Microstrip Antennas Questions and Answers

Q: What is a microstrip patch antenna?

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.

Q: What is the usual resonant length of a rectangular patch?

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.

Q: Which parts of a rectangular patch actually radiate?

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.

Q: Why is a patch's resonant frequency lower than a simple half wavelength calculation predicts?

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.

Q: Why does a high permittivity substrate make a patch smaller?

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.

Q: What is the typical impedance bandwidth of a simple patch antenna?

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.

Q: How can the bandwidth of a patch antenna be increased?

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.

Q: What is a surface wave in a microstrip antenna?

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.

What is a microstrip patch antenna?

A.A metal patch printed over a grounded dielectric substrate
B.A slot cut in the ground plane of a printed circuit board and fed by a line
C.A wire antenna printed on a flexible substrate and folded into a small volume
D.A resonant cavity machined from metal and covered with a dielectric window

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.

Discuss in Forum

What is the usual resonant length of a rectangular patch?

A.About one tenth of a free space wavelength, which makes it very compact
B.About half a wavelength in the substrate
C.About one full wavelength measured in free space at the operating frequency
D.About a quarter of a wavelength in the substrate beneath the patch

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.

Discuss in Forum

Which parts of a rectangular patch actually radiate?

A.The two side edges running parallel to the direction of current flow
B.The ground plane beneath the patch rather than the patch itself
C.The fringing fields at the two open ends
D.The centre of the patch where the current is at its maximum value

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.

Discuss in Forum

Why is a patch's resonant frequency lower than a simple half wavelength calculation predicts?

A.The dielectric substrate absorbs energy and slows the resonance down
B.The ground plane adds inductance that lowers the resonant frequency
C.The feed line loads the patch and pulls its resonance below the design value
D.Fringing makes the patch appear electrically longer than it is

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.

Discuss in Forum

What is the effective dielectric constant of a microstrip patch?

A.A value between one and the substrate permittivity
B.Exactly the permittivity of the substrate material used beneath the patch
C.Always equal to one, since most of the field is in the air above the patch
D.A value larger than the substrate permittivity because of the ground plane

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.

Discuss in Forum

A patch on a substrate of effective permittivity 4 resonates at half a wavelength. How long is it at 3 GHz?

A.About 12.5 mm, once the fringing extension is subtracted
B.About 25 mm
C.About 50 mm
D.About 100 mm

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.

Discuss in Forum

Why does a high permittivity substrate make a patch smaller?

A.The field is pushed further out of the substrate and into the air above it
B.The ground plane becomes electrically closer to the patch at high permittivity
C.The wavelength in the substrate falls as the square root of permittivity
D.The higher permittivity increases the capacitance and lowers the current needed

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.

Discuss in Forum

What is the typical impedance bandwidth of a simple patch antenna?

A.About thirty per cent, comparable with a broadband horn antenna
B.Less than a tenth of one per cent, similar to a high quality crystal filter
C.About one full octave
D.A few per cent

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.

Discuss in Forum

How can the bandwidth of a patch antenna be increased?

A.By using a thicker substrate of lower permittivity
B.By using a thinner substrate of higher permittivity to confine the field
C.By making the patch physically longer than its resonant length
D.By increasing the size of the ground plane beneath the substrate

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.

Discuss in Forum

10.

What is a surface wave in a microstrip antenna?

A.A wave reflected from the edge of the ground plane back towards the patch
B.A wave trapped in the substrate travelling parallel to the ground plane
C.A current flowing across the upper surface of the patch conductor itself
D.A standing wave formed between the patch and its feeding transmission line

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.

Discuss in Forum

Loading…

Topics

Other Subjects

Tags: Microstrip Antennas Questions and Answers || Microstrip Antennas MCQ Questions and Answers || Microstrip Antennas GK Questions and Answers || Microstrip Antennas GK MCQ Questions || Microstrip Antennas Multiple Choice Questions and Answers || Microstrip Antennas GK || GK on Microstrip Antennas || Antenna and Wave Propagation Questions and Answers || Antenna and Wave Propagation MCQ Questions and Answers || Antenna and Wave Propagation GK Questions and Answers || GK on Antenna and Wave Propagation

Microstrip Antennas Questions and Answers

Topics

Other Subjects

About Us

Stay informed about all important Dates & Notifications