a real improvement.

Answer: C

Bandwidth is inversely proportional to the total quality factor, and lowering that factor by adding loss widens the response while sending the added energy into heat instead of into space. The antenna looks better matched and radiates less. The same trap appears whenever a resistive element is added to flatten a response. Asking where the energy goes, rather than what the match looks like, is the reliable test of whether a bandwidth improvement is genuine.

Answer: D

A dielectric cover of the right thickness forms a partially reflecting surface that resonates with the ground plane and concentrates the radiation, raising gain by several decibels. A wrongly chosen thickness does the opposite. Radomes over patches must therefore be designed rather than merely fitted. Absorption is small in a good dielectric, the pattern remains broadside, and the ground plane is essential. A layer that looks like mere protection can change the electrical behaviour substantially.