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MICROSTRIP ANTENNA

Outline

Overview of microstrip antennas Feeding methods Radiation Mechanism Radiation from Patch Antenna Improving bandwidth

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Overview of Microstrip AntennasAlso called “patch antennas”

One of the most useful antennas at microwave frequencies (f > 1 GHz). It usually consists of a metal “patch” on top of a grounded dielectric substrate. The patch may be in a variety of shapes, but rectangular and circular are the

most common.

3Microstrip line feed Coax feed

Overview of Microstrip Antennas

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Common Shapes

Rectangular Square Circular

Elliptical

Annular ring

Triangular

Most popular in wireless communication network.

Invented by Bob Munson in 1974 (but earlier work by G.A. Dechamps goes back to1953).

Became popular starting in the 1970s.

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Overview of Microstrip Antennas

History

Advantages of Microstrip Antennas

Low profile (can even be “conformal,” i.e. flexible to conform to a surface).

Easy to fabricate (use etching and photolithography). Easy to feed (coaxial cable, microstrip line, etc.). Easy to use in an array or incorporate with other microstrip circuit

elements. Patterns are somewhat hemispherical, with a moderate directivity

(about 6-8 dB is typical).

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Overview of Microstrip Antennas

Disadvantages of Microstrip Antennas

Low bandwidth (but can be improved by a variety of techniques). Bandwidths of a few percent are typical. Bandwidth is roughly proportional to the substrate thickness and inversely proportional to the substrate permittivity.

Efficiency may be lower than with other antennas. Efficiency is limited by conductor and dielectric losses*, and by surface-wave loss**.

Only used at microwave frequencies and above (the substrate becomes too large at lower frequencies).

Cannot handle extremely large amounts of power (dielectric breakdown).

* Conductor and dielectric losses become more severe for thinner substrates.

** Surface-wave losses become more severe for thicker substrates (unless air or foam is used).

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Overview of Microstrip Antennas

Applications

Satellite communications

Microwave communications

Cell phone antennas

GPS antennas

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Applications include:

Overview of Microstrip Antennas

Microstrip Antenna Integrated into a System: HIC Antenna Base-Station for 28-43 GHz

Filter

Diplexer

LNAPD

K-connectorDC supply Micro-D

connector

Microstrip antenna

Fiber input with collimating lens

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Overview of Microstrip Antennas

Overview of Microstrip Antennas

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Arrays

Linear array (1-D corporate feed)

22 array

2-D 8X8 corporate-fed array 4 8 corporate-fed / series-fed array

Wraparound Array (conformal)

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Overview of Microstrip Antennas

The substrate is so thin that it can be bent to “conform” to the surface.

x

y

hL

W

Note: L is the resonant dimension (direction of current flow). The width W is usually chosen to be larger than L (to get higher bandwidth).

However, usually W < 2L (to avoid problems with the (0,2) mode).

r

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Overview of Microstrip AntennasRectangular patch

W = 1.5L is typical.

Js

Circular Patch

x

y

h

a

r

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Overview of Microstrip Antennas

The location of the feed determines the direction of current flow and hence the polarization of the radiated field.

Feeding Methods

Some of the more common methods for feeding microstrip antennas are shown.

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The feeding methods are illustrated for a rectangular patch, but the principles apply for circular and other shapes as well.

Coaxial Feed

A feed along the centerline is the most common

(minimizes higher-order modes and cross-pol).

x

y

L

W

Feed at (x0, y0)

Surface current

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xr h

z

Feeding Methods

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Advantages: Simple Directly compatible with coaxial cables Easy to obtain input match by adjusting feed position

Disadvantages: Significant probe (feed) radiation for thicker substrates Significant probe inductance for thicker substrates (limits

bandwidth) Not easily compatible with arrays

Coaxial Feed

xr h

z

Feeding Methods

x

y

L

W 0 0,x y

Advantages: Simple Allows for planar feeding Easy to use with arrays Easy to obtain input match

Disadvantages: Significant line radiation for thicker substrates For deep notches, patch current and radiation pattern may show distortion

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Inset Feed

Microstrip line

Feeding Methods

Advantages: Allows for planar feeding Less line radiation compared to microstrip feed Can allow for higher bandwidth (no probe inductance, so

substrate can be thicker)

Disadvantages: Requires multilayer fabrication Alignment is important for input match

Patch

Microstrip line

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Feeding MethodsProximity-coupled Feed

(Electromagnetically-coupled Feed)

Top view Microstrip line

Advantages:

Allows for planar feeding

Feed-line radiation is isolated from patch radiation

Higher bandwidth is possible since probe inductance is eliminated (allowing for a thick substrate), and also a double-resonance can be created

Allows for use of different substrates to optimize antenna and feed-circuit performance

Disadvantages: Requires multilayer fabrication Alignment is important for input match

Patch

Microstrip line

Slot

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Feeding MethodsAperture-coupled Patch (ACP)

Top view

Slot

Microstrip line

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Mechanism of RadiationLet us consider a Rectangular patch as in the figure:-

Assumptions:-

As h << lambda, field variation along the substrate thickness is neglected.

Also if it is assumed that there is no variation of electric field along the width of the patch then the electric field of the patch can be expressed as :

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Thus in essence the electric field of the patch vary along the length of the patch which is approximately lambda/2.

Radiation occurs from the fringing fields existing at the open circuited end or edges of the patch.

The fringing field at the two open circuited ends of the patch can be resolved into two components :

One normal to the ground plane andOther parallel to the ground plane. From the figure above it can be seen that the electric fields are out

of phase and hence radiation due to these components cancel out in far region.

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However in the Broadside direction, these electrical fields are in phase and hence add together in far region.Thus the maximum radiation is in the broadside direction.

Thus the rectangular patch can be represented by two slots which are lambda/2 apart. They are excited in phase and their radiations are normal to the plane of the patch or the ground plane.

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Radiation from Patch antenna

A rectangular patch(with coaxial feed) with length L and width W is shown in the figure:-

Substrate thickness is t and its permittivity is epsilon.

The Transverse Magnetic (TM) mode is characterized by H_z = 0 and E_z /= 0.

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If the boundaries at x=0;L and y=0,W be considered as open circuits then the tangential component of magnetic field will be zero at x=0;L and y=0,W.

Since h << lambda, there’ll be no field variation with z.Under these conditions the solution for electric field can be written as:

Therefore, the summation of various modes for all values of m and n will give rise to the general solution of the field.Now the equivalent magnetic current in the open circuited side walls (apertures) can be obtained as:

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As per the boundary condition H_t = 0 at the side walls thus the equivalent electric current will be zero i.e., n x H = 0. As already mentioned that the thickness of the substrate is very small and hence the total magnetic current h J_m could be considered as a line source just above the conducting ground plane.

Substituting m=1 and n=0 in equation we’ll get,

And therefore the line magnetic-current sources responsible for the radiation will be

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Now it is evidently clear that the magnetic current sources at y=0 and y=W are equal and opposite and their contribution in radiation field in far zone cancel out.Therefore, these sources produces no radiation in the z- direction.

However, the field radiated from the line source at x=0 and x=L are in phase and are add up in the broadside direction.

The magnitude vector potentials for these line sources can be, obtained by

Which can be used to evaluate the radiation field in the far zone as :

Improving Bandwidth

Some of the techniques that have been successfully developed are illustrated here.

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The literature may be consulted for additional designs and variations.

L-shaped probe:

Capacitive “top hat” on probe:

Top view

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Improving BandwidthProbe Compensation

As the substrate thickness increases the probe inductance limits the bandwidth – so we

compensate for it.

SSFIP: Strip Slot Foam Inverted Patch (a version of the ACP).

Microstrip substrate

Patch

Microstrip line Slot

Foam

Patch substrate

Bandwidths greater than 25% have been achieved. Increased bandwidth is due to the thick foam substrate and

also a dual-tuned resonance (patch+slot).

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Improving Bandwidth

Note: There is no probe inductance to worry about here.

Bandwidth increase is due to thick low-permittivity antenna substrates and a dual or triple-tuned resonance.

Bandwidths of 25% have been achieved using a probe feed. Bandwidths of 100% have been achieved using an ACP feed.

Microstrip substrate

Coupling patch

Microstrip lineSlot

Patch substrates

Top Patch

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Improving BandwidthStacked Patches

The introduction of a U-shaped slot can give a significant bandwidth (10%-40%).

(This is due to a double resonance effect, with two different modes.)

“Single Layer Single Patch Wideband Microstrip Antenna,” T. Huynh and K. F. Lee, Electronics Letters, Vol. 31, No. 16, pp. 1310-1312, 1986.

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Improving BandwidthU-Shaped Slot

A 44% bandwidth was achieved.

Y. X. Guo, K. M. Luk, and Y. L. Chow, “Double U-Slot Rectangular Patch Antenna,” Electronics Letters, Vol. 34, No. 19, pp. 1805-1806, 1998.

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Improving BandwidthDouble U-Slot

A modification of the U-slot patch.

A bandwidth of 34% was achieved (40% using a capacitive “washer” to compensate for the probe inductance).

B. L. Ooi and Q. Shen, “A Novel E-shaped Broadband Microstrip Patch Antenna,” Microwave and Optical Technology Letters, vol. 27, No. 5, pp. 348-352, 2000.

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Improving BandwidthE Patch

Multi-Band Antennas

General Principle:

Introduce multiple resonance paths into the antenna.

A multi-band antenna is sometimes more desirable than a broadband antenna, if multiple narrow-band channels are to be covered.

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Dual-band E patch

High-band

Low-band

Low-band

Feed

Dual-band patch with parasitic strip

Low-band

High-band

Feed

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Multi-Band Antennas

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