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41The Challenges of Handset Antenna Design and Computer Aided Design,Development and Fabrication of Circular Microstrip Antenna

The Challenges of Handset AntennaDesign and Computer Aided Design,

Development and Fabricationof Circular Microstrip Antenna

R D Kanphade*, D G Wakade** and N T Markad***

2011 IUP. All Rights Reserved.

* Principal, Dhole Patil College of Engineering, Wagholi, Tal-Haveli, Pune, India. E-mail: [email protected]

** Director, P R Patil College of Engineering and Management, Pote Estate, Kathora Road, Amaravati,India. E-mail: [email protected]

*** Assistant Professor, Bharati Vidyapeeth College of Engineering, New Delhi, India; and is thecorresponding author. E-mail: [email protected]

IntroductionThe increasing effort in miniaturization of mobile communication equipments has

inspired the development of small, low profile antenna suitable for implementation

in portable devices. Whereas in the past a single antenna element has been used for

mobile transceivers, the desire to combat multi-path fading has led to the use of

multiple elements arranged in a suitable diversity scheme. When more than one

element is used, an important design consideration is the effect of mutual coupling

on the antenna performance. Early handset treated the antenna as a bolt-on item,

but the current trend is to integrate the antenna within the body of the handset.

The handsets are becoming smaller or more functionality is being packing into these

units. This leaves little room for the antenna.

Design and realization of microstrip antennas in S-band at 2.4 GHz is reported in this

research paper. It is shown that the design adopted for circular microstrip antenna is quite

accurate. By using the conventional MIC fabrication technology compact light weight

microstrip antenna can be realized. The desired narrow band achieved the circular microstrip

antenna. Antennas are designed and fabricated on the substrate of dielectric constant 4.22

and thickness of 1.6 mm. Simulation is done using the microwave software to achieve the

desired results. The purpose of this paper is to introduce the reader to the challenges of

designing an antenna integrated into modern handset. It provides the reader with insight

into all the details associated with handset antenna design and the type of measurements

used to characterize handset antennas.

Keywords: Circular microstrip, Antenna, Impedance, Balun, VSWR, Return loss, Smith chart


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The IUP Journal of Telecommunications, Vol. III, No. 2, 201142

A wide variety of antennas have been used in handsets, but they can split in the

following categories:

External helical antenna

Printed internal helical antenna

Printed inverted F-type antenna

L-type antenna

Circular, square, triangular, rectangular patch antenna

Ceramic antenna

Meander line antenna

Dual planar inverted L-type antenna

These antenna types have been covered in numerous papers and books. For the

reader not familiar with above antennas further reading is suggested (Wheeler,

1975; and Balanis, 2005). Conventional antenna theory uses an image technique to

allow in infinite ground plane. This cannot be used for electrically small ground

plane, as all the antennas mentioned above are effected to some extent by the

electrically small ground plane, the use of a simulator is recommended to examine

the current flow in the ground. The current distribution on the handset provides

a useful insight into the positioning of co-axial feeds to the antenna and coupling

into other assemblies on the handset.

The undergraduate antenna designer will start the antenna modeling by using

a simple wire grid model of handset substrate ground. The current trend is to

integrate the antennas within the handset for western market. This meansantenna engineers must be familiar with the mechanics design. It is rare on

modern handsets for the antenna designer to be given a space for exclusive items

such as the loud speaker and its associated acoustic cavity, a camera or electrical

connector. Not only can these items significantly reduce the volume available for

the antenna but they normally degrade the performance of the antenna. The use

of plastic within the handset may be chosen for its mechanical properties and

low cost rather than the dielectrics and loss tangent parameter. This is especially

true inside the handsets where sub-assemblies can be held in place using low-

cost glass reinforced plastic.The antenna designer may have to educate the

mechanical designer on the effect of their design decisions. Lossy plastics are an

obvious area of concern, but cost drives means that it is not normally possible

to use low loss plastic throughout the design. Instead, other techniques may have

to be employed, such as the use of stands offs, the underlined holes in the support

structure repositioning ribs away from high field strength areas etc. The

grounding scheme used for different parts of the handset needs to be considered

(Wheeler, 1975; and Balanis, 2005).


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The single biggest challenge with designing handset antenna is the time scale.

The antenna can only be tuned once the plastic cases, electronic sub-assemblies and

substrates become available. That means the early investigative studies have to use

prototype components and this can cause errors. Once the correct components andplastics become available, there is usually very little time to optimize the antenna and

validate its performance. Circular microstrip antenna structure is planar in

configuration and enjoys all the advantages of substrate technology. The feed lines

and matching circuits are fabricated simultaneously with antenna structure. The solid

state components can be added directly on microstrip antenna substrate and hence

such antennas are compatible with modular design. It is small in size, low in weight,

easy to manufacture on mass scale with low manufacture ring cost. Can also be

applied to the metallic surface on an aircraft or missile and do not disturb aerodynamic

flow and thus have better aerodynamic properties. Liner and circular polarizations

are possible with simple change in feed position and dual frequency antennas can

be made possible. The first application of circular microstrip antennas was thoserequiring thin, conformal antennas. The telemetry and communication antennas on

missile are often of microstrip type. Small arrays of microstrip radiator are used for

radar altimeter antennas and other aircrafts related applications included satellite and

mobile telephone commutation. Circular microstrip antenna has also been used as

communication link between ship and satellites such as Geostationary Operational

Environmental Satellites (GOES). Smart weapon systems used circular microstrip

antenna because of its thin profile and low cost. One of the most important

applications of circular microstrip antenna at present is in GPS system. Circular

microstrip antennas are also used in RFID, TAGS, mobiles, and WIFI applications

(Radiation from Microstrip Radiator, 1969).

Feed Network

The feed used for this circular microstrip antenna is of the transformer microstrip

type. By using formulae for W, L, eff

, the terms of feed dimensions are calculated.

L1 length of transformer feed comes out to be 7 mm, W1 width of transformer feed

are found to be 3.2 mm. To design the matching transformer between the antennas

and feed line, the input of antennas must be calculated. The input impedance of

the antennas is controlled to a little extent by the width of patch. After calculating

the input impedance of the patch, it is used to match the impedance of the antennas

and the feed line (Balanis, 2005).

Antenna Configuration

The design of the circular microstrip patch antenna begins with choice of the

substrate, selecting feed mechanism, determining patch length L, determining patch

width W and selecting the feed location. Figure 1 shows one of the common ways


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of feeding the patch antenna with transformer feed line. The length L of the patch

is selected such that it satisfies the condition of resonance. It is usually chosen close

to 1/2 such that the input impedance of the patch is pure real at the desired

frequency. Since the two ends of the patch are open, an open end correction is done

for calculating the physical length of the patch. Since the design is a square patch,

the length and width of the patch is the same. To design the matching between

the antenna and the feed line, the input impedance of the patch is controlled to

a little extent by the width of the patch. After calculating input impedance of the

patch, quarter wave transformer is used to match the impedance of the antenna

and feed line. Once the theoretical design is complete, the simulation is done on the

microwave software to get the return loss of the antenna less than 10 dB. Puff

software is used for cross verification of the calculated length and width of the feed

line and matching transformer design equations for the transformer coupled patch

antenna.

Dielectric constant: 4.22, height of substrate: 1.6 mm

Design of frequency: 2.4 GHz

Length of quarter wave transformer (here length of the patch) is equal to

width W of patch. Circular patch antenna is the popular design other than

the rectangular patch. In certain applications such as the design of the

arrays, circular geometry provides a more beneficial shape over the other

Figure 1: Actual Fabricated Antenna

L2. W2

L1W1


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patches shapes as the feed can be connected at any point along the

periphery.

At the desired resonance frequency, the design of the circular patch requires

calculation of the radius a. The calculations based on these equations are not very

accurate at 2.4 GHz design frequency (Balanis, 2005). The complete design

dimensions are given below while the transformer feed circular patch antenna is

shown in Figure 1.

L1 = 7 mm

W1 = 3.2 mm

L2 = 15 mm

W2 = 0.2 mm

a = 17.5 mm

1

2

2

1

rfr

W

...(1)

Length of the patch

Lfr

L

eff

22

1

...(2)

L2. W2

L1

W1

L2. W2

L1W1

Figure 1 (Cont.)


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Effective dielectric constant

W

h

rreff 12

1

1

2

1

2

1

...(3)

Open end correction length L

80.0

264.00.3142.0

h

W

h

W

h

L

eff

eff

0.258...(4)

Characterizing the Circular Microstrip Antenna

Historically the return loss or impedance of the antenna was the main parameter

considered. The return loss is still a key parameter to be measured. The impedance

is a passive measurement, that is a coaxial feed needs careful consideration to ensure

that the coaxial line does not significantly alter the current distribution on the ground.

It is recommended that the current distribution on the ground is examined to

determine the optimum connection point between the feed and the ground. Some

current is still likely to flow along the outer of the feed, so the use of either ferrites

or baluns (or both) is recommended. The antennas impedance should be measured

in a variety of conditions that reflect the handsets different operating scenarios.

Ideally the impedance should be measured:

1. In free space;

2. Next to head phantom (left and right sides);

3. Next to a head and with hand phantom;

4. With a hand phantom only;

5. On a belt clip (next to a body phantom); and

6. On a metal plate (a car roof simulation).

Different handset manufacturers use different specifications for the above

scenarios, but aiming to achieve 6 dB return loss for conditions 1-6 listed above would

be a good starting point. Readers familiar with large antennas (such as vase stations)will be used to seeing return loss specifications better than 14 dB (VSWR less than

1.5:1), the handset specification equates to a VSWR of less than 3:1. At first glance,

this appears to be trivial to meet but the small size of the antenna and the variety of

handset operating scenarios make the requirements nontrivial for multi-band antennas.


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Figure 2: Simulated Return Loss

From simulated return loss plot of circular microstrip antenna it is seen that at the

frequency of 2.4 GHz return loss is 15. 4124 dB and from tested return loss of circular

microstrip antenna on network analyzer it is seen that at the frequency of 2.37 GHz

return loss is 30 dB. This means that simulated and tested results matched and circularpatch microstrip antenna will radiate satisfactory and sufficient radiation. From

simulated VSWR plot of antenna it is seen that at the frequency of 2.4 GHz VSWR

seen is 1.4084 and from tested VSWR results of antenna it is seen that at the frequency

of 2.37 GHz VSWR seen is 108. This means that simulated and tested results matched

means that circular patch microstrip antenna design and fabrication is practically

correct. From simulated Smith chart it is seen that impedance offered by circular patch

microstrip antenna is totally inductive and from measured plot of Smith chart it is seen

that impedance offered by the antenna is real, capacitive as well as inductive. Plot of

simulated return loss is shown in Figure 2. Plot of measured return loss is shown in Figure

3. Plot of simulated VSWR is shown in Figure 4 and plot of measured VSWR shown in

Figure 5. Plot of simulates Smith chart is shown in Figure 6 and measured Smith chartin Figure 7. Some of the antennas do not have a terminal impedance of 50 ohms. So

some form of matching is required. In case of circular microstrip antenna matching line

of length of 7 mm and width of 3.2 mm is used. It is not necessary for the impedance

measurements to be performed in an anechoic chamber with care the impedance may

be measured in a normal laboratory environment.


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Figure 4: Simulated VSWR

Figure 3: Measured Return Loss


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Figure 5: Measured VSWR

Figure 6: Simulated Smith Chart

100 90 8070

60

50

40

30

20

10

0

10

20

30

4050

6070

8090100110

120

130

140

150

160

170

180

170

160

150

140130

120110

0 0.20 0.50 1.00 2.00 5.00

5.00

2.00

1.00

0.50

0.20

0.20

0.50

1.002.00

5.00


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Radiated Measurements

The main reasons for performing radiated measurements on the handset are to check

of the antennas efficiency, directivity and radiation pattern. Radiation pattern of the

circular microstrip antenna is shown in Figure 8.

Figure 7: Measured Smith Chart


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Radiation Pattern

These measurements may be performed either passively causing a coaxial feed oractively, where the antenna contains its own generator. The radiated measurements

should be performed in an anechoic chamber. The aim should be to have an efficient

antenna with near omni-directional far field radiation pattern. The target values for

efficiency vary with antenna manufacturers and sometimes network operations specify

the efficacy as acceptance criteria. The required efficiency tends to be in the range

of 2.5 dB to 5 dB, with higher efficiency required at the lower frequencies. The

pattern should be close to omni-directional so that the user doses not have to orientate

the handset in any particular direction during use. The FCC specify that the maximum

radiated power in any direction shall not exceed 33 dBm, so there is limit on the

maximum directivity in GSM 1900 band .The radiated measurements can be performed

in all the operating scenarios listed for the impedance measurements, but effort is

normally concentrated in free space and next to a head phantom (left and right side).

Walk Test

This test is performed by a user with a handset walking around a predetermined

route while the RSSI (received signal strength) at the base station is recorded.

Figure 8: Radiation Pattern

0

30

60

90

120

150

180

150

120

90

60

30

7.00

10.50

14.00

3.50


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The handset should remain under the control of one base station and the power

control at the base station should be turned off. The test requires the cooperation

of a network operator.

During the antennas initial development phase, passive measurements arerecommended. Passive measurements are performed with a coaxial line feeding the

antenna directly. The same comments apply as in the previous section regarding

the placing of feed point and the use of ferrites and baluns and the use of

matching line which is used in the circular microstrip antenna. Any current flowing

along the outer of the coaxial feed will radiate and cause an error in the

measurement. Passive measurements have the advantage of being rapid to

perform, and they may be readily made early during development when no

functioning substrate is available and all frequency bands may be covered during

a single measurement.

Other Antenna Measurements

The use of an anechoic chamber is recommended to characterize the efficiency and

radiation pattern of antenna. It is appreciated that it is not always passive to use

the ideal test equipment, and other methods are used for comparing efficiency of

different antennas. A couple of these are given below.

Reverberation Box

A reverberation box is the opposite of an anechoic chamber in that the box

designed to be a multipath environment. The box is a metal enclosure about the

size of a domestic fridge (for antenna testing). The antenna along with handset

is placed inside the box and the signal received on three orthogonal antennas,

metal paddled are rotated inside the chamber. Reverberation boxes allow both

active and passive measurements to be performed on the handset antenna. Thechamber is portable and so allows efficiency measurements to be performed at

the antenna designers laboratory. Bluetest is one company to offer such a

chamber (Hegge et al., 2004).

Conclusion

The purpose of this paper is to present some of the challenges of designing

antennas integrated into modern handset to characterize handset antenna. It can

be seen that the design adopted for antenna in this research paper is accurate.

This antenna can be used at 2.4 GHz frequency for industrial or mobile

communication applications. For an antenna to work properly the visor must be

less than 2 and the return loss must be less than 10 dB. Only then will the antennaradiate or receive the power with minimum reflection. As designed antenna has

the return loss 30 dB and VSWR 1.1 with the slight shift of frequency at 2.37 GHz,

this antenna is good with sufficient bandwidth. The shift in actual designed

frequency and actual measured frequency could be attributed to low quality


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substrate used. The light weight antennas are ideal for the mobile satellite

application where the weight is the main constraint. &Acknowledgment: The authors would like to thank DRDO for permission to use their anechoic

chamber and vector network analyses.

References

1. Balanis C A (2005),Antenna Theory Analysis and Design, 3rd Edition, John Wiley

& Sons.

2. Hegge N, Orlenius C and Kildal P (2004), Development of Reverberation Chamber

for Accurate Measurements of Mobile Phones and Mobile Phone Antennas, IEEE

Antenna Measurement and SAR Conference, pp. 55-58.

3. Radiation from Micro Strip Radiator (1969), IEEE Transactions on Microwave

Theory and Techniques, Vol. 1.7, No. 4.

4. Wheeler H A (1975), Small Antennas, IEEE Trans. Antenna and Propagation,

Vol. 23, No. 4, pp. 462-469.

Reference # 70J-2011-05-05-01


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