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Microstrip antennasMicrowave antennas(100MHz -

100GHZ)

Where Size, weight, cost,performance, ease of installation, and

aerodynamic profile are constraints.

Applications like :aircraft, WLAN,

spacecraft, satellite, and missile, cell

phone applications.Amanpreet Kaur, Assistant Prof. , ECED , TU

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Amanpreet Kaur, Assistant Prof. , ECED , TU

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Numerous substrates with dielectric constantsusually in the range of 2.2 or 12.

Thick substrates with lower dielectric constant-> provide better efficiency.

Larger bandwidth, loosely bound fields for

radiation into space, -> at the expense of largerelement size

Thin substrates with higher dielectric constantsare desirable for microwave circuitry -> they

require tightly bound fields to minimizeundesired radiation and coupling, and lead tosmaller element sizes.

But because of their greater losses, they are less

efficient and have relatively smaller bandwidths.Amanpreet Kaur, Assistant Prof. , ECED , TU

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The radiating elements and the feed lines are usuallyphoto etched on the dielectric substrate.

The radiating patch may be square, rectangular, thinstrip (dipole), circular, elliptical, triangular, or anyother configuration.

The microstrip patch is designed so its patternmaximum is normal to the patch (broadside radiator).

This is accomplished by properly choosing the mode(field configuration) of excitation beneath the patch

End-fire radiation can also be accomplished byjudicious mode selection.


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Advantages :

low profile, conformable to planar and non planarsurfaces, simple and inexpensive to manufacture

using modern printed-circuit technology, mechanically robust when mounted on rigid

surfaces.

With a particular patch shape and mode selected,they are very versatile in terms of resonantfrequency, polarization, pattern, and impedance.

Disadvantages:

low efficiency, poor polarization purity, poorscanperformance, spurious feed radiation andvery narrow frequency bandwidth, which is 1-5%around the center frequency.


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Feeding Methods

There are many configurations that can be

used to feed microstrip antennas. The four most popular are:

The microstrip line, coaxial probe, aperture

coupling, and proximity coupling


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Microstrip feed line The microstrip feed line is a conducting strip,

usually of much smaller width compared to thepatch.

The microstrip-line feed is easy to fabricate,

simple to match by controlling the insetposition and rather simple to model.

Disadvantage:

as the substrate thickness increases, surfacewaves and spurious feed radiation increase,which for practical designs limit the bandwidth(typically 25%).


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Coaxial-line feeds,:

Here the inner conductor of the coax is

attached to the radiation patch while the outer

conductor is connected to the ground plane.

The coaxial probe feed is also easy to fabricate

and match, and it has low spurious radiation.

However, it also has narrow bandwidth and it

is more difficult to model, especially for thick

substrates (h > 0.020).


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Both the microstrip feed line and the probepossess inherent asymmetries which generate

higher order modes which produce cross-polarized radiation.

Aperture coupled feed:

Easier to model and has moderate spuriousradiation, but is difficult to fabricate.

The aperture coupling consists of twosubstrates separated by a ground plane. On

the bottom side of the lower substrate there is

A microstrip feed line whose energy is coupledto the patch through a slot on the ground

plane separating the two substrates.Amanpreet Kaur, Assistant Prof. , ECED , TU

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This arrangement allows independentoptimization of the feed mechanism and the

radiating element.

Typically a high dielectric material is used for

the bottom substrate, and thick low dielectricconstant material for the top substrate.

The ground plane between the substrates alsoisolates the feed from the radiating elementand minimizes interference of spuriousradiation for pattern formation and

Polarization purity.Amanpreet Kaur, Assistant Prof. , ECED , TU

T i ll t hi i f d b t lli th

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Typically matching is performed by controlling thewidth of the feed line and the length of the slot.

The coupling through the slot can be modeled usingthe theory of Bethe .

In this theory the slot is represented by anequivalent normal electric dipole to account for thenormal component (to the slot) of the electric field,and an equivalent horizontal magnetic dipole to

account for the tangential component (to the slot)magnetic field.

If the slot is centered below the patch, whereideally for the dominant mode the electric field is

zero while the magnetic field is maximum themagnetic coupling will dominate.

Doing this also leads to good polarization purityand no cross-polarized radiation in the principal

planesAmanpreet Kaur, Assistant Prof. , ECED , TU

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Of the four feeds described here, the

PROXIMITY COUPLING has the largestbandwidth (as high as 13 percent), is

somewhat easy to model and has low spurious

radiation.

Has a planar geometry.

Here microstrip line is etched into the bottom

of lower substrate.

Advantage that the radiation from the feed line

is negligible.


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Lower most layer is ground substrate andabove that we have two substrates of differentdielectric constants.

Microstrip line on the lower layer and patchantenna on the upper layer.

Feed line terminates in an open end

underneath a patch. Also known as electromagnetically coupled

feed line.

Coupling is capacitive in nature. Lower layer is thin with high dielectric

constant.

Higher is thick with low dielectric constant.Amanpreet Kaur, Assistant Prof. , ECED , TU

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Parasitic ArrayYagi-Uda

Array antennas can be used to increase directivity.

Parasitic array does not require a direct connection to eachelement by a feed network.

The parasite elements acquire their excitation from near field

coupling by the driven element. A Yagi-Uda antenna is a linear array of parallel dipoles.

The basic Yagi unit consists of three elements:

1. Driver or driven element

2. Reflector 3. Director


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Yagi-Uda Antenna

Optimum spacing for gain of a reflector and driven

element is 0.15 to 0.25 wavelengths Director- Driven element spacing of 0.1 to 0.15

wavelengths

Director to director spacing are 0.2 to 0.35

wavelengths apart.

Reflector length is typically 0.05 wavelengths longeror a length 1.05 that of the driven element.

The driven element is calculated at resonancewithout the presence of parasitic elements. Drivenelement is a wave dipole.

The directors are usually 10 to 20% shorter than at

resonance. Amanpreet Kaur, Assistant Prof. , ECED , TU

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All the elements are clamped at centre because voltage is minimum at the centreAmanpreet Kaur, Assistant Prof. , ECED , TU

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Yagi-Uda

Metal booms can be implemented because voltage is at zero

midway through the element.

Other factors that effect resonant lengths:

1. A comparatively large boom will require parasitic

elements to increase their length.

2. Length to diameter ratio of the elements.


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Operation : Directors with length less than /2 capacitive

reactance

current leads the induced EMF.

Reflectors with length greater than /2 Inductivereactance current lags the induced EMF.

So directors act like elements of array carryingequal currents and currents in phase maintained by

proper spacing between elements

Reflectors add the radiation fields in the direction

away from reflector and towards the directorsAmanpreet Kaur, Assistant Prof. , ECED , TU

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Mathematics

Impact on input impedance

Total electric filed at a point P


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Isotropic antenna

Isotropic antenna or isotropicradiator is a hypothetical (notphysically realizable) concept,used as a useful reference to

describe real antennas. Isotropic antenna radiates

equally in all directions. Its radiation pattern is represented

by a sphere whose center

coincides with the location of theisotropic radiator.

Source: NK Nikolova

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In case we replace infinitesimal small dipoles withpractical half wave dipoles, it is called a Georgebrown Turnstile antenna

Methods of feeding:

Two dipoles connected to non resonant lines ofunequal length one greater then the other by

/4.

Introduce reactance in series with the dipoles

Directivity can be increased by using an array of

turnstile antennas mounted on a mast coincidentwith the axis of turnstile.

Mathematics: produce larger wave front as

compared to waveguidemore directivity.Amanpreet Kaur, Assistant Prof. , ECED , TU

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Horn Antennas


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For a moderate power gainhorn antennas

Disadvantages of using a waveguide for

microwave frequencies:

Reflections and diffraction around the edges

Waveguides mouth is opened to form a Horn

Antenna- improves directivity & reduces

diffraction

Types : Sectoral horn, Pyramidal Horn, conical

horn.

Mathematics: to provide larger wave fronts to

get a greater directivity.Amanpreet Kaur, Assistant Prof. , ECED , TU

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L A

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Loop Antenna:

Loop antennas take many different forms such

as a rectangle, square, triangle, ellipse, circle,and many other configurations. Because of the

simplicity in analysis and construction, the

circular loop is the most popular and hasreceived the widest attention.


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Loop antennas are usually classified into two

categories, electrically small and electrically large.

Electrically small antennas are those whose overalllength (circumference) is usually less than about

one-tenth of a wavelength (C < /10).

However, electrically large loops are those whose

circumference is about a free-space wavelength (C

).

Most of the applications of loop antennas are in the

HF (330 MHz), VHF (30300 MHz), and UHF(3003,000 MHz) bands.

When used as field probes, they find applications

even in the microwave frequency rangeAmanpreet Kaur, Assistant Prof. , ECED , TU

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They are also used as probes for fieldmeasurements and as directional antennas forradio wave navigation.

The field pattern of electrically small antennasof any shape is similar to that of an infinitesimaldipole with a null perpendicular to the plane of

the loop and with its maximum along the planeof the loop.

As the overall length of the loop increases andits circumference approaches one free-spacewavelength, the maximum of the pattern shiftsfrom the plane of the loop to the axis of theloop which is perpendicular to its plane.

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