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Patch Antenna analysis

using Ansoft Designer

By: Pejman TaslimiShahed University ofTehran IR IRAN

Represented to: Dr. MoghadasiTehran - August 2005

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2. Design procedure2.1. General dimensions: W and L

At the start of design procedure, we need to decide about some parameters, mayproblem or practical and commercial factors affect. So I assume that the specified

information includes dielectric constant of substrate (relative permittivity), the resonant

frequency, and the height of the substrate. In this simulation, these parameters arecmhGHzfrr 1588.0102.2 === respectively.

For an efficient radiator, a practical width that leads to good radiation efficiencies

is:1

2

21

2

2

1 0

00+

=+

=rrrr

f

v

fW

where v0 is free-space velocity of light. Here

we have W = 1.186 cm.

Then I need to determine the effective dielectric constant of the microstrip

antenna using: 972.11212

1

2

12/1

=

+

+

+=

W

hrreff

Once W is found, I must calculate effective length to find physical length as is

described below.

cmLf

L

cmL

h

W

h

W

h

L

effr

eff

eff

906.022

1

081.0

8.0)258.0(

264.0)3.0(

412.0

00

=

=

+

++

=

2.2. Resonant input resistanceWith the inset feed-point moving from the edge towards the centre of the patch, re

resonant input impedance decreases monotonically and reaches zero at the centre. When

the value of the inset feed-point approaches the centre of the patch (y0 = L / 2), the

)/(cos 02

Ly function varies very rapidly; therefore the input resistance also changes

rapidly with the position of the feed point. To maintain very accurate values, a close

tolerance must be preserved.

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Now Im going to find the position of the feed-point where the input impedance is

50 ohms:

cmyyL

ohmsRGsiemensG

dLkJ

Wk

GGG

R

in

in

3126.0)(cos3508.22850

3508.228101683.600157.0

sin)sin(cos

cos2

sin

120

1

)(2

1

00

2

4

121

0

3

00

2

0

212

121

==

===

=

=

2.3. Feed line widthTwo parameters must be found to finish design procedure: W0 and W1. W1 must

be as small as the construction procedure let. Ive chosen it 0.2 millimetres.

W0 must be found to fit the characteristic needed. Here I use an optimisation

procedure to find it with best radiation pattern. As Ill show later, W0 must be 0.4 mm.

3. Results3.1. Drawing layout

While I am not sure about W0, I draw all parts of the patch leaving the width of

microstrip feed parametric. Then I can define an optimisation procedure to find best value

or I can set parameter sweeping to tune it manually. I select the latter case to find outeffect of changing line width. When there are numbers of factor interfering, response to

changing line width is quite nonlinear. The antenna in Layout Editor is shown.

This consists of four rectangle and one excitation port named my_p1.

Stackup:Name Material Thickness Lower Level Roughness

t1 Copper 10um 1.583mm 0mm

d1 My_d1 0.1588cm 0mm 0mmg1 Copper 0mm 0mm 0mm

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As you can see, Ive defined my own material for substrate, with its relativepermittivity 2.2.

The 3D layout

3.2. Simulation setupI choose fixed mesh to simulate this antenna. Its frequency must be kept as small

as possible, because for each change in parameter, the whole setup will be analysed and itmust be fast enough. I think 5.00 GHz is fast enough to let us change two parameters and

accurate enough to let us find desired variable. The mesh overlay for 5.00 GHz is like

this:

When I select desired variables, Ill go on with a more accurate mesh; itsfrequency equals 25.00 GHz. Its mesh overlay is also shown. As you can, it has more

complex configuration.

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3.3. OptimisationThe parameter W0 is changing between .2 mm to 1.8 mm. The total far field

criterion finds optimise value for W0. Figure below shows the procedure.

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3.4. First resultsWhen analysis finished, first results can be surface current distribution, near and

far field of the antenna. Following images shows these.

Surface current distribution

Total electric near field

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Total electric far field

You can also see animations for these results, where frequency sweep from 8 GHz

to 12 GHz is animated. Note that there are two different far field animations. The first

shows total electric far field simply, but next one is a normalised pattern to compare gainfor different frequencies.

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3.5. Final resultsNow it is time to view final results. Radiation pattern in different planes and input

impedance are most important results. More results can be obtained if needed. From

scattering parameters results we can see if the calculations in previous sections wereaccurate or not.

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References: Antenna theory: analysis and design Constantine A. Balanis The Basics of Patch Antennas - D. Orban and G.J.K. Moernaut