Microstrip Antennas Questions and Answers

Q: Consider the following pairings for microstrip antennas. Thicker substrate - wider bandwidth Higher permittivity - smaller patch Higher permittivity - more surface wave loss Thicker substrate - lower probe inductance How many of the pairs given above are correctly matched?

The first three are correct. A thicker substrate makes the probe longer and so raises its inductance rather than lowering it, which is one of the practical obstacles to widening the bandwidth by thickening the board. The pattern of trades is worth holding as a whole: thickness buys bandwidth but costs surface waves and probe inductance, and permittivity buys size but costs bandwidth, efficiency and tolerance.

Q: Why is substrate permittivity tolerance important for patch designs?

The guided wavelength varies as the inverse square root of permittivity, so a few per cent of material variation shifts a narrowband patch by more than its bandwidth. Antenna grade laminates are therefore specified with tighter permittivity tolerance and cost more than ordinary board. Strength, power handling and etching are separate properties. When a resonator's bandwidth is small, every parameter that sets its frequency must be controlled to a comparable precision.

Q: What lies beneath the substrate of a microstrip patch antenna?

The ground plane completes the resonator, reflects the radiation forward and carries the return current. Without it the structure would be a printed dipole rather than a patch. An absorber would waste half the power, a second driven patch describes a different antenna, and an unbacked patch is not a patch at all. The three layers of metal, dielectric and metal are what define the microstrip family.

Q: Which shape of patch is most commonly used?

The rectangular patch is by far the commonest, because its two dimensions independently set the resonant frequency and the impedance and bandwidth, and its analysis is straightforward. Circular patches are the next most common. Triangular, annular and elliptical shapes all exist and have their uses, and a nearly elliptical patch can indeed give circular polarisation, but none of them is the standard starting point for a design.

Q: What is the patch of a microstrip antenna usually made of?

Patches are etched from the copper cladding of a laminate by the same photolithographic process used for any printed circuit, which is exactly why they are cheap and reproducible. Conductive inks are used for very low cost or flexible antennas at some loss in conductivity, and bonded metal is occasionally used for large elements. Manufacturability by an existing process is the principal commercial advantage of the whole technology.

Q: What is the chief mechanical advantage of a printed antenna?

Being thin, light and produced by ordinary board processes makes printed antennas cheap in volume and easy to integrate with the circuitry behind them. Temperature capability is generally worse than metal, field repair is not practical on an etched structure, and a connector or a transition is still needed somewhere. Cost and integration, rather than any electrical superiority, are why the technology displaced others in consumer and vehicle applications.

71.

Consider the following pairings for microstrip antennas.

Thicker substrate - wider bandwidth

Higher permittivity - smaller patch

Higher permittivity - more surface wave loss

Thicker substrate - lower probe inductance

How many of the pairs given above are correctly matched?

A.Only two
B.All four
C.Only three
D.Only one

Answer: C

The first three are correct. A thicker substrate makes the probe longer and so raises its inductance rather than lowering it, which is one of the practical obstacles to widening the bandwidth by thickening the board. The pattern of trades is worth holding as a whole: thickness buys bandwidth but costs surface waves and probe inductance, and permittivity buys size but costs bandwidth, efficiency and tolerance.

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72.

Three of the following widen the bandwidth of a patch antenna. Which is the odd one out?

Raising the substrate permittivity

Increasing the substrate thickness

Adding a stacked parasitic patch

Cutting a U shaped slot in the patch

A.Increasing the substrate thickness
B.Adding a stacked parasitic patch
C.Cutting a U shaped slot in the patch
D.Raising the substrate permittivity

Answer: D

Thickness, stacking and slot loading all widen the response by increasing the radiated energy relative to the stored energy or by adding a second resonance. Raising the permittivity does the opposite, binding more field in the substrate and narrowing the band while making the patch smaller. It is the one entry that trades bandwidth away rather than buying it, and recognising which direction each design variable pushes is most of what patch design consists of.

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73.

Assertion (A): A patch antenna on a thin high permittivity substrate has a very narrow bandwidth.

Reason (R): Most of the field is confined in the dielectric, so stored energy is large compared with radiated energy.

Select the correct answer from the options below.

A.Both A and R are true, and R is the correct explanation of A
B.Both A and R are true, but R is not the correct explanation of A
C.A is true but R is false
D.A is false but R is true

Answer: A

Both are true and the second is the reason. Bandwidth is inversely proportional to the quality factor, which is the ratio of stored to radiated energy, so confining the field in a thin dense substrate raises that ratio and narrows the band. Every technique for widening a patch's bandwidth works by attacking that ratio, whether by letting more field escape or by adding a second coupled resonance. Seeing bandwidth as a statement about energy makes the whole family of remedies obvious.

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74.

Statement I: A patch antenna radiates chiefly from the fringing fields at its edges.

Statement II: A patch antenna radiates chiefly from the current at the centre of the patch.

In the light of the above statements, choose the correct answer.

A.Statement I is incorrect but Statement II is correct
B.Statement I is correct but Statement II is incorrect
C.Both Statement I and Statement II are correct
D.Both Statement I and Statement II are incorrect

Answer: B

The first is correct and the second is exactly wrong. The centre is a current maximum but a voltage null, and it is the fringing field at the voltage maxima on the two open ends that escapes and radiates. Modelling those two regions as slots half a wavelength apart predicts the broadside pattern correctly. Confusing where the current is largest with where the radiation comes from is a common error and it leads to quite wrong conclusions about feed placement.

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75.

A patch array is built to a proven design but resonates three per cent low in frequency.

What is the most likely cause?

A.The ground plane is smaller than the design intended it to be
B.The elements have been spaced slightly too far apart on the board
C.The substrate permittivity differs from the assumed value
D.The feed network has been printed with the wrong line impedances

Answer: C

Resonant frequency depends on the patch length and the effective permittivity, so a substrate whose permittivity is a few per cent above the datasheet value, or a batch variation, shifts the resonance by about half that in frequency. Etching tolerance is the other usual suspect. Feed impedance affects the match, ground plane size affects back radiation, and element spacing affects the pattern, none of which moves the resonance. Matching the symptom to the parameter it depends on narrows the search immediately.

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76.

Why is substrate permittivity tolerance important for patch designs?

A.It determines the mechanical strength of the finished antenna board
B.It sets the maximum power the antenna is able to handle before breakdown
C.It controls the thickness to which the copper can be etched reliably
D.The resonant frequency depends directly on it

Answer: D

The guided wavelength varies as the inverse square root of permittivity, so a few per cent of material variation shifts a narrowband patch by more than its bandwidth. Antenna grade laminates are therefore specified with tighter permittivity tolerance and cost more than ordinary board. Strength, power handling and etching are separate properties. When a resonator's bandwidth is small, every parameter that sets its frequency must be controlled to a comparable precision.

Discuss in Forum

77.

What lies beneath the substrate of a microstrip patch antenna?

A.A continuous conducting ground plane
B.A second patch of the same size fed in antiphase with the first one
C.An absorbing layer that prevents radiation into the lower half space
D.Nothing at all, since the patch radiates equally on both of its faces

Answer: A

The ground plane completes the resonator, reflects the radiation forward and carries the return current. Without it the structure would be a printed dipole rather than a patch. An absorber would waste half the power, a second driven patch describes a different antenna, and an unbacked patch is not a patch at all. The three layers of metal, dielectric and metal are what define the microstrip family.

Discuss in Forum

78.

Which shape of patch is most commonly used?

A.Elliptical, because it gives circular polarisation without any feeding tricks
B.Rectangular
C.Triangular, because it packs most efficiently into an array lattice
D.Annular

Answer: B

The rectangular patch is by far the commonest, because its two dimensions independently set the resonant frequency and the impedance and bandwidth, and its analysis is straightforward. Circular patches are the next most common. Triangular, annular and elliptical shapes all exist and have their uses, and a nearly elliptical patch can indeed give circular polarisation, but none of them is the standard starting point for a design.

Discuss in Forum

79.

What is the patch of a microstrip antenna usually made of?

A.Conductive paint applied to the substrate through a printed stencil
B.Silver plated steel, chosen for its strength at microwave frequencies
C.Copper etched from the cladding of the board
D.Aluminium sheet cut to size and bonded onto the substrate afterwards

Answer: C

Patches are etched from the copper cladding of a laminate by the same photolithographic process used for any printed circuit, which is exactly why they are cheap and reproducible. Conductive inks are used for very low cost or flexible antennas at some loss in conductivity, and bonded metal is occasionally used for large elements. Manufacturability by an existing process is the principal commercial advantage of the whole technology.

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80.

What is the chief mechanical advantage of a printed antenna?

A.It withstands higher temperatures than a machined metal antenna does
B.It can be repaired in the field by replacing individual components
C.It requires no connector because the feed is printed onto the board
D.It is flat, light and made by an existing manufacturing process

Answer: D

Being thin, light and produced by ordinary board processes makes printed antennas cheap in volume and easy to integrate with the circuitry behind them. Temperature capability is generally worse than metal, field repair is not practical on an etched structure, and a connector or a transition is still needed somewhere. Cost and integration, rather than any electrical superiority, are why the technology displaced others in consumer and vehicle applications.

Discuss in Forum

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