Microstrip Antennas Questions and Answers

Q: Why is the back radiation of a patch antenna low?

A continuous conducting ground plane reflects the field and prevents radiation into the half space behind it, so front to back ratios of fifteen to twenty decibels are usual on a board of reasonable size. Diffraction at the board edges is what limits it. The substrate is chosen for low loss rather than absorption, the edge fields add rather than cancel behind, and the feed network is not a reflector. Ground plane size is therefore a real design parameter and not merely a mechanical convenience.

Q: What happens to a patch's front to back ratio on a small ground plane?

Waves reaching the edge of a small ground plane diffract around it and radiate backwards, so the front to back ratio falls and the pattern develops ripples. A ground plane extending a wavelength or so beyond the patch is a common rule. Surface wave energy is worse rather than better on a small board because the edge is nearer, the ground plane is electrically essential, and the pattern remains directional. Edge diffraction is the recurring penalty of finite structures.

Q: What is a stacked patch antenna?

Stacking places a second resonant patch above the fed one, and the two coupled resonances give a much wider impedance bandwidth or a second operating band. Bandwidths of fifteen to twenty per cent are achievable against two or three for a single patch. Side by side patches form an array, printing on both sides describes a different structure, and a second ground plane above would short the radiation out. Coupling two resonators to broaden a response is a standard filter technique applied here to an antenna.

Q: How does a stacked patch widen the bandwidth?

Two resonators tuned slightly apart and coupled give a double humped response that is flatter and wider than either alone, exactly as in a two pole filter. The spacing and the coupling set how wide and how flat. Shielding, permittivity and absorption are not the mechanism, and absorbing reflected power would be a loss rather than a gain in bandwidth. Recognising the antenna as a resonant circuit lets the whole vocabulary of filter design be brought to bear on its bandwidth.

Q: What is a slot loaded patch used for?

Cutting a U shaped or straight slot in the patch lengthens the current path for one mode and so introduces a second, lower resonance that can be placed at a wanted band. It is widely used in handset antennas covering several cellular bands. Gain follows the aperture, cross polar performance depends on the feed and symmetry, and surface waves live in the substrate rather than in the patch metal. Perturbing a resonator to add a mode is a compact alternative to adding a second resonator.

Q: What is a planar inverted F antenna?

Shorting one edge places a voltage null there, so the structure resonates as a quarter wave rather than a half wave and is about half the size of an equivalent patch. The shorting pin or wall and the feed pin close together give the F shaped side view that names it. It became the standard internal handset antenna for exactly this size reason. Halving a resonator by exploiting a symmetry plane is a technique that recurs throughout antenna design.

Q: Why is a planar inverted F antenna common in mobile handsets?

Its quarter wave resonance makes it small, it lies flat against the board, and slots and parasitic branches let one structure cover several cellular bands. Efficiency is modest rather than the best available, the pattern is broad so the user's hand and head do absorb appreciably, and the phone's ground plane is an essential part of the radiating structure. Fitting the available volume, rather than maximising any single figure of merit, is what drives handset antenna design.

21.

How does a dual feed produce circular polarisation on a patch?

A.Two orthogonal feeds are driven in quadrature by a hybrid coupler
B.Two feeds at the same edge are driven in phase with equal amplitudes
C.One feed drives the patch while the second absorbs the reflected wave
D.The two feeds are switched alternately at the modulation rate of the signal

Answer: A

Driving two orthogonal modes with equal amplitude and ninety degrees of phase difference, supplied by a branch line or ring hybrid, gives circular polarisation over a much wider band than any single feed technique. The hybrid also absorbs the reflected power in its isolated port, which flattens the input match. In phase feeding gives a diagonal linear polarisation, and the other options describe arrangements that would not produce a rotating field at all.

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22.

What is the typical gain of a single rectangular patch element?

A.Around 20 dBi, which is why patches are used for satellite reception
B.Around 6 to 8 dBi
C.Around 15 dBi
D.Around 2 dBi, the same as a half wave dipole in free space

Answer: B

A patch over a ground plane radiates into a hemisphere with a broad beam, giving 6 to 8 decibels over isotropic depending on the substrate and the ground plane size. Higher gain requires an array of patches, which is exactly how printed antennas reach the twenty five or thirty decibels a satellite terminal needs. A single element is modest, and recognising that fixes expectations before a design is attempted.

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23.

What is the radiation pattern of a patch antenna over a large ground plane?

A.An end fire beam directed along the plane of the substrate itself
B.An omnidirectional pattern in the plane containing the ground plane
C.A broad beam directed normal to the patch
D.A doughnut shaped pattern with a null perpendicular to the patch surface

Answer: C

The two radiating edges add in phase broadside, and the ground plane suppresses radiation backwards, so the pattern is a single broad lobe pointing away from the board. That makes patches natural for conformal mounting on a surface that should radiate outward. A doughnut belongs to a dipole, end fire to a travelling wave structure, and an omnidirectional pattern in the board plane to a monopole. The pattern follows directly from the two slot model.

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24.

Why is the back radiation of a patch antenna low?

A.The substrate absorbs any energy that travels backwards from the patch
B.The two radiating edges cancel exactly in the backward direction only
C.The feed network behind the board reflects the backward radiation forward
D.The ground plane screens the region behind the substrate

Answer: D

A continuous conducting ground plane reflects the field and prevents radiation into the half space behind it, so front to back ratios of fifteen to twenty decibels are usual on a board of reasonable size. Diffraction at the board edges is what limits it. The substrate is chosen for low loss rather than absorption, the edge fields add rather than cancel behind, and the feed network is not a reflector. Ground plane size is therefore a real design parameter and not merely a mechanical convenience.

Discuss in Forum

25.

What happens to a patch's front to back ratio on a small ground plane?

A.It worsens because of diffraction at the board edges
B.It improves because less surface wave energy is generated in the substrate
C.It stays the same, since the ground plane merely supports the patch mechanically
D.It becomes undefined because the pattern is no longer directional at all

Answer: A

Waves reaching the edge of a small ground plane diffract around it and radiate backwards, so the front to back ratio falls and the pattern develops ripples. A ground plane extending a wavelength or so beyond the patch is a common rule. Surface wave energy is worse rather than better on a small board because the edge is nearer, the ground plane is electrically essential, and the pattern remains directional. Edge diffraction is the recurring penalty of finite structures.

Discuss in Forum

26.

What is a stacked patch antenna?

A.A patch with a second ground plane above it to shape the radiation pattern
B.A parasitic patch mounted above the driven one on a second layer
C.Several patches printed side by side and fed from a common line
D.A patch printed on both sides of the same substrate for dual polarisation

Answer: B

Stacking places a second resonant patch above the fed one, and the two coupled resonances give a much wider impedance bandwidth or a second operating band. Bandwidths of fifteen to twenty per cent are achievable against two or three for a single patch. Side by side patches form an array, printing on both sides describes a different structure, and a second ground plane above would short the radiation out. Coupling two resonators to broaden a response is a standard filter technique applied here to an antenna.

Discuss in Forum

27.

How does a stacked patch widen the bandwidth?

A.The extra substrate raises the effective permittivity of the structure
B.The parasitic patch absorbs the reflected power at band edges
C.Two coupled resonances merge into a broader response
D.The upper patch shields the lower one from the surrounding air

Answer: C

Two resonators tuned slightly apart and coupled give a double humped response that is flatter and wider than either alone, exactly as in a two pole filter. The spacing and the coupling set how wide and how flat. Shielding, permittivity and absorption are not the mechanism, and absorbing reflected power would be a loss rather than a gain in bandwidth. Recognising the antenna as a resonant circuit lets the whole vocabulary of filter design be brought to bear on its bandwidth.

Discuss in Forum

28.

What is a slot loaded patch used for?

A.Increasing the gain of the patch without enlarging its physical area
B.Improving the cross polar performance of a linearly polarised patch
C.Reducing the surface wave loss by interrupting the substrate currents
D.Creating a second resonance for dual band operation

Answer: D

Cutting a U shaped or straight slot in the patch lengthens the current path for one mode and so introduces a second, lower resonance that can be placed at a wanted band. It is widely used in handset antennas covering several cellular bands. Gain follows the aperture, cross polar performance depends on the feed and symmetry, and surface waves live in the substrate rather than in the patch metal. Perturbing a resonator to add a mode is a compact alternative to adding a second resonator.

Discuss in Forum

29.

What is a planar inverted F antenna?

A.A quarter wave patch with one edge shorted to the ground plane
B.A patch bent into an F shape to fit inside a handset casing
C.A pair of patches arranged in an F pattern for dual band operation
D.An inverted patch mounted beneath the ground plane of a circuit board

Answer: A

Shorting one edge places a voltage null there, so the structure resonates as a quarter wave rather than a half wave and is about half the size of an equivalent patch. The shorting pin or wall and the feed pin close together give the F shaped side view that names it. It became the standard internal handset antenna for exactly this size reason. Halving a resonator by exploiting a symmetry plane is a technique that recurs throughout antenna design.

Discuss in Forum

30.

Why is a planar inverted F antenna common in mobile handsets?

A.It requires no ground plane, which saves space inside the casing
B.It is compact, low profile and easily tuned to several bands
C.It has the highest efficiency of any antenna that will fit in a handset
D.It radiates only away from the user and so avoids any absorption

Answer: B

Its quarter wave resonance makes it small, it lies flat against the board, and slots and parasitic branches let one structure cover several cellular bands. Efficiency is modest rather than the best available, the pattern is broad so the user's hand and head do absorb appreciably, and the phone's ground plane is an essential part of the radiating structure. Fitting the available volume, rather than maximising any single figure of merit, is what drives handset antenna design.

Discuss in Forum

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