Microstrip Antennas Questions and Answers

Answer: A

A patch antenna is a conducting area, usually rectangular or circular, printed on one side of a thin dielectric whose other side is a continuous ground plane. It radiates from the fringing fields at its edges. Slot antennas in a ground plane are a related but distinct family, printed wire antennas are another, and a machined cavity is not a printed structure at all. Being flat, light and made by ordinary circuit board processes is what made the patch so widely used.

Answer: B

The patch behaves as a half wavelength resonator in the dielectric, with the two ends acting as open circuits and the fringing fields there doing the radiating. Because the wavelength in the substrate is shorter than in free space, the patch is smaller than a half wavelength in air. A quarter wave version exists if one edge is shorted, which is the basis of the planar inverted F antenna. The half wavelength resonance is the starting point for every patch calculation.

Answer: C

The open ends are voltage antinodes where the field fringes out beyond the patch, and those two fringing regions act as a pair of slots radiating in phase. The centre is a current maximum but a voltage null and radiates little, and the fields at the side edges largely cancel. The ground plane carries the return current and reflects, giving the broadside pattern. Modelling the patch as two slots half a wavelength apart predicts both the pattern and the impedance usefully.

Answer: D

The fields extend a little beyond each end, so the effective length exceeds the physical one by roughly a substrate thickness at each edge and the resonance falls accordingly. Standard formulas add this length extension explicitly. Dielectric loss reduces the quality factor rather than the frequency, the ground plane is part of the resonator rather than an added inductance, and feed loading is a smaller secondary effect. Any resonator whose field escapes at its ends behaves as though it were longer than it is.

Answer: A

Part of the field travels in the substrate and part in the air above, so the wave sees a weighted average lying between one and the substrate value. A thin substrate of high permittivity confines more of the field and pushes the effective value towards the material figure. It can never exceed the substrate permittivity or fall to one for a real structure. This mixed medium behaviour is what makes microstrip formulas empirical rather than exact.

Answer: B

The free space wavelength at 3 GHz is 100 mm, and the substrate shortens it by the square root of the permittivity:

\[ L = \tfrac{100}{2\sqrt{4}} = 25 \]

millimetres, that is about 25 mm before the small fringing correction is subtracted. The square root dependence means a permittivity of nine halves the size again, which is why high permittivity substrates are chosen when compactness matters more than bandwidth or efficiency.

Answer: C

Wavelength in a dielectric is the free space value divided by the square root of the permittivity, so a patch resonating at half a guided wavelength shrinks in the same proportion. The penalty is that more of the field is bound in the substrate rather than fringing out, so radiation efficiency and bandwidth both fall and surface wave losses rise. Choosing the substrate is therefore the first and most consequential decision in a patch design, and it trades size directly against performance.

Answer: D

A thin substrate patch gives only one to three per cent for a two to one standing wave ratio, because it is a high quality factor resonator with a small radiating volume. Thicker and lower permittivity substrates, stacked patches and parasitic elements all widen it, at the cost of size, surface waves or complexity. Narrow bandwidth is the patch's defining weakness, and almost every refinement of the basic design is an attempt to do something about it.

Answer: A

A thicker, lower permittivity substrate lets more field fringe into the air, which raises the radiated energy relative to the stored energy and so lowers the quality factor. The costs are a larger antenna, greater surface wave excitation and a probe feed inductance that must be compensated. Thin high permittivity substrates do the opposite, patch length sets the frequency, and ground plane size affects the back radiation rather than the bandwidth.

Answer: B

Some of the energy is trapped in the dielectric slab and travels sideways until it diffracts at the board edge, radiating in unwanted directions and coupling into neighbouring elements. Thick, high permittivity substrates excite it most strongly, which is one of the limits on making a patch broadband by thickening the board. Patch currents, feed standing waves and edge reflections are separate phenomena. Surface waves are the hidden loss mechanism in printed antenna design.