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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
13
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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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.
