microstrip antenna report 1 (2)

7/28/2019 Microstrip Antenna Report 1 (2)

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MGM COET

2013

MICROSTRIP ANTENNA

Submitted by:-

Ashish kumar pandit

M G M C O E T

N O I D A - 6 2


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3.rectangular patch

The most commonly employed microstrip antenna is a rectangular patch. The rectangular

patch antenna is approximately a one-half wavelength long section of rectangular microstrip

transmission line. When air is the antenna substrate, the length of the rectangular microstrip

antenna is approximately one-half of a free-space wavelength. As the antenna is loaded with

a dielectric as its substrate, the length of the antenna decreases as the relative dielectric

constant of the substrate increases. The resonant length of the antenna is slightly shorter

because of the extended electric "fringing fields" which increase the electrical length of the

antenna slightly. An early model of the microstrip antenna is a section of microstrip

transmission line with equivalent loads on either end to represent the radiation loss.

4.specifications

The dielectric loading of a microstrip antenna affects both its radiation pattern and impedance

bandwidth. As the dielectric constant of the substrate increases, the antenna bandwidth

decreases which increases the Q factor of the antenna and therefore decreases the impedance

bandwidth. This relationship did not immediately follow when using the transmission line

model of the antenna, but is apparent when using the cavity model which was introduced in

the late 1970s by Lo et al. The radiation from a rectangular microstrip antenna may be

understood as a pair of equivalent slots. These slots act as an array and have the highest

directivity when the antenna has an air dielectric and decreases as the antenna is loaded by

material with increasing relative dielectric constant.

The half-wave rectangular microstrip antenna has a virtual shorting plane along its center.

This may be replaced with a physical shorting plane to create a quarter-wavelength microstrip

antenna. This is sometimes called a half-patch. The antenna only has a single radiation edge

(equivalent slot) which lowers the directivity/gain of the antenna. The impedance bandwidth

is slightly lower than a half-wavelength full patch as the coupling between radiating edges

has been eliminated.

5.Other types

Another type of patch antenna is the Planar Inverted F Antenna (PIFA) common in cellular

phones with built-in antennas.(The Planar Inverted-F antenna (PIFA) is increasingly used in

the mobile phone market. The antenna is resonant at a quarter-wavelength (thus reducing the

required space needed on the phone), and also typically has good SAR properties. This

antenna resembles an inverted F, which explains the PIFA name. The Planar Inverted-F

Antenna is popular because it has a low profile and an omnidirectional pattern. The PIFA is

shown from a side view in Figure 4.) These antennas are derived from a quarter-wave half-patch antenna. The shorting plane of the half-patch is reduced in length which decreases the


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resonance frequency.Often PIFA antennas have multiple branches to resonate at the various

cellular bands. On some phones, grounded parasitic elements are used to enhance the

radiation bandwidth characteristics.

The Folded Inverted Conformal Antenna (FICA)has some advantages with respect to the

PIFA, because it allows a better volume reuse.

6.rectangular microstrip antenna properties

Rectangular Microstrip Antenna

Introduction to Patch Antennas

Microstrip or patch antennas are becoming increasingly useful because they can be printed

directly onto a circuit board. Microstrip antennas are becoming very widespread within themobile phone market. Patch antennas are low cost, have a low profile and are easily

fabricated.

Consider the microstrip antenna shown in Figure 1, fed by a microstrip transmission line. The

patch antenna, microstrip transmission line and ground plane are made of high conductivity

metal (typically copper). The patch is of length L, width W, and sitting on top of a substrate

(some dielectric circuit board) of thickness h with permittivity . The thickness of the

ground plane or of the microstrip is not critically important. Typically the height h is much

smaller than the wavelength of operation, but not much smaller than 0.05 of a wavelength.

(a) Top View of Patch Antenna
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In the above, k is the free-spacewavenumber, given by . The magnitude of the

fields, given by:

The fields of the microstrip antenna are plotted in Figure 2 forW=L=0.5 .

Figure 2. Normalized Radiation Pattern for Microstrip (Patch) Antenna.

The directivity of patch antennas is approximately 5-7 dB. The fields are linearly polarized,

and in the horizontal direction when viewing the microstrip antenna as in Figure 1a (we'll see

why in the next section). Next we'll consider more aspects involved in Patch (Microstrip)

antennas.

Fringing Fields for Microstrip Antennas
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Consider a square patch antenna fed at the end as before in Figure 1a. Assume the substrate is

air (or styrofoam, with a permittivity equal to 1), and that L=W=1.5 meters, so that the patch

is to resonate at 100 MHz. The height h is taken to be 3 cm. Note that microstrips are usually

made for higher frequencies, so that they are much smaller in practice. When matched to a

200 Ohm load, the magnitude ofS11 is shown in Figure 3.

Figure 3. Magnitude of S11 versus Frequency for Square Patch Antenna.

Some noteworthy observations are apparent from Figure 3. First, the bandwidth of the patch

antenna is very small. Rectangular patch antennas are notoriously narrowband; the bandwidth

of rectangular microstrip antennas are typically 3%. Secondly, the microstrip antenna was

designed to operate at 100 MHz, but it is resonant at approximately 96 MHz. This shift is due

to fringing fields around the antenna, which makes the patch seem longer. Hence, when

designing a patch antenna it is typically trimmed by 2-4% to achieve resonance at the desired

frequency.

The fringing fields around the antenna can help explain why the microstrip antenna radiates.Consider the side view of a patch antenna, shown in Figure 4. Note that since the current at

the end of the patch is zero (open circuit end), the current is maximum at the center of the

half-wave patch and (theoretically) zero at the beginning of the patch. This low current value

at the feed explains in part why the impedance is high when fed at the end (we'll address this

again later).

Since the patch antenna can be viewed as an open circuited transmission line, the voltage

reflection coefficient will be -1 (see thetransmission line tutorialfor more information).

When this occurs, the voltage and current are out of phase. Hence, at the end of the patch the

voltage is at a maximum (say +V volts). At the start of the patch antenna (a half-wavelength

away), the voltage must be at minimum (-V Volts). Hence, the fields underneath the patch
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will resemble that of Figure 4, which roughly displays the fringing of the fields around the

edges.

Figure 4. Side view of patch antenna with E-fields shown underneath.

It is the fringing fields that are responsible for the radiation. Note that the fringing fields near

the surface of the patch antenna are both in the +y direction. Hence, the fringing E-fields on

the edge of the microstrip antenna add up in phase and produce the radiation of themicrostrip antenna. This paragraph is critical to understanding the patch antenna. The

current adds up in phase on the patch antenna as well; however, an equal current but with

opposite direction is on the ground plane, which cancels the radiation. This also explains why

the microstrip antenna radiates but the microstrip transmission line does not. The microstrip

antenna's radiation arises from the fringing fields, which are due to the advantageous voltage

distribution; hence the radiation arises due to the voltage and not the current. The patch

antenna is therefore a "voltage radiator", as opposed to thewire antennas, which radiate

because the currents add up in phase and are therefore "current radiators".

As a side note, the smaller is, the more "bowed" the fringing fields become; they extend

farther away from the patch. Therefore, using a smaller permittivity for the substrate yields

better radiation. In contrast, when making a microstrip transmission line (where no power is

to be radiated), a high value of is desired, so that the fields are more tightly contained

(less fringing), resulting in less radiation. This is one of the trade-offs in patch antenna

design. There have been research papers written were distinct dielectrics (differentpermittivities) are used under the patch antenna and transmission line sections, to circumvent

this issu

Inset Feed

Previously, the patch antenna was fed at the end as shownhere. Since this typically yields a

high input impedance, we would like to modify the feed. Since the current is low at the ends

of a half-wave patch and increases in magnitude toward the center, the input impedance

(Z=V/I) could be reduced if the patch was fed closer to the center. One method of doing this

is by using an inset feed (a distance R from the end) as shown in Figure 1.
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Figure 1. Patch Antenna with an Inset Feed.

Since the current has a sinusoidal distribution, moving in a distance R from the end will

increase the current by cos(pi*R/L) - this is just noting that the wavelength is 2*L, and so the

phase difference is 2*pi*R/(2*L) = pi*R/L.

The voltage also decreases in magnitude by the same amount that the current increases.

Hence, using Z=V/I, the input impedance scales as:

In the above equation, Zin(0) is the input impedance if the patch was fed at the end. Hence,

by feeding the patch antenna as shown, the input impedance can be decreased. As an

example, if R=L/4, then cos(pi*R/L) = cos(pi/4), so that [cos(pi/4)]^2 = 1/2. Hence, a (1/8)-

wavelength inset would decrease the input impedance by 50%. This method can be used totune the input impedance to the desired value.

Fed with a Quarter-Wavelength Transmission Line

The microstrip antenna can also be matched to a transmission line of characteristic

impedance Z0 by using a quarter-wavelength transmission line of characteristic impedance

Z1 as shown in Figure 2.


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Figure 2. Patch antenna with a quarter-wavelength matching section.

The goal is to match the input impedance (Zin) to the transmission line (Z0). If the

impedance of the antenna is ZA, then the input impedance viewed from the beginning of the

quarter-wavelength line becomes

This input impedance Zin can be altered by selection of the Z1, so that Zin=Z0 and the

antenna is impedance matched. The parameter Z1 can be altered by changing the width of the

quarter-wavelength strip. The wider the strip is, the lower the characteristic impedance (Z0)

is for that section of line.

Coaxial Cable or Probe Feed

Microstrip antennas can also be fed from underneath via a probe as shown in Figure 3. The

outer conductor of the coaxial cable is connected to the ground plane, and the center

conductor is extended up to the patch antenna.


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Figure 3. Coaxial cable feed of patch antenna.

The position of the feed can be altered as before (in the same way as the inset feed, above) to

control the input impedance.

The coaxial feed introduces an inductance into the feed that may need to be taken into

account if the height h gets large (an appreciable fraction of a wavelength). In addition, the

probe will also radiate, which can lead to radiation in undesirable directions.

Coupled (Indirect) Feeds

The feeds above can be altered such that they do not directly touch the antenna. For instance,

the probe feed in Figure 3 can be trimmed such that it does not extend all the way up to the

antenna. The inset feed can also be stopped just before the patch antenna, as shown in Figure4.


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Figure 4. Coupled (indirect) inset feed.

The advantage of the coupled feed is that it adds an extra degree of freedom to the design.

The gap introduces a capacitance into the feed that can cancel out the inductance added by

the probe feed.

Aperture Feeds

Another method of feeding microstrip antennas is the aperture feed. In this technique, the

feed circuitry (transmission line) is shielded from the antenna by a conducting plane with a

hole (aperture) to transmit energy to the antenna, as shown in Figure 5.

Figure 5. Aperture coupled feed.

The upper substrate can be made with a lower permittivity to produce loosely bound fringing

fields, yielding better radiation. The lower substrate can be independently made with a high

value of permittivity for tightly coupled fields that don't produce spurious radiation. The

disadvantage of this method is increased difficulty in fabrication.

All of the parameters in a rectangular patch antenna design (L, W, h, permittivity)

control the properties of the antenna. As such, this page gives a general idea of

how the parameters affect performance, in order to understand the design

process.

First, the length of the patch L controls the resonant frequency as seenhere. This istrue in general, even for more complicated microstrip antennas that weave around -

the length of the longest path on the microstrip controls the lowest frequency of

operation. Equation (1) below gives the relationship between the resonant

frequency and the patch length:

(1)
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Second, the width Wcontrols the input impedance and the radiation pattern (see

the radiation equationshere). The wider the patch becomes the lower the input

impedance is.

The permittivity of the substrate controls the fringing fields - lowerpermittivities have wider fringes and therefore better radiation. Decreasing thepermittivity also increases the antenna'sbandwidth. The efficiency is alsoincreased with a lower value for the permittivity. The impedance of the antenna

increases with higher permittivities.

Higher values of permittivity allow a "shrinking" of the patch antenna. Particularly

in cell phones, the designers are given very little space and want the antenna to bea half-wavelength long. One technique is to use a substrate with a very high

permittivity. Equation (1) above can be solved forL

to illustrate this:

Hence, if the permittivity is increased by a factor of 4, the length required

decreases by a factor of 2. Using higher values for permittivity is frequentlyexploited in antenna miniaturization.

The height of the substrate h also controls the bandwidth - increasing the heightincreases the bandwidth. The fact that increasing the height of a patch antennaincreases its bandwidth can be understood by recalling the general rule that "an

antenna occupying more space in a spherical volume will have a wider bandwidth".

This is the same principle that applies when noting that increasing the thickness ofa dipole antenna increases its bandwidth. Increasing the height also increases theefficiency of the antenna. Increasing the height does induce surface waves that

travel within the substrate (which is undesired radiation and may couple to othercomponents).

The following equation roughly describes how the bandwidth scales with these

parameters:
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microstrip antenna report 1 (2)

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