Microstrip Antennas Questions and Answers

Answer: C

Energy that leaves sideways is lost to the intended beam, diffracts from the board edges into stray radiation and travels between elements of an array as unwanted coupling. Photonic bandgap ground planes and cavity backing are among the remedies. The resonant frequency is set by the patch dimensions, the cross polar effect is one symptom among several, and bandwidth is not improved by loss of this kind. A single mechanism producing three separate symptoms is worth recognising as one problem.

Answer: D

The radiating edge is a voltage maximum and a current null, so the impedance there is high, typically two to three hundred ohms. The centre is the opposite, a current maximum and a voltage null, giving essentially zero impedance. Moving the feed point between the two lets any intermediate value be selected, which is exactly how a patch is matched to fifty ohms. Knowing where the voltage and current maxima lie predicts the impedance at every point on the structure.

Answer: A

Cutting a slot into the patch lets a microstrip line reach inward to a point where the impedance is fifty ohms, giving a match without any separate transformer. The depth of the inset is the design variable. Aperture coupling, probe feeding and proximity coupling are the other standard feed methods, each with its own advantages. The inset feed is popular because it is entirely planar and needs no extra components or drilling.

Answer: B

A probe through the substrate connects to the patch from below, so the feed line and any matching circuitry stay on the far side of the ground plane and add no spurious radiation. The probe position sets the impedance. Its drawbacks are a series inductance that grows with substrate thickness and the need to drill and solder. Bandwidth is not improved, edge feeding presents a high impedance whatever the method, and the ground plane is essential to the structure.

Answer: C

The probe is itself a short vertical conductor, and its inductance is proportional to its length, so a thick substrate leaves a substantial series reactance at resonance. A series capacitive gap or a modified probe head is used to cancel it. Permittivity is a material property independent of thickness, the resonance depends on the patch dimensions, and the ground connection is a low impedance. This inductance is one of the practical obstacles to making a patch broadband simply by thickening the board.

Answer: D

In an aperture coupled design the feed line runs on a separate substrate below, and energy passes to the patch through a small slot in the ground plane between them. The two substrates can then be chosen independently, a thin high permittivity one for the line and a thick low permittivity one for the patch. There is no direct connection to solder, and the feed radiation is shielded by the ground plane. It is the most flexible of the feeding methods and the most complex to build.

Answer: A

Because the feed network and the patch sit on opposite sides of a common ground plane, each can use the substrate that suits it: a thin dense board for compact low radiation circuitry and a thick light one for an efficient wideband patch. The slot sets the coupling strength through its length and position rather than acting as a universal transformer, the coupling does involve the dielectrics, and both structures work at the same frequency. Separating two requirements onto two substrates is the neat idea here.

Answer: B

In proximity or electromagnetic coupling the microstrip line is sandwiched inside the dielectric stack and couples capacitively to the patch above it without touching. It gives wider bandwidth than a direct edge feed and avoids the probe inductance, but the multilayer construction is harder and alignment matters. Parasitic patch coupling and waveguide transitions are separate techniques. The family of feeding methods is best remembered by whether a galvanic connection is made and where the line sits.

Answer: C

Truncating opposite corners splits the degenerate pair of orthogonal modes slightly in frequency, so that at the design frequency they are equal in amplitude and ninety degrees apart in phase. A nearly square patch fed on the diagonal achieves the same effect. Centre feeding excites nothing, a slab is a superstrate rather than a polariser, and rotation changes the orientation of a linear polarisation rather than its nature. Splitting a degenerate resonance is the single feed route to circular polarisation.

Answer: D

The two split modes are in exact quadrature at only one frequency, so the axial ratio degrades quickly on either side and is usually acceptable over a band narrower than the impedance bandwidth. A dual feed with a ninety degree hybrid holds the axial ratio over a much wider range at the cost of the extra network. Gain is unaffected, the size is essentially unchanged, and feed position tolerance is a general patch issue rather than a specific one here.