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International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

DOI : 10.5121/ijngn.2011.3301 1

The Performance of a Cylindrical Microstrip

Printed Antenna for

TM

10

Mode as a Function of

Temperature for Different Substrates

A. Elrashidi *, K. Elleithy

* and Hassan Bajwa†

*Department of Computer and Electrical Engineering

† Department of Electrical Engineering

University of Bridgeport, 221 University Ave,

Bridgeport, CT, USA

aelrashi@Bridgeport.edu

A

BSTRACT

A temperature is one of the parameters that have a great effect on the performance of microstrip antennas

for TM

10

mode at 2.4 GHz frequency range. The effect of temperature on a resonance frequency, input

impedance, voltage standing wave ratio, and return loss on the performance of a cylindrical microstrip

printed antenna is studied in this paper. The effect of temperature on electric and magnetic fields are also

studied. Three different substrate materials RT/duroid-5880 PTFE, K-6098 Teflon/Glass, and Epsilam-10

ceramic-filled Teflon are used for verifying the new model.

K

EYWORDS

Temperature, Voltage Standing Wave Ratio VSWR, Return loss S11, effective dielectric constant,

Transverse Magnetic TM

10

model.

1. I

NTRODUCTION

Due to unprinted growth in wireless applications and increasing demand of low cost solutions for

RF and microwave communication systems, the microstrip flat antenna, has undergone

tremendous growth recently. Though the models to analyze microstrip structures have been

widely accepted, effect of curvature on dielectric constant and antenna performance has not been

studied in detail. Low profile, low weight, low cost and its ability of conforming to curve surfaces

[1], conformal microstrip structures have also witnessed enormous growth in the past few years.

Applications of microstrip structures include Unmanned Aerial Vehicle (UAV), planes, rocket,

radars and communication industry [2]. Some advantages of conformal antennas over the planer

microstrip structure include, easy installation (randome not needed), capability of embedded

structure within composite aerodynamic surfaces, better angular coverage and controlled gain,

depending upon shape [3, 4]. While Conformal Antenna provide potential solution for many

applications it has some drawbacks due to bedding [5], those drawbacks include phase,

impedance, and resonance frequency errors due to the stretching and compression of the

dielectric material along the inner and outer surfaces of conformal surface. Changes in the

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

2

dielectric constant and material thickness also affect the performance of the antenna. Analysis

tools for conformal arrays are not mature and fully developed [6]. Dielectric materials suffer from

cracking due to bending and that will affect the performance of the conformal microstrip antenna.

In some applications, a microstrip antenna is required to operate in an environment that is close to

what is defined as room or standard conditions [7]-[11]. However, antennas often have to work in

harsh environments characterized by large temperature variations [12]. In this case, the substrate

properties suffer from some variations. The effect of that variation on the overall performance of

a microstrip conformal antenna is very important to study under a wide range of temperature.

2.

B

ACKGROUND

Conventional microstrip antenna has a metallic patch printed on a thin, grounded dielectric

substrate. Although patch can be of any shape rectangular patches, as shown in Figure 1 [13], are

preferred due to easy calculation and modeling.

Figure. 1. Rectangular microstrip antenna

Fringing field has a great effect on the performance of a microstrip antenna. In microstrip

antennas the electric field in the center of the patch is zero. The radiation is due to the fringing

field between the periphery of the patch and the ground plane. For rectangular patch shown in the

Figure 2, there is no field variation along the width and thickness. The amount of fringing field is

a function of the dimensions of the patch and the height of the substrate. Higher the substrate the

more is the fringe fields.

Due to effect of fringing a microstrip patch antenna would look electrically wider compared to its

physical dimensions. As shown in Figure 2, waves travel both in substrate and air. Thus an

effective dielectric constant ε

reff

is to be introduced. The effective dielectric constant ε

reff

take in

account both the fringing and the wave propagation in the line.

Figure 2. electric field lines (Side View).

The expression for the effective dielectric constant is introduced by A. Balanis [13], as shown in

Equation 1.

(1)

h

L

W

r

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

3

y

x

L

ε

r

R

z

The length of the patch is extended on each end by ∆L is a function of effective dielectric

constant and the width to height ratio (W/h). ∆L can be calculated according to a practical

approximate relation for the normalized extension of the length [14], as in Equation 2.

(2)

The effective length of the patch is L

eff

and can be calculated as in Equation 3.

L

eff

= L+2∆L (3)

By using the effective dielectric constant (Equation 1) and effective length (Equation 3), we can

calculate the resonance frequency of the antenna f

r

and all the microstrip antenna parameters.

Figure. 3. Physical and effective lengths of rectangular microstrip patch.

Cylindrical-Rectangular Patch Antenna

All the previous work for a conformal rectangular microstrip antenna assumed that, the curvature

does not affect the effective dielectric constant and the extension on the length. Effect of

curvature on the resonant frequency has been presented previously [15]. In this paper we present

the effect of fringing field on the performance of a conformal patch antenna. A mathematical

model that includes the effect of curvature on fringing filed and on antenna performance is

presented. The cylindrical-rectangular patch is the most famous and popular conformal antenna.

The manufacturing of this antenna is easy with respect to spherical and conical antennas.

Figurer 4: Geometry of cylindrical-rectangular patch antenna

W

∆L

L

∆L

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

4

Effect of curvature of conformal antenna on resonant frequency been presented by Clifford M.

Krowne [15] as:

(4)

Where 2b is a length of the patch antenna, a is a radius of the cylinder, 2θ is the angle bounded

the width of the patch, ε represents electric permittivity and µ is the magnetic permeability as

shown in Figure 4.

3. T

EFLON AS A

S

UBSTRATE IN

M

ICROSTRIP

P

RINTED

A

NTENNAS

T. Seki et.al. introduced a highly efficient multilayer parasitic microstrip antenna array that is

constructed on a multilayer Teflon substrate for millimeter-wave system [16]. This antenna

achieves a radiation efficiency of greater than 91% and an associated antenna gain 11.1 dBi at 60

GHz. The antenna size is only 10 mm × 10 mm. So, using Teflon as a substrate material in

microstrip antennas is highly recommended nowadays, especially in conformal microstrip

antennas for its ability to bend over any surface [17].

4.

E

FFECT OF

T

EMPERATURE ON A

T

EFLON

S

UBSTRATE

P. Kabacik et.al. studied the effect of temperature on substrate parameters and their effect on

microstrip antenna performance [18]. Dielectric constant and dispersion factor are plotted as a

function of temperature for a wide temperature ranges equivalent to those in airborne

applications. The authors used Teflon-glass and ceramic-Teflon materials as a substrate for

microstrip antenna. Also, the authors conclude that, the measured dielectric constant value was

greater than the one specified in the data sheets.

The effect of temperature on Teflon material on the electrical properties is studied by A.

Hammoud et.al. [19]. In this work, the authors indicated that the dielectric properties of Teflon is

temperature dependence as illustrated in the next chapter.

The effect of high temperature on a Teflon substrate material on electrical properties, dielectric

constant, mechanical properties, and thermal properties are also studied [20] - [23].

5. T

EMPERATURE

E

FFECT ON THE

A

NTENNA

P

ERFORMANCE

For a microstrip antenna fixed on a projectile that fly at a long distance, the temperature will be

an issue for the performance of that antenna. A large variation of temperature (-25

0

C, 25

0

C and

75

0

C) will be considered during the studying. The effect of the temperature on the substrate

material of the microstrip antenna is studied in this paper [24].

The Temperature affects the dielectric constant of the substrate and also affects expansion of the

material which increase or decrease the volume of the dielectric with increasing or decreasing the

temperature [25]. The recorded dielectric constant of the Teflon at low frequencies is 2.07 at

room temperature but due to the dependency of the dielectric constant on the operating frequency

[26], the dielectric constant decreases to be around 2.02 at the range of Giga hertz.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

5

The measured relationship between temperature and dielectric constant is given in [26] as shown

in the Figure 3 as an actual data, and the fitted data that we already did using MATLAB software,

as a linear relation, is also shown bellow.

The linear Equation for that relation is illustrated in the following Equation:

(5)

Linear thermal expansion can be calculated as in the following formula [25]:

(6)

where: ∆L

thermal

is the expansion in length.

L is the original length at certain temperature.

α is the coefficient of thermal expansion.

∆T is the difference of temperature.

So, the linear thermal expansion which is represents the ratio between ∆L

thermal

and L is given by

[26], which is shown in Figure 4. The actual and fitted curves and the fitted Equation are given

bellow:

(7)

Figure 3. Dielectric constant vs. temperature for Teflon substrate at the range of GHz.

Hence, we can calculate the effect of temperature on the expansion of the dimensions of the

substrate and on the dielectric constant of the microstrip antenna. The new length or width of the

microstrip antenna will be due to the effect of fringing field and thermal expansion, so the new

length or width will take the form of Equation (8):

(8)

Also, the effect of fringing field and temperature on the dielectric constant of the substrate will be

considered in the calculations of antenna parameters.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

6

Figure 4. Linear thermal expansion vs. temperature for Teflon substrate.

6. R

ESULTS

For a range of GHz, the dominant mode is TM

10

for h<<W which is the case. Also, for the

antenna operates at the range of 2.4 GHz we can get the following dimensions; the original length

is 41.5 mm, the width is 50 mm, substrate height is 0.8 mm and for different lossy substrate we

can get the effect of curvature on the effective dielectric constant and the resonance frequency.

Three different substrate materials RT/duroid-5880 PTFE, K-6098 Teflon/Glass, and Epsilam-10

ceramic-filled Teflon are used for verifying the new algorithm. The dielectric constants for the

used materials are 2.07, 2.5 and 10 respectively with a tangent loss 0.0015, 0.002 and 0.0004

respectively.

The relation between the effective dielectric constant and radius of curvature for different values

of temperature, -25, 25 and 75

0

C is shown in Figure 5.

The relation between curvature and effective dielectric constant was introduced in [27], and by

using the generated model in [27] we can caudate the input impedance, VSWR and return loss.

6.1

RT/duroid-5880 PTFE substrate

Resonance frequency for TM

10

mode is shown in Figure 5. Due to temperature, decreasing in

resonance frequency for every 50

0

C in temperature is almost 40 MHz at radius of curvature 50

mm.

The real and imaginary parts of input impedance are shown in Figure 6 and 7 consequently. The

peak value of input impedance for a real part is 1800 Ω which is higher than the values of TM

01

mode by 800 Ω.

VSWR is given in Figure 8, and gives a minimum value around 4 which is higher than the value

from TM

01

mode. Return loss is around -12 dB, as in Figure 9, which is higher than the value

from TM

01

mode by -10 dB. So, one can note that, better performance can be obtained in case of

TM

01

mode than in case of TM

10

mode, lower return loss and lower VSWR.

The effect of temperature is almost the same for TM

01

and TM

10

modes in normalized electric and

magnetic fields. The effect of temperature is given in Figures 10 and 11 for normalized electric

and magnetic fields respectively.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

7

Increasing temperature increases the angle of radiation of the radiation pattern for electric and

magnetic fields by very small amount for a large value of temperature from -25

0

C to 150

0

C.

Figure 5. Resonance frequency versus radius of curvature for cylindrical-rectangular

and flat microstrip printed antenna at different temperatures 75, 25 and -25

0

C.

Figure 6. Real part of the input impedance as a function of frequency at different

temperatures 75, 27 and -25

0

C and radius of curvature 50 mm.

Figure 7. Imaginary part of the input impedance as a function of frequency at

different temperatures 75, 27 and -25

0

C and radius of curvature 50 mm.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

8

Figure 8. VSWR versus frequency at different temperatures 75, 27 and -25

0

C and

radius of curvature 50 mm.

Figure 9. Return loss (S11) as a function of frequency at different temperatures 75, 27

and -25

0

C and radius of curvature 50 mm.

Figure 10. Normalized electric field for different temperatures 150, 75, 27 and -25

0

C

at θ=0:2π and φ=0

0

and radius of curvature 50 mm

.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

9

Figure 11. Normalized magnetic field for different temperatures 150, 75, 27

and -25

0

C at θ=0:2π and φ=0

0

and radius of curvature 50 mm.

6.2 K-6098 Teflon/Glass substrate

Effect of temperature on a performance of K-6098 Teflon/Glass material is studied in this section.

The effect of temperature on the effective dielectric constant is shown in [23]. Using Figure 12,

we can note that, increasing in temperature leads to increasing in the value of effective dielectric

constant by 0.0007 for each Celsius degree.

Effective dielectric constant increases with increasing the temperature due to two reasons:

1. Increasing temperature leads to increasing the collision between atoms and electrons inside

the material and hence the speed of light inside the material will decrease which leads to

increases the effective dielectric constant.

2. Increasing temperature expands the dielectric material and hence, the distance which electric

field goes inside the substrate increases which means, the effective dielectric constant

increases.

Resonance frequency for TM

10

is shown in Figure 12. AS clearly notice from the Figure the

resonance frequency decreases by 30 MHz for increasing in temperature b 50

0

C. The peak value

of real part of input impedance is higher than in case of TM

01

mode by 300 Ω as shown in Figure

13. Imaginary part is also given in Figure 14.

The value of VSWR is between 2 and 3 as shown in Figure 15 and the value of return loss is

almost -21 dB as shown in Figure 16.

Normalized electric and magnetic fields are shown in Figures 17 and 18 respectively. The same

results are almost obtained as in case of TM

01

mode, small change in radiation patterns due to

temperature change.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

10

Figure 12. Resonance frequency versus radius of curvature for cylindrical-rectangular

and flat microstrip printed antenna at different temperatures 75, 25 and -25

0

C.

Figure 13. Real part of the input impedance as a function of frequency at different

temperatures 75, 27 and -25

0

C and radius of curvature 50 mm.

Figure 14. Imaginary part of the input impedance as a function of frequency at

different temperatures 75, 27 and -25

0

C and radius of curvature 50 mm.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

11

Figure 15. VSWR versus frequency at different temperatures 75, 27 and -25

0

C and

radius of curvature 50 mm.

Figure 16. Return loss (S11) as a function of frequency at different temperatures 75, 27

and -25

0

C and radius of curvature 50 mm.

Figure 17. Normalized electric field for different temperatures 150, 75, 27 and -25

0

C.

at θ=0:2π and φ=0

0

and radius of curvature 50 mm.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

12

Figure 18. Normalized magnetic field for different temperatures 150, 75, 27 and -25

0

C.

at θ=0:2π and φ=0

0

and radius of curvature 50 mm.

6.3 Epsilam-10 ceramic-filled Teflon substrate

For Epsilam-10 ceramic-filled Teflon substrate; the same parameters are also studied in this

section. Due to temperature, the effective dielectric constant increases by 0.00068 for increasing

temperature by one Celsius degree. This value is less than the other two substrates, which is

0.0007 for K-6098 Teflon/Glass and 0.00074 for RT/duroid-5880 PTFE substrate. Hence; we can

conclude that, as the dielectric constant increases, the effect of temperature on the effective value

of dielectric constant decreases.

The resonance frequency for TM

10

mode is shown in Figure 19. The difference between

resonance frequencies due to increasing in temperature by 50

0

C is almost 10 MHz. The effect of

temperature on the real and imaginary parts of input impedance is not as in case of using

substrates of lower dielectric constants. As shown in Figures 20 and 21 respectively, the peak

value is less than in case of TM

01

mode by almost 150 Ω for real and imaginary parts of input

impedance. VSWR and Return loss are shown in Figures 22 and 23 consequently. As in the

previous substrates, the performance in case of TM

01

is better than in case of TM

10

.

Normalized electric and magnetic fields are shown in Figures 24 and 25. As shown in the Figures,

the effect of temperature is very small in case of using substrates have high dielectric constant. In

both transverse magnetic modes and for high range of temperatures, the effect is almost vanishing

for both electric and magnetic fields radiation patterns.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

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Figure 19. Resonance frequency versus radius of curvature for cylindrical-rectangular

and flat microstrip printed antenna at different temperatures 75, 25 and -25

0

C.

Figure 20. Real part of the input impedance as a function of frequency at different

temperatures 75, 27 and -25

0

C and radius of curvature 50 mm.

Figure 21. Imaginary part of the input impedance as a function of frequency

at different temperatures 75, 27 and -25

0

C and radius of curvature 50 mm.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

14

Figure 22. VSWR versus frequency at different temperatures 75, 27 and -25

0

C and

radius of curvature 50 mm.

Figure 23. Return loss (S11) as a function of frequency at different temperatures 75, 27

and -25

0

C and radius of curvature 50 mm.

Figure 24. Normalized electric field for different temperatures 150, 75, 27 and -25

0

C.

at θ=0:2π and φ=0

0

and radius of curvature 50 mm.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

15

Figure 25. Normalized magnetic field for different temperatures 150, 75, 27 and -25

0

C.

at θ=0:2π and φ=0

0

and radius of curvature 50 mm.

7. C

ONCLUSION

The effect of temperature on the performance of a conformal microstrip printed antenna used for

a projectile flight on a high distance is very important to study. The temperature affects the three

different substrates effective dielectric constant and hence affect the operating resonance

frequency for TM

10

mode. The effect of temperature on input impedance, VSWR and return loss

are also studied for a radius of curvature of 50 mm. We notice that, as the temperature increases,

the effective dielectric constant is also increases for different materials used. On the other hand,

the resonance frequency decreases with increasing temperature. VSWR and return loss are

decreasing as the temperature increases.

The change in resonance frequency is between 40 MHz for TM

10

mode. This shift is very small

for a wide range of temperature used, but it is very effective in case of using frequency hopping

technique.

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EFERENCES

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International Journal of Wireless Communications and Networking, July-Dec 2011,

Accepted

Authors

Ali Elrshidi, Ali is Ph.D student in University of Bridgeport. Ali received the Bachelor in

communication engineering from the University of Alexandria, Egypt 2002. He got his

master degree in fiber optics field in 2006 from the same university under supervision of

Prof: Ali Okaz, Prof. Moustafa Hussien, and Dr: Keshk. He works in a project funded by

US Army, to control the motion of a small projectile using two small stepper motors. Also,

Ali has designed a microstip printed antenna works at 2.4 GHz and gives a high

performance.

Dr. Elleithy is the Associate Dean for Graduate Studies in the School of Engineering at the

University of Bridgeport. He has research interests are in the areas of network security,

mobile communications, and formal approaches for design and verification. He has

published more than one hundred fifty research papers in international journals and

conferences in his areas of expertise. Dr. Elleithy is the co-chair of the International Joint

Conferences on Computer, Information, and Systems Sciences, and Engineering (CISSE).

CISSE is the first Engineering/Computing and Systems Research E-Conference in the world to be

completely conducted online in real-time via the internet and was successfully running for four years.Dr.

Elleithy is the editor or co-editor of 10 books published by Springer for advances on Innovations and

Advanced Techniques in Systems, Computing Sciences and Software.

Dr. Elleithy received the B.Sc. degree in computer science and automatic control from Alexandria

University in 1983, the MS Degree in computer networks from the same university in 1986, and the MS

and Ph.D. degrees in computer science from The Center for Advanced Computer Studies at the University

of Louisiana at Lafayette in 1988 and 1990.

International Journal of Next-Generation Networks (IJNGN) Vol.3, No.3, September 2011

18

Hassan Bajwa, Ph.D., is an Assistant Professor of Electrical Engineering at The

University of Bridgeport. He received his BSc degree in Electrical Engineering from

Polytechnic University of New York in 1998. From 1998 to 2001 he worked for

Software Spectrum and IT Factory Inc, NY. He received his MS from the City College

of New York in 2003, and his Doctorate in Electrical Engineering from City University

of New York in 2007. Dr. Hassan research interests include low power sensor networks,

flexible electronics, RF circuit design, Antennas, reconfigurable architecture, bio-

electronics, and low power implantable devices. He is also working on developing

biomedical instruments and computation tools for bioinformatics.