2008 IEEE INTERNATIONAL RF AND MICROWAVE CONFERENCE PROCEEDINGS 2008 IEEE INTERNATIONAL RF AND MICROWAVE CONFERENCE PROCEEDINGS 2008 IEEE INTERNATIONAL RF AND MICROWAVE CONFERENCE PROCEEDINGS 2008 IEEE INTERNATIONAL RF AND MICROWAVE CONFERENCE PROCEEDINGS December 2December 2December 2December 2----4, 2008, Kuala Lumpur, MALAYSIA4, 2008, Kuala Lumpur, MALAYSIA4, 2008, Kuala Lumpur, MALAYSIA4, 2008, Kuala Lumpur, MALAYSIA

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Design of a Spiral Antenna for Wi-Fi Applications

M. F. Abdul Khalid

1, M.A.Haron

1, A. Baharudin

1, and A. A. Sulaiman

2

1Microwave Technology Centre (MTC), Universiti Teknologi MARA, Selangor, Malaysia 2Faculty of Electrical & Electronic Engineering, Universiti Sains Malaysia

Engineering Campus, Pulau Pinang, Malaysia mfarid@ieee.org, adib2201@yahoo.com, atiq_slim@yahoo.com, asari100@yahoo.com

Abstract – The main objective of this project is to

design a spiral antenna at a frequency range between

2.4 to 2.5 GHz. The spiral antenna is fed by a

microstrip line and RT Duroid 5870 is used with

dielectric constant, εr and thickness, tsub of 2.33 and 0.5

mm respectively. For the spiral antenna design and

simulation, the Genesys software is utilised. A

prototype of a circular microstrip antenna is fabricated

and measured. Simulated results like return loss,

VSWR and input impedance are compared with those

obtained from measurements where good agreements

are shown.

Keywords - Spiral antenna, Wi-Fi

1. Introduction

Wi-Fi networks use radio technologies. A person

with a Wi-Fi enabled device such as a PC, cell phone

or PDA can connect to the Internet when in proximity

of an access point. The region covered by one or

several access points is called a hotspot. Hotspots can

range from a single room to many square miles of

overlapping hotspots. Wi-Fi can also be used to create

a mesh network. Both architectures are used in

community networks [1].

Nowadays, Wi-Fi pollution, or an excessive

number of access points in the area, especially on the

same or neighboring channel, can prevent access and

interfere with the use of other access points by others,

caused by overlapping channels in the 802.11g/b

spectrum, as well as with decreased signal-to-noise

ratio (SNR) between access points. This can be a

problem in high-density areas, such as large apartment

complexes or office buildings with many Wi-Fi access

points. Additionally, other devices use the 2.4 GHz

band, microwave ovens, cordless phones, baby

monitors, security cameras, and Bluetooth devices can

cause significant additional interference.

In order to avoid the use of different antennas for

the different services, one multiband or extremely

broadband antenna has to be used. Spiral antennas, that

show frequency independent characteristics at a relatively

small size, are promising candidates for this application.

Additionally planar spiral antennas give the possibility of

conformal integration of the antenna into existing surfaces

such as the car bodywork for reasons of aerodynamics,

stability and aesthetics. Only in recent years the interest in

this type of antenna for communications applications has

increased. The bandwidth of the antenna, which is intended

to cover the radio services range from 800MHz to 2.2GHz.

[2]. In that case, for WiFi application, the bandwidth of the

antenna at the range from 2.4 GHz to 2.5 GHz.

Applications that require low-profile, light weight, easily

manufactured, inexpensive, conformable antennas often

use some form of a microstrip radiator.

2. Design of The Single Arm Spiral Antenna

The spiral antenna which is shown in Fig. 1 consists of

one arm which forms the spiral structure. The arm has

width, w and the separation between the lines is described

by the parameter s. The number of turns of the middle line

is denoted by N and the maximum radius is specified by

rout. The spiral is printed on the dielectric substrate with a

specific thickness tsub and a dielectric constant εr. The input

impedance of the antenna is affected by the line width w,

the distance between the lines s, the dielectric constant εr,

and the substrate thickness tsub.

Fig 1: Geometry of a single arm spiral antenna from the outer

edge.

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2.1 Size The radiation efficiency of the patch antenna

depends largely on the permittivity (εr) of the

dielectric. Ideally, a thick dielectric, low εr and low

insertion loss is preferred for broadband purposes and

increased efficiency. The substrate that used in this

project is 0.5mm thickness and a dielectric constant

value, εr=2.33. Simulation results show suitable

radiation patterns for frequency range between 2.4GHz

and 2.5GHz. The center frequency is at 2.455GHz.

The resonant rectangular patch antenna is on the

order of λ/2 in extent. The length of the element is

reduced by a factor of two if the half-wave patch is

short-circuited to ground at the zero potential planes.

The result is a resonant rectangular patch with length

on the order of λ/4, called a quarter-wave patch.

Certain applications require further size reduction. In

these cases, the shape of the patch is altered to achieve

acceptable performance from rectangular patches with

resonant length less than λ/4.

Fig. 2, 3 and 4 shows the VSWR, return loss

and input impedance graph respectively using three

different microstrip sizes. The length will be constant

and the width is different. The length is constant

because it is the minimum size to ignore the fringing

effect. The size is; (a) 125mm x 115mm, (b) 125mm x

113mm, (c) 125 x 120mm. The best result is the (a)

size.

Fig. 2: VSWR graph for three different antenna sizes.

Table 1: VSWR value compared with different size.

Frequency

GHz Graph (a)

Graph (b)

Graph (c)

2.412 6.792 3.758 4.978

2.455 1.097 12.839 6.550

2.494 6.330 19.418 27.874

The (a) graph shows the best result, VSWR is

1.097 at frequency 2.455GHz. As the width size

decrease, graph (b) show the VSWR is 12.839 at frequency

2.455GHz. Lastly, if the width size is increase, graph (c)

give VSWR value at 2.455GHz is 6.55.

From these entire three graphs, we can see that the

VSWR graph will shift to the left if the width size is

decrease or increase.

Fig. 3 The return loss for three different antenna sizes.

Table 2: Return Loss value with different size.

Frequency

GHz

Graph (a)

Graph (b)

Graph (c)

2.412 -2.576 dB -4.736 dB -3.538 dB

2.455 -26.711 dB -1.356 dB -2.673 dB

2.494 -2.766 dB -0.895 dB -0.623 dB

At frequency 2.455GHz, the return loss for graph

(a) gives the sharpest graph and the value is -26.711 dB.

The graph should be sharp at the center frequency, so that

the minimum return loss will be given for the antenna

design at the center frequency. Graph (a) give the best

result and as the width size is increase or decrease, the

graph will be shift to the left and the graph is not sharp as

the graph (b) compare to graph (a).

Fig. 4: The input impedance for three different antenna sizes.

(a) 125 x 115

(b) 125 x 113

(c) 125 x 120

(a) 125 x 115

(b) 125 x 113

(c) 125 x 120

(a) 125 x 115

(b) 125 x 113

(c) 125 x 120

432

Table 3: Input Impedance value compare to different

size.

Frequency

GHz

Graph (a)

Graph (b)

Graph (c)

2.412 196.932 166.077 224.844

2.455 46.363 41.490 31.440

2.494 20.740 48.949 10.578

For the input impedance graph, the value should

be 50 at center frequency because the transmission line

is loaded with a 50ohm resistor at the end. The nearly

value that the simulation can achieve is at 46.353 for

the (a) graph.

As the size is increase or decrease, the input

impedance graph lower the maximum value and the

graph is shift to the right. At the center frequency, the

input impedance value for both graph {graph (b) and

graph (c)} is less than graph (a).

From this analysis, the best result show at graph

(a). The best size for the antenna design is 125mm x

115mm.

2.2 Feeder Network The four most popular feed techniques used are

the microstrip line, coaxial probe (both contacting

schemes), aperture coupling and proximity coupling

(both non-contacting schemes). In this spiral design, it

use microstrip line feed technique.

In this type of feed technique, a conducting

strip is connected directly to the edge of the microstrip

patch as shown in Fig. 5. The conducting strip is

smaller in width as compared to the patch and this

kind of feed arrangement has the advantage that the

feed can be etched on the same substrate to provide a

planar structure.

Fig. 5: Microstrip Line Feed.

The purpose of the inset cut in the patch is to

match the impedance of the feed line to the patch

without the need for any additional matching element.

This is achieved by properly controlling the inset

position. Hence this is an easy feeding scheme, since it

provides ease of fabrication and simplicity in modeling as

well as impedance matching.

However as the thickness of the dielectric

substrate being used, increases, surface waves and spurious

feed radiation also increases, which hampers the bandwidth

of the antenna. The feed radiation also leads to undesired

cross polarized radiation. In simulation, it gives different

result while matching the patch and the feeder line. Fig. 6

shows three different matches to feeder line.

Fig. 6: Three different matching to feeder line. Figure (a)

show the matching feeder that used in the design.

From this three different matching, Fig. 7, 8 and 9

show the result for VSWR, return loss and input

impedance graph respectively.

Fig. 7: Graph VSWR with varying matching with feeder line.

Table 4: VSWR value compared to different feed matching.

Frequency

GHz

Graph (a)

Graph (b)

Graph (c)

2.412 6.792 19.257 30.310

2.455 1.097 6.593 8.694

2.494 6.333 1.443 28.207

(a)

(b)

(c)

Picture (c)

Picture (a) Picture (b)

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From this graph, it shows that there is an

effect for the different matching with the feeder line. If

the patch is nearly align with the y-axis feeder line, as

shown in figure (b), the graph was shifted to the right

and the value at the canter frequency is 6.593.

In figure (c), the patch is over the feeder line

the VSWR graph was shifted to the left and higher

than the reference graph (graph (a)).

Thus, the best graph is the graph (a), for

figure (a). The patch is at the edge of the feeder line.

The VSWR value is 1.097.

Fig 8: Return Loss Graph for different feeder line

matching.

Table 5: Return Loss value compared

to different matching feed line.

Frequency

GHz

Graph (a) Graph (b) Graph (c)

2.412 2.576 dB -0.903 dB -0.573 dB

2.455 -26.711 dB -2.656 dB -2.007 dB

2.494 -2.766 dB -14.834 dB -0.616 dB

Same cases for the return loss graph, Fig 10.

The minimum value for S11 in graph (b) was shifted

to the right and the graph (c) curve is not sharp

compare to the other two graphs. The best graph is still

the graph (a), using figure (a) simulation.

On an edge-fed spiral, the current travels

from the input to the center of the spiral antenna and

gradually emits electromagnetic waves into the air.

Depending on the number of turns and diameter of the

spiral a portion of the bounded electromagnetic waves

reaches the center and then it reflected back towards

the input. The reflected wave travels in the opposite

direction and radiates the opposite circular

polarization.

3. Measurement of Single Arm Spiral Antenna

The single arm spiral antenna was fabricated and its

performance is presented in this section. For the prototype

antenna the width w and spacing s is equal that is 3 mm the

spiral given the outer radius 27mm and consists of 4 turns.

The antenna was fabricated on a rectangular (125mm x

115mm) substrate with εr = 2.33 and a thickness tsub =

0.5mm.

3.1 Measured Return Loss

The return loss of the prototype antenna was

measured from 2.3 – 2.6GHz using a vector analyzer. In

Fig. 12 the simulated and measured result was plotted. The

simulation predicts the band of operation rather accurately.

From the measured graph, compared to the simulated

graph, it is slightly shifted to the right. The minimum

return loss is not at the centre frequency.

The discrepancy between the measured and

simulated results stems from the fact that in the simulations

the antenna substrate and the ground plane are assumed

infinite. The second reason for the observed discrepancy is

the difficulty in connecting a standard 50-Ohm connector

to coplanar feed line of the antenna (see Fig 7). Thus the

feed line is adversely affecting the return loss. Note that

the antenna is not simulated with the connector.

Another reason for the discrepancy can be the gap

width of the coplanar waveguide line at the antenna center.

The return loss is sensitive to small variations of the gap

width as shown in paper. The copper etching cannot

maintain a constant 3mm gap. There is considerable

undercut causing some variations in the characteristic

impedance of the coplanar waveguide.

Fig. 9: Simulated and measured return loss of single arm

spiral antenna.

Table 6: Simulation and measured return loss value.

Frequency

GHz

Simulated

Graph

Measured

Graph

2.412 2.576 dB -5.031 dB

2.455 -26.711 dB -10.024 dB

2.494 -2.766 dB -2.188 dB

simulated

measured

(a)

(b)

(c)

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3.2 Measured VSWR The VSWR measurement and simulation

result was compared in Fig. 10. The measured result is

also slightly shifted to the right. For ideal antenna, the

value for VSWR is equal to 1. Low values of VSWR

indicates the antenna have a small value of reflection

coefficient.

The discrepancies between measurement and

simulation are caused by the assumption of the infinite

substrate and the ground plane in the simulations and

superiors radiations caused by the feed cable and

scattering in the chamber.

A reason for the discrepancy between the

simulated and measured VSWR can be the

measurement setup. A small amount of asymmetry in

the antenna structure, the feed, and the nearby objects

such as the cable feeding the antenna can cause

noticeable errors on the VSWR result.

The other reason for the discrepancy between

measurement and simulation is caused by the

fabrication process. The printer used to print the circuit

layout from the CAD simulation must have high

resolution and high quality printouts. The fabrication

process needs to be handled with care so that the

circuit layouts are precisely constructed on the

microstrip. It is also have to make sure that all part of

the microstrip antenna is grounded.

Fig. 10: Simulated and measured VSWR of single arm

spiral antenna.

Table 7: Simulation and measured VSWR value.

Frequency

GHz

Simulated

Graph

Measured

Graph

2.412 6.792 3.549

2.455 1.097 1.921

2.494 6.333 8.638

6. CONCLUSION

A single arm microstrip antenna topology is

introduced. This spiral antenna is fed from outside edge by

a coplanar waveguide transmission line to achieve a

completely planar antenna that does not require feed

assembly and external matching network. Full-wave

numerical simulations were carried out to optimize the

physical parameters of the antenna. A prototype antenna

above a ground plane was designed, fabricate and its

performance was measured. By optimizing the antenna

gain, the expected results between measure and simulation

are considerably closed. For the future development, the

spiral antenna can vary the arm. Different arm spiral

antenna gives different application and results. Single arm

spiral antenna also can be extended for array single arm

spiral antenna. It can use microstrip antenna fed for the

feeding line. This could be difficult especially during

fabrication process. Another development is the feeding

line, if the microstrip spiral antenna can fed using coaxial

probe. The patch shapes are symmetric and their radiation

is easy to model.

References [1] http://www.Wikipedia.htm/Wi-Fi, 30 July 2007

[2] Johnna Powell and Anantha Chandrakasan, “Spiral Slot Patch

Antenna and Circular Disc Monopole Antenna for 3.1-10.6 GHz Ultra Wideband Communication”, Massachusetts Institute of

Technology, 50 Vassar St. Rm 38-107, Cambridge, MA 02139.

[3] Brad A. Kramer, Chi-Chih Chen and John L. Volakis, “Design and

Performance of an Ultra Wideband Ceramic-Loaded Slot Spiral”,

ElectroScience Laboratory, Electrical and Computer Engineering Dept. The Ohio State University.

[4] Dominikus J. Müller, and Kamal Sarabandi, “Design and Analysis of a 3-Arm Spiral Antenna”, IEEE Transactions On Antennas And

Propagation, vol. 55, No. 2, pp 258-266,

[5] Chang Won Jung, Bedri A. Cetiner And Franco De Flaviis, “A

Single-Arm Circular Spiral Antenna With Inner/Outer Feed

Circuitry For Changing Polarization And Beam Characteristics”, Department Of Electrical And Computer Engineering University Of

California, Irvine, CA, 92697, USA

[6] E. Gschwendtner, W. Wiesbeck, “Low-Cost Spiral Antenna with

Dual-Mode Radiation Pattern for Integrated Radio Services”,

Institut für Höchstfrequenztechnik und Elektronik, University of Karlsruhe, Kaiserstraße 12, 76128 Karlsruhe, Germany

[7] L. Schreider, X. Begaud, M. Soiron, B. Perpere and C. Renard, “Broadband Archimedean spiral antenna above a loaded

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simulated

measured

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