Design of dual band patch antenna

7/29/2019 Design of dual band patch antenna

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N.W.F.P University of Engineering and Technology

Mardan Campus

Department of Telecommunication Engineering

Term Project

Report

Project Title

Design of dual band patch antenna


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Project By:

Haseeb Ahmed Khan

Email id: [email protected]

Project coordinator

Dr. SadiqullahAdvisor, ProfessorTelecommunication Engineering


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Starting and simulation stage:

First it needs to choose which feeding technique that will be used. A small size and wide

bandwidth are two properties with high priority. A microstrip line is not the best choicesince the microstrip line increase the size of the antenna and the bandwidth decrease if a

thick substrate is used. An aperture-coupled patch would not either be the best solutionsince it gives narrow bandwidth and difficult to fabricate.

Therefore a feeding technique that feed the antenna below the patch will be the bestalternative since it decrease the size (width or length) of the antenna. Example of these

feeding techniques is coaxial probe or coaxial probe with capacitive feed. The coaxialprobe gives narrow bandwidth and coaxial probe with capacitive feed gives widerbandwidth. Than the choice will be coaxial probe with capacitive feed. The disadvantage

with this feeding technique is the manufacturing process. It can be difficult to match theantenna.

Since the reader antenna that is used in tests is right handed circular polarized thedesigned antenna also need to be right handed circular polarized for low polarization loss

factor as possible. If the designed antenna is LHCP and reader antenna is RHCP thepolarization loss factor will be maximal, which means they can not communicate with

each other. To simplify the manufacturing process of the antenna truncated corner areused instead of the dual feed patch.

When feed and polarization technique have been chosen the design continuous with aalready existing large patch antenna for 868 MHz. It has a ground plane with the

dimension of w*L*H= 255*255*2.5 mm and with patch dimension

W*L*H= 153*153*2.5 mm. The used feed technique is coaxial probe with capacitive

feed with air substrate of height 13 mm. The patch has also truncated corner. Simulationsresult for the antenna shows that it has a bandwidth of 90 MHz (810-890 MHz) with RL

< -10 db. For the 2.45 GHz frequency band the antenna is detuned and not matched.

Between 1.9 GHz and 2.287 GHz the RL< -6 db.

The simulated antenna was detuned in frequency so it operates best for 828 MHz withcircular polarization straight out from the patch and linear polarization at the short sides

(circular close to antenna). For 2.45 GHz the antenna should radiate from the short sidesof the antenna. The simulated antenna radiates with around 5 dBi at 2.15 GHz at the sides

of the antenna.


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Minimizing stage:

To reduce the size of the antenna three techniques been used: shorting wall, change of

substrate and use of available antenna parameters. First a shorting wall is used to

minimize the antenna. That is an effective method and reduced the size of the patch with50%. By using this method to reduce the size of the antenna the bandwidth will decreasesince the bandwidth is dependent of the ratio between the width and length of the patch,

L > W. By reducing W the bandwidth decreases.

Air is an excellent choice as substrate if high bandwidth is a priority. The minimum

bandwidth that is needed is at least 2 MHz bandwidth for the antenna. The goal was toget higher than 2 MHz bandwidth to compensate detuning changes of frequency. A

substrate with higher dielectric constant lowers the resonance frequency but it will alsodecrease the bandwidth. A substrate with high dielectric constant is Rogers Corporation

TMM 10i, Er = 9.8 and tan theta 0.002 . The advantage with this substrate is that Rogers

Corporation has many different heights of the substrate available. A 12.7 mm thicksubstrate is needed. The antenna was simulated with TMM 10i which gave a resonancefrequency lower than 868 MHz. To increase the frequency the antennas size can then be

minimized. The antenna dimension after minimizing it with TMM 10i is

W*L*H= 108*52*3.5 mm. The minimized antenna is matched for 868 MHz, RL868

MHz= -42 db. The bandwidth for 868 MHz is 24 MHz (856-880 MHz) for RL< -10 db.For the 2.45 GHz frequency band the antenna is detuned and not matched. At 2.138 GHz

the RL< -10.7 db and for 2.75 GHz the RL< -4.8 db.

There occurs one big problem with the minimized antenna. The substrate (TMM 10i) has

a delivery time of 7-8 weeks which is not acceptable for the current project. The wholeproject time is 20 weeks and when the materials are delivered the project would nearly be

ended. Therefore it was necessary to change substrate or design. Therefore the substrate

is changed to FR-4. FR-4 is a very common material that is cheaper2

than TMM 10i and

with Er= 4.3 and tan theta = 0.02. FR-4 is only available in few thicknesses (0.8-1.5mm).To get a height of 12-13 mm several layers need to be stacked.

The consequence with FR-4 instead of TM10i is that FR-4 has lower dielectric constantwhich increases the size of the patch antenna (formulas 2.1, 2.2, 2.6). The antenna will

also get higher bandwidth with FR4 instead of TMM 10i since it has lower dielectricconstant gives also a recommendation for the width of the patch. In this design the ratio

between the length and width of the patch is exactly two. The ground plane affects thesize of the antenna. To minimize the total physical size of the antenna the ground plane is

kept as small as possible. The ground plane should be extended with the factor 6hantenna(81.21 mm) for the antenna. This will increase the total physical size of the

antenna significantly. The reason why the extension part (6h antenna) is so long dependson the large thickness of the antenna.

The recommended size of the ground plane will not be used. To decide the size of the


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ground plane, simulation result and the manufacturing process has been taking intoaccount.

Each FR-4 plate that is bought is 160 mm long. To simplify the manufacturing process,the choosing length of the antenna is 160 mm. Then the ground plane is larger than the

patch and will not increase the resonate frequency as much as possible. The width isdetermined by the simulation results so the antenna is match for the right frequency.

Design errors:

From the beginning the simulated model of the antenna for 868 MHz has a bandwidth

about 25 MHz for RL < -10 dB. When the antenna was built the network analyzer3

showed that the antenna has a resonance frequency around 650 MHz and 1.3 GHz. By

change the dimensions of the patch, radius of feed circle (from 3 mm to 10 mm) andradius of the hole in the patch (from 0.5 mm to 1.5 mm) the design get more stable and

easier to build.

After these changes the antenna works much better and increase the frequency closer to

868 MHz but the antenna did not work as the simulations. The simulation software (CST)showed that the antenna is matched for the resonant frequency at 838 MHz (about 10 mm

from metal), 16 MHz bandwidth for RL < -10dB but the network analyzer showed thatthe antenna is matched for the resonance frequency 880 MHz (around 10 mm frommetal), that is 42 MHz difference.

The final test bed needed two RFID antennas. When the second antenna was built fewerrors was found. In CST the patch circle was not centered over the feed circle which it

were drilled and for CST the radius was 3 mm, not diameter of 3 mm.

Simulation shows that the built antenna is better match then the simulation for the designwith errors but there are still 44 MHz difference between the built antennas S11

parameter, see figure 5.12a and the simulated version, red curve in figure 4.2.


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Layout:

Figure 4.3-4.7 shows the structure from five different views of the antenna. The antennais also built with the given parameters in figure 4.3-4-7.


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Figure 4.3 shows the top view on feed with substrate behind. Figure 4.4 shows the topview from ground. From this view only the ground is shown and the coaxial feed in the

middle of the ground. Figure 4.5 shows the patch layers form with the substrate behind.Figure 4.6 shows the antenna form the long side. The antenna is built with nine layers

FR-4, 1.5 mm height each. Figure 4.7 shows the antenna from the short side. Thesubstrate is divided into nine sub layers (SL) as figure 4.6.


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General simulations settings in CST:

CST microwave studio is used as simulation software. The simulation settings in CST

(mesh shells properties, frequency range, accuracy etc.) affect the simulations result alittle, special the S11parameter. Therefore the S11parameter looks a bit different

depending on which settings are used.

Following simulation settings have been used:

Frequency range: 800-870 MHz, 2.22-2.5 GHz, 0.7-2.5 GHz

Solver settings (Accuracy): -30dB

Global Mesh Properties

Lines per wavelength: 15 Lower mesh limit: 5Meshline ratio limit: 20Simulations result:

Following section presents simulation results from CST for the S11parameter, smith

chart, far field pattern and axial ratio for both frequency band, 868 MHz and 2.45 GHz.

The simulation results are for the built antenna dimensions. There is a different betweenthe simulated result and the measured result from the network analyzer.


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S11 parameter

Figure 5.1 shows that the antenna is not matched for 868 MHz.

RL836MHz= 22 dB gives that 0.6 % of the incident power is reflected.RL868MHz=

2.06 dB gives that 62.2 % of the incident power is reflected.


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Figure 5.2 shows that the antenna is not match for 2.45 GHz.

RL2.284 GHz= 22.5 dB gives that 0.3 % of the incident power is reflected. RL2.402

GHz= 2.06 dB gives that 62.2 % of the incident power is reflected.


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Figure 5.3 shows the S11parameter with both frequency bands (868 MHz and 2.45 GHz)

in the same figure.

RL836MHz= 11.3 dB gives that 7.4 % of the incident power is reflected.RL868MHz=

3.5 dB gives that 44.7 % of the incident power is reflectedRL2.284 GHz= 25.3 dB

gives that 0.3 % of the incident power is reflected.RL2.402

GHz= 1.96 dB gives that

63.7 % of the incident power is reflected..


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Smith chart:

Figure 5.4 shows that the antenna is good matched at 836 MHz since the blue point is

nearZS=1 and worse matched at 868 MHz since the black point is far fromZS=1.

Figure 5.4 shows even thatZA = 46 + 6.25i at 836 MHz and for 868 MHz,ZA = 6.74

+ 18.66i Figure 5.5 shows that the antenna is good matched at 2.284 GHz

since the blue point is nearZS= 1 and worse matched at 2.402 GHz since the black


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pointisfarfromZS=1.Figure5.5showseventhatZA =562.5i at2.284GHzand for 2.402

GHz,ZA = 165.9 + 203.1i

Far field pattern:

Figure 5.6 shows the far field pattern for 868 MHz. The antenna radiates with 5.92 dBi in

front of the antenna and worse at the sides (0 dbi). The radiation efficiency is - 0.4844dB, the total efficiency is 4.708 dB and the directivity is 5.924 dBi. For 868 MHz there

are much losses in the antenna since the ratio between the radiation efficiency and totalefficiency is not near 1.


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Figure 5.7-5.8 shows that the antenna radiates with 7.63 dBi at best from the both shortsides of the antenna for 2.402 GHz. The radiation efficiency is -2.287 dB, the total


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efficiency is -6.605 dB and the directivity is 7.631 dBi. For 2.402 GHz there are muchlosses in the antenna since the ratio between the radiation efficiency and total efficiency

is not near 1.

Axial ratio:

The simulation results show that the antenna is much better matched for 836 MHz than

868 MHz. A result of this is shown in figure 5.9 a-b. Figure 5.9a and 5.9b.shows that theantenna has much better circular polarization for 836 MHz than 868 MHz since the axial

ratio value (dB) is closer to zero and it has also lower value for axial ratio at more angelsin the far field pattern.


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Figure 5.10 that the antenna has circular polarization at the left side close to the antennaand linear polarization far away from the antenna. Figure 5.11 show that the antenna has


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not so good circular polarization at the right side close to the antenna and linearpolarization far away from the antenna.

Documents

Design of dual band patch antenna