Microstrip Antennas Questions and Answers

Q: What are the consequences of strong surface wave excitation?

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.

Q: Where along a patch is the input impedance highest?

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.

Q: What is an inset feed on a patch antenna?

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.

Q: What is the advantage of a coaxial probe feed for a patch?

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.

Q: Why does a probe feed become inductive on a thick substrate?

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.

Q: What is an aperture coupled patch?

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.

Q: Why does aperture coupling allow the substrates to be chosen independently?

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.

Q: What is proximity coupling of a patch?

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.

Q: How is circular polarisation obtained from a single square patch?

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.

Q: What is the drawback of single feed circular polarisation on a patch?

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.

11.

What are the consequences of strong surface wave excitation?

A.An increase in the cross polar level with no other measurable effect
B.A rise in the impedance bandwidth accompanied by a loss of gain
C.Reduced efficiency, distorted patterns and higher mutual coupling
D.A shift in the resonant frequency of the patch towards a higher value

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.

Discuss in Forum

12.

Where along a patch is the input impedance highest?

A.At the exact centre of the patch where the current is at a maximum
B.At the side edges parallel to the direction of the current flow
C.At the point directly above the feed connector in the ground plane
D.At the radiating edge

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.

Discuss in Forum

13.

What is an inset feed on a patch antenna?

A.A notch cut into the patch so the line reaches a lower impedance point
B.A feed line passing beneath the substrate and coupling through a slot
C.A coaxial probe soldered to the patch through a hole in the substrate
D.A separate parasitic patch placed close to the driven one to feed it

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.

Discuss in Forum

14.

What is the advantage of a coaxial probe feed for a patch?

A.It removes the need for a ground plane beneath the patch entirely
B.The feed network is kept behind the ground plane and does not radiate
C.It gives a wider impedance bandwidth than any other feeding method
D.It allows the patch to be fed at its radiating edge without a transformer

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.

Discuss in Forum

15.

Why does a probe feed become inductive on a thick substrate?

A.The patch resonance shifts upward and leaves the feed off tune
B.The ground plane connection becomes resistive at microwave frequencies
C.The probe is a length of wire carrying current through the dielectric
D.The dielectric constant rises with thickness and adds reactance to the feed

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.

Discuss in Forum

16.

What is an aperture coupled patch?

A.A patch with an aperture cut in its centre to lower its resonant frequency
B.A patch fed by a waveguide aperture placed directly beneath the substrate
C.A pair of patches coupled through an aperture in a shared ground plane
D.A patch excited through a slot in the ground plane from a line below

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.

Discuss in Forum

17.

Why does aperture coupling allow the substrates to be chosen independently?

A.The ground plane separates the feed circuit from the radiating patch
B.The slot acts as a transformer that matches any two substrate materials
C.The coupling is magnetic and so does not depend on the dielectric at all
D.The patch and the line resonate at different frequencies by design

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.

Discuss in Forum

18.

What is proximity coupling of a patch?

A.A method of coupling energy from a waveguide into a printed structure
B.A feed line buried between two dielectric layers under the patch
C.Two patches placed side by side so that one excites the other by coupling
D.A feed that touches the patch only at a single point near its centre

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.

Discuss in Forum

19.

How is circular polarisation obtained from a single square patch?

A.By placing a dielectric slab of quarter wave thickness above the patch
B.By rotating the whole patch through forty five degrees on the substrate
C.By truncating two opposite corners to split the resonance into two modes
D.By feeding the patch at its exact centre so that both modes are excited

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.

Discuss in Forum

20.

What is the drawback of single feed circular polarisation on a patch?

A.The gain is three decibels lower than a linearly polarised patch gives
B.The patch must be made physically larger than a linear one at the same frequency
C.The feed point becomes extremely sensitive to manufacturing tolerance in position
D.Good axial ratio is obtained over only a very narrow band

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.

Discuss in Forum

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