Indian Journal of Radio Space Physics
Vol. 34, April, 2005, pp. 131-134
Parametric study of temperature sensitivity for microstrip patch antenna
Surendra Kumar Roy 1, Santosh Kumar Jha 2 & Lalan Jha 1
2
1
University Department of Physics, L N Mithila University, Darbhanga 846 004
Department of Electronics Engineering, UNS lnstituie of Engineering and Technology
YBS Purvanchal University, Jaunpur (U.P.)
Received 28 November 2003; revised 19 August 2004; accepted I November 2004
Microstrip substrates were exposed to large temperature variations and their temperature dependent properties were
measured. A parametric study of the temperature sensitivity for circular microstrip patch antenna utili sing a substrate with a
lower dielectric constant and thermal coefficient is found to be less sensitive to the temperature variations.
Key words: Environmental conditions, Microstrip patch antenna, Temperature sensitivity
PACS No.: 84.40 Ba
IPC Code: HOIQ9/00; H01Q21/00; H01Q23/00
1 Introduction
Due to their significant merits, microstrip patch
antennas have widely been used in various
communication systems. These antennas, being low
profile and light-weight, are becoming popular for
various applications. In some of these applications, a
patch antenna is required to operate in an environment
that is close to what is defined as room or standard
conditions. However, antennas often have to work in
harsh environments characterized by large temperature
variations. The result is that the electrical properties of
microstrip patch antennas suffer from unwanted
variations.
In the common approach to the design of microstrip
antennas, the designers rely on data provided in the
manufactures' specifications, even though such
specifications are confined to standard environmental
conditions. In practice, the electrical parameters of the
substrates may deviate from the manufacturers' data,
thus making the antenna designer adopt a deficient
design strategy.
In thi s paper mjcrostrip substrates were exposed to
large temperature variations and their temperature
dependent properties were analyzed for the analysis of
characteristic behaviours of microstrip patch antennas.
A parametric study of the temperature sensitivity for
microstrip patch antenna for different types of
substrates has been carried out.
2 Theoretical analysis
The resonant frequency of a microstrip antenna is
sensitive to large temperature variations. There are
two major factors affecting the resonant frequency of
a microstrip radiator exposed to large temperature'.
2.1 Metallic expansion or contraction
The metallic expansion or contraction of the
radiating patch due to a change in temperature affects
the resonant frequency. With an increase in
temperature, the metallic patch expands, making the
effective resonant dimension longer and, therefore,
decre~sing the operating frequency. The relative
frequency change for dimensional changes may be
expressed in terms of linear dimensions or in terms of
2
temperature change as follows ·3 :
of I fo =- oL I L = ad oT
.. . ( I )
where
of
oL
ad
oT
Change in resonant frequency
Change in effective resonant dimension
Thermal coefficient of expansion
Temperature change in °C.
2.2 Effective dielectric constant change
Most of the substrates which are generally used for
microwave applications like polytetra fluroethyl ene
based materials, teflon/fiberglass reinforced materials
and ceramic powder filled TFE (epsilam) materials
exhibie a decrease in dielectric constant with an
increase in temperature. The change in operating
frequency of a microstrip antenna due to a small
change in E, can be expressed as follows 2 :
INDIAN J RADIO & S PACE PHYS, APRIL 2005
132
8flfo=-1128E,!E,=+ 1/2ae 8T
... (2)
where, OE r is the change in E r and ae the thermal
coefficient of dielectric constant.
The change in resonant frequency due to a
temperature variation 8T is given b/-6
8f/ fo =(-ad + 1/2 oF) 8T
3 Temperature
parameters
dependence
... (3)
of
substrate
Of the various materials used in microstrip
ceramic
and
polystyrene
technology,
teflon,
laminates, dielectric foams and honey comb
composites have already found wide acceptance.
The investigated substrates were divided into four
categories, namely, A, B, C and D according to the
composition of the material and the temperature
dependence of dielectric constant. Categories A and B
included teflon-glass microwave laminates. Substrates
of category A were characterized by dielectric
constants whose values decrease with temperature
where the drop had approximately constant gradients,
which varied slightly when a phase transition of glass
occurred. We note that th e phase trarlSltron
phenomenon involves changes in molecular thermal
mobility , which is associated with transformation s of
th e amorphous phase of mate rials from a glassy to a
rubbery state 6 . Phase transition in g lass occurs at 20300C (68-86"F) and is accompanied by a variation in
th e dielectric constant and di ss ipati o n factor and by an
increase in the thermal expansion coefficient.
Category B laminates were characterized by a
dielectric constant with a grad ie nt val ue diffe ring
markedly in, at least, two tem perature subranges of
the diel ectric constant plot. Regardless of this effect,
the laminates experienced phase transition. It is
worthwhile to menti on that th e category 8 lam inates
had dielectric constants, which were almost
independe nt of temperature in th e lower temperature
subrange and showed a re markable temperature
dependance in the upper subrange.
Category C materials were laminates with
uniformly dispersed ceramic filter in a teflon matrix.
Typical examples of this category are RT/duroid 60 I 0
and AR I 000. The measured dielectric constant value
changed by 6.9%. There was negligible variation in
the dielectric constant with frequency in the
temperature range of -60°C to +25°C (77"F) and a
slight variation with frequency in the higher
temperature range, reaching a maximum of 0.8 % at
+SO"C.
One of the recently recommended category D
materials for use in microstrip antennas is a quartzbased composite, which consists of short quartz fibres
bonded together. The development of this material
was originally stimulated by the needs of thermal
insulation engineering.
Compared to the materials of category A, B, or C,
the dielectric constant value of the category D
material varied only slightly, i.e. from 1.718 at 120"C to 1.645 at + 160°C (4.3%). The dissipation
factor turned out to be ultra low, varying from
0 .00043 at- 120"C to 0.0005 at+ 160"C.
4 Results and discussion
In the analysis, E, is used for smaller hlr and Edyn
is used for larger h/r since the capacitive effect of a
circular microstrip antenna (CMSA) increases with
lzlr (h being substrate thickness and r the radius of
CMSA). Simulations have been carried out for
different radii and substrate parame ters for th e
CMSA. The reported 6 8 and simulated results are
shown in Table I. The feed point locat ions are
spec ified only for the last four cases in the reported
results and hence, the feed point locations are
calculated for the other cases to match a 50 Q input
impedance.
The measured temperature characteristics of th e
dielectric constant for two laminates representin g
categories A and 8 , respectively , are plotted in Fig. I.
As shown in Fig. I, the measured values for material
A ranged from 2 .82 at -60°C to 2.56 at +80°C. There
was a noticeable phenomenon of g lass phase
transition as observed at 23 °C. For the material
representing category 8, th e measured dielectric
constant ranged from 2.46 at -60°C to 2.45 at +20"C
and 2.27 at + 80"C. The changes in the dielectric
constant as a function o f frequency for laminates A
and 8 were less noticeable. The relative change in th e
dielectric constant as a function of frequency was less
than 0.15% for material A and less than 0.5 % for
material B. The measured temperature c haracteristics
of the dissipation factor for both laminates are
presented in Fig. 2. The measured value for laminate
A approached 0.005 and was almost frequency
independent. This value was much lowe r than the one
quoted in the manufacturer's specification. The
highest measured value of the dissipation factor for
la minate B was close to 0.0017 at temperatures
between + 30"C and + SO"C and compiled with the
data sheets only for a limited temperature range.
133
ROY et a/.: PARAMETRIC STUDY OF TEMPERATURE SENSITIVITY FOR MICROSTRIP PATCH ANTENNA
Table I--Comparison of results for CMSA
Case no.
2
3
4
5
6
7
8
9
10
II
12
13
em
h
em
Feed point
em
3.493
1.270
3.493
13.894
4.950
3.975
2.990
2.000
1.040
0.770
6.700
3.000
4. 190
0. 1588
0.0794
0.3 175
1.2700
0.2350
0.2350
0.2350
0.2350
0.2350
0.2350
0.1590
0. 1590
0.1590
2.43
1.65
2.30
9.00
3.75
3.03
2.25
1.48
0.73
0.51
5.02
2.05
3. 17
r
E,
2.50
2.59
2.50
2.70
4.55
4.55
4.55
4.55
4.55
4.55
2.62
2.20
2.50
%error
Simulated
Reported
frequ ency, MHz frequency, MHz
1570
4070
1510
378
825
1030
1360
2003
3750
4945
798
1920
1304
- 1.53
+0.98
+1 .65
- 1.06
- 1.2 1
-2.33
+0.51
+0.45
-0.93
- 1.23
+0.88
- 1.1 5
-0.46
1547
4111
1536
375
816
1007
1368
2013
3716
4885
806
1899
1298
0.002
0.
---- ----------------- - -- - ------Material
A
(specifKalion I
f-
~
VJ
z
f-
z
0
0
u
u
;:::
!--
Vi
Ul
i5
Cl.0012
<
0..
~
u
Vl
_J
0.001
Ul
5
0.
Material
Malorial
0
B
tspeciicalion I
A
20
LO
60
eo
TEMPERATURE , •c
TEMPERATURE. "C
Fig. 1-Measured di electric constant for two categori es (A and B)
of glass-reinforced teflon laminates
Hav in g meas ured the values of th e dielectric constant
for lam inates A and B, it was interestin g to compare
them with the values specified by the manufacturers.
5 Conclusions
The parametric study of c ircul ar microstrip patch
antenn a has been carri ed out. The simulated resonant
frequency values are in good agreement with the
reported values. The error in the resonant frequency
for most of the cases is around I%, however, the error
in the worst case is 2.33%.
A thorough study is made to see the performance
of microstrip patch an tenn as th at are exposed to large
temperature variations . The obtain ed results allow the
following general comments. For microwave
Fig. 2-Measured dissipation factor (tan 8) for two categories (A
and B) of glass-re inforced tefl on laminates
laminates, the measured dielectric constant and
dissipation factor often differ from the values
reco mmended in the data sheets. What is more, in all
of the investigated cases the measured dielectric
constant value was greater than the one specified in
tne data sheets. In general, the man ufacturers'
specifications are valid only for a limited temperature
range. Hence, the values provided by re levant data
sheets are inadequate when the substrate is exposed to
large temperature variations. This deficiency is the
temperature depende nce of the dielectric constant of
th e microwave laminates studied.
In consequence, the electrical characteristics of
microstrip patch antennas that involve layered
dielectrics
are
considerably
influenced
by
temperature. A microstrip patch antenna utili sing a
INDIAN J RADIO & SPACE PHYS, APRIL 2005
134
substrate with a lower dielectric constant and thermal
coefficient is less sensitive to the temperature
variations.
Acknowledgement
The authors are thankful to CSIR, New Delhi, for
sponsoring the project.
2
3
4
5
6
7
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