ORIGINAL PAPER

Miniaturization of GPS patch antennas based on novel dielectricceramics Zn(12x)MgxAl2O4 by sol–gel method

Huda Abdullah • Wan Nasarudin Wan Jalal •

Mohd Syafiq Zulfakar

Received: 19 September 2013 / Accepted: 3 December 2013 / Published online: 24 December 2013

� Springer Science+Business Media New York 2013

Abstract The need for miniaturization and weight

reduction of GPS patch antennas has prompted the search

for new microwave dielectric materials. In this study, a

sol–gel method was used to prepare Zn(1-x)MgxAl2O4 thin

films and fabricate GPS patch antennas at a low annealing

temperature (700 �C). X-ray diffraction (XRD) patterns,

field emission scanning electron microscopy images, Fou-

rier transform infrared spectra, and optical band gap anal-

yses confirmed the nanostructure of (Mg/Zn)Al2O4. The

XRD patterns displayed the characteristic peaks of (Mg/

Zn)Al2O4 with a face-centered cubic structure. Mg addition

decreased the crystallite size, surface morphology, and

lattice parameters of the resultant films, evidently affecting

their density and dielectric constant (er). Based on the

material investigated and microwave antenna theory, GPS

patch antennas were fabricated using Zn(1-x)MgxAl2O4 and

then studied using a PNA series network analyzer. The

fabricated patch antennas with different er ceramics

decreased in size from 12.5 to 10.8 cm2. The patch

antennas resonated at a frequency of 1.570 GHz and pro-

vided a return loss bandwidth between -16.6 and

-20.0 dB; their bandwidth also improved from 90 to

255 MHz. The GPS patch antenna fabricated from

Zn0.70Mg0.30Al2O4 showed an excellent combination of

return loss (-20.0 dB), small size (10.8 cm2), and wide

bandwidth (255 MHz). Therefore, addition of Mg

improves antenna performance and decreases the dimen-

sions of the device.

Keywords Mg–Zn–Al2O3 � Patch antennas � Dielectric

constant � Sol–gel method

1 Introduction

Nanostructured transition metal-oxide spinels are important

in many applications because of their low temperature

sinterability, high thermal resistance, and increased hard-

ness [1]; these properties make spinels promising catalytic

or carrier materials and precursors for mixed oxides. Spi-

nels have enormous potential to generate well-dispersed,

active, and very stable catalysts. Zinc aluminate (ZnAl2O4)

and magnesium aluminate (MgAl2O4) are members of a

class of mineral gahnite and the spinel family (AB2O4).

The normal spinel AB2O4 contains A2? and Al3? ions in

tetrahedral and octahedral sites, respectively. Spinels have

a close-packed face-centered-cubic (fcc) structure of oxy-

gen ions [2]. Spinel oxides are wide-band gap semicon-

ductors; for example, the reported optical band gaps of

ZnAl2O4 and MgAl2O4 are in the ranges of 3.8 eV [3, 4] to

4.0 eV [5] and 5.36 eV [2] to 7.8 eV [6], respectively.

These wide-band-gap structures are useful in photoelectric

and optical applications and currently under study as can-

didate materials for reflective optical coatings in aerospace

applications [2, 3]. Considering their wide band gap, spinel

oxides have attracted much interest among researchers for

their potential applications in the telecommunications

industry.

The operating frequency ranges of microwaves for GPS

applications has expanded to the range of millimeter-wave

applications [7]. New ceramic materials that can reduce the

size of GPS components are urgently required. Miniaturi-

zation of GPS patch antennas can be done using a material

with high dielectric constant (er). However, Lei et al. [8]

H. Abdullah (&) � W. N. W. Jalal � M. S. Zulfakar

Department of Electrical, Electronic and System Engineering,

Faculty of Engineering and Built Environment, Universiti

Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

e-mail: huda@eng.ukm.my

123

J Sol-Gel Sci Technol (2014) 69:429–440

DOI 10.1007/s10971-013-3239-7

and Huang [9] reported that materials with er [ 15 are not

particularly interesting because the operating frequency of

GPS has reached the millimeter-wave operation range.

Tsunooka et al. [10] and Kim et al. [11] also reported that

special microwave dielectric ceramics used in millimeter-

wave technology must have ceramics properties with low er

and high quality (Q). High Q can be achieved using

materials with very low dielectric loss and low er [12, 13]

and is necessary to ensure good selection frequency char-

acteristics; this parameter is determined from the dielectric

loss factor of the material. Shannon and Rossman [14]

measured the dielectric loss factor and er of single crystal

MgAl2O4 using a two-terminal method. Microwave

dielectric materials with low er can minimize cross cou-

pling with conductors and shorten the time required for

electronic signal transition [9]. According to Narang and

Bahel [15], low er in the range of 4–12 are used for mil-

limeter-wave applications and as microwave substrates.

Thus, a material with low er and high Q is more functional

than a material with high er and low Q. High-performance

microwave dielectric materials with er \ 15 and high Q are

needed for fabricating GPS patch antennas.

Both ZnAl2O4 and MgAl2O4 ceramics have good

dielectric properties and therefore garnered increased

attention as potential microwave dielectric ceramics for

millimeter-wave applications. Surendran et al. reported that

ZnAl2O4 [16] and MgAl2O4 [17] ceramics exhibit interest-

ing dielectric properties of er = 8.5, Qxf = 56,300 GHz

and er = 8.8, Qxf = 68,900 GHz, respectively. Microwave

dielectric ceramics are suitable and effective for creating

smaller devices by improving the er. The effects of substi-

tuting Mg with Zn in the MgAl2O4 framework have been

studied by several authors [18, 19]. Zheng et al. [18]

improved the Q 9 f value up to a maximal value of 106,000

GHz and showed a er that varied from 7.90 to 8.56. How-

ever, information on the preparation of ZnAl2O4 as well as

its modification by Mg doping as microwave dielectric

ceramics is lacking. Doping of the parent lattice with suit-

able cations and methods during synthesis is done to modify

and improve ceramic properties. Mg was selected as a

doping material because of its relatively high Q, high er,

high melting point (2,130 �C), and stability at service tem-

peratures; these properties indicate that Mg is a suitable

candidate for the production of GPS patch antennas.

In this study, the microwave dielectric properties of

ZnAl2O4 spinels were determined. Tailoring of ZnAl2O4

spinel properties was performed by molar mixing with Mg

to form Zn(1-x)MgxAl2O4 and develop an alternate micro-

wave dielectric ceramic for GPS antennas. The sol–gel

method was used to prepare Zn(1-x)MgxAl2O4 microwave

dielectric ceramics and the GPS antenna. The effects of the

sol–gel method on the structural, morphological, optical,

chemical, and dielectric properties of Zn(1-x)MgxAl2O4 thin

films were investigated. Finally, a microstrip patch antenna

for GPS application was fabricated and its performance was

measured.

2 Experimental procedure

2.1 Preparation of Zn(1-x)MgxAl2O4

Zinc acetate dehydrate Zn(O2CCH3)2, aluminum nitrate

nonahydrate Al(NO3)3�9H2O, and magnesium nitrate

Mg(NO3)2�6H2O were dissolved in absolute ethanol

(C2H5OH). Substitution of Zn2? by Mg2? in the framework

of ZnAl2O4 was completed by the sol–gel method with

different concentrations (x) of Mg (x = 0.00, 0.05, 0.10,

0.15, 0.20, 0.25, 0.30). Calculated amounts of

Zn(O2CCH3)2, Al(NO3)3�9H2O, and Mg(NO3)2�6H2O were

dissolved separately. Initially, Al(NO3)3�9H2O was dis-

solved in 60 mL of absolute ethanol (C2H5OH) and then in

0.5 mL of ethylene glycol (chelating agent). Subsequently,

Mg(NO3)2�6H2O and Zn(O2CCH3)2 were added to the

solution. After storing at approximately 75 �C for 1 h,

0.36 mL of nitric acid was used to prepare a homogeneous

solution. The solution was continuously heated at 75 �C

with constant stirring by a magnetic stirrer for another 1 h

until a clear solution was obtained following a chemical

reaction. The solution containing Zn(1-x)MgxAl2O4 was

stirred for 0.5 h at 180 �C to obtain a transparent and stable

sol.

Zn(1-x)MgxAl2O4 thin films were deposited using the

sol–gel spin coating method. The transparent and stable sol

of Zn(1-x)MgxAl2O4 was prepared on a fluorine tin oxide

(FTO) substrate by the spin coating technique at 3,500 rpm/

min for 35 s and then treated at 85 �C for 15 min. The

coating procedure were repeated 10 times to obtain satis-

factory thickness and remove organic residuals. This tech-

nique was conducted in a clean room facility to avoid

contamination of particles and obtain films with good

optical quality. The thin films were annealed at 700 �C for

1 h at 1 �C/min to eliminate the remaining organic matter.

The nanostructure of ZnAl2O4-doped Mg was charac-

terized by X-ray diffraction (XRD, Siemens D-500) using

CuKa radiation of WL1 = 1.541 A, WL2 = 1.544 A, and

WL3 = 1.392 A. The XRD pattern was collected over the

2h range from 20� to 60� with a selected step scan and step

width of 0.025�. The density of the annealed thin films was

measured by the Archimedes method. The structural mor-

phology and chemical bonding of the thin films was

observed by field-emission scanning electron microscopy

(FESEM, Zeisz Supra 15 kV), atomic force microscopy

(AFM, NTEGRA Prima), and Fourier transform infrared

spectroscopy (FTIR). The er was measured using an LCR

spectrometer (HP 4284A) in the frequency range of 20 Hz

430 J Sol-Gel Sci Technol (2014) 69:429–440

123

to 1 MHz at room temperature. Patch antennas were suc-

cessfully fabricated on a glass substrate, and their return

loss was measured by PNA series network analyzer

(E8358A, Agilent).

3 Results and discussion

3.1 X-ray diffraction studies (XRD)

The ceramics were expected to be generated as the solid

solution Zn(1-x)MgxAl2O4 because ZnAl2O4 and MgAl2O4

have the same spinel structure. The XRD patterns of

Zn(1-x)MgxAl2O4 thin films annealed at 700 �C for 1 h as a

function of Mg content are shown in Fig. 1. The X-ray

diffraction patterns of as-prepared thin films showed the

formation of the same phases of a known Mg/ZnAl2O4

spinel structure. The added Mg was successfully doped into

the ZnAl2O4 lattice and emerged as the same phase in

samples with higher Mg loading. The nanostructure of

ZnAl2O4 corresponded to the standard pattern of fcc [20]

with a dominant peak (311) that could be indexed using

JCPDS File Card No. 00-005-0669. Mg/ZnAl2O4 peaks

appeared at 2h = 31.17� (220), 36.75� (311), 45.52� (400),

48.57� (331), 55.55� (422), 58.92� (511). The observed

diffraction peaks of ZnAl2O4 are consistent with those

reported by Abdullah et al. [21] and Martins and Serra [22],

who also prepared ZnAl2O4 by sol–gel method.

With increasing Mg concentration in the ZnAl2O4, no

additional phase was formed following the reaction

between the two spinels. Therefore, the peaks corre-

sponding to MgAl2O4 (JCPDS File Card No. 01-077-0435)

show the same peaks as ZnAl2O4, except for peak (311).

Peak (311) disappeared with increasing Mg concentration

because this peak does not belong to the MgAl2O4 struc-

ture, as reported by Chandradass and Kim [23] and Su et al.

[24]. XRD analyses confirm that the Mg/ZnAl2O4 peak

appears when Mg is added and suggest that increasing

amounts of Mg increase the intensity of the ZnAl2O4 dif-

fraction peaks [24] because the ionic radius of Mg2?

(0.65 A) [25] is smaller than that of Zn2? (0.74 A) [26].

The diffraction peaks in the XRD spectra shift toward

higher angles in Fig. 1, which implies that a solid solution

is formed with increasing x and that the unit-cell parameter

decreases. This peak shift and distortion occurred because

of the incorporation of Mg2? and Zn2? [27], which have

different ionic radii.

The lattice parameter a, unit cell volume, density, and

structural parameters are presented in Table 1. The crys-

tallite size of MgxZn(1-x)Al2O4 heat-treated at 700 �C was

calculated from the XRD data. The crystallite size was

calculated from the broadening peak (311) of the XRD data

using Scherrer’s equation:

D ¼ 0:94 k= b Cos hð Þð Þ ð1Þ

where k is the X-ray wavelength (0.15418 nm) and b is the

full-width at half maximum intensity of the diffraction line.

The calculated crystallite size of ZnAl2O4 is 21.44 nm,

consistent with that reported by Da Silva et al. [28] and

Charinpanitkul et al. [29]. By increasing the amount of Mg

loading, the crystallite size decreases linearly from 19.17 to

16.33 nm. This decrease also occurs when ZnAl2O4 is

doped with cobalt [30]. The reduction in crystallite size

with increasing Mg load may be due to the high energy

required by Mg2? ions to enter the lattice host and get

substituted for complete grain crystallization and growth.

Furthermore, crystallinity decreased with the incorporation

of Mg2? ions because of the large degree of disorder the

Fig. 1 Shows the XRD patterns

of Zn(1-x)MgxAl2O4 thin films

annealed at 700 �C for 1 h

J Sol-Gel Sci Technol (2014) 69:429–440 431

123

ions produce in the lattice host as a result of ionic radius

mismatches between Mg2?, Zn2?, and Al3? ions [1]. The

crystallite size may be affected by the ionic radius of Mg2?

(0.65 A) [25], which is smaller than that of Zn2? (0.74 A).

The linear relationship between a and the crystallite size

with Mg addition was fairly consistent with Vegard’s law

[31].

The experimental a of the fcc spinel phase of ZnAl2O4

in the films are shown in Table 1. The a of fcc

Zn(1-x)MgxAl2O4 was calculated based on the XRD pat-

terns using the following equation:

a ¼ dhkl h2 þ k2 þ l2� �

ð2Þ

where d is the interplanar distance (A), and h, k, and l are

the Miller indices of the peak (311). The calculated a of

ZnAl2O4 (8.0848 A) was in agreement with the reported

value a = 8.083–8.095 A [16, 32, 33], theoretical value

(8.05 A), and JCPDS File Card No. 00-005-0669. The

a value changed because peak 311 was shifted upon the

substitution of Zn2? by Mg2? in the ZnAl2O4 framework

(Fig. 1). The experimental values of a upon Mg substitu-

tion of Zn decreased from 8.078 A (x = 0.05) to 8.021 A

(x = 0.30). The relationships between crystallite size and

a show a linear relationship with Mg loading, as illustrated

in Fig. 2.

The densities of Zn(1-x)MgxAl2O4 thin films are also

shown in Table 1. The density of annealed pure ZnAl2O4

was measured as 4.61 g/cm3, slightly higher than the theo-

retical density (4.58 g/cm3) [34]. The density of the annealed

thin films was measured by the Archimedes method from the

XRD data. The densities increased with increasing Mg

concentrations (x) until x = 0.10 and then slightly decreased

at x = 0.15 and x = 0.20. The densities increased again

thereafter. This phenomenon is attributed to the increase in

volume ratio of MgAl2O4 and the simultaneous decrease in

volume ratio of ZnAl2O4, which indicates that the particles in

the (Mg/Zn)Al2O4 film are not compact and less uniform.

This behavior can be observed in the FESEM images. This

phenomenon could also occur because of smaller grain sizes

(Fig. 2), ion volatilization, and other defects that appear in

(Mg/Zn)Al2O4 ceramics, which could lead to a decrease in

density. Such behavior can be expected because the density

of MgAl2O4 (3.58 g/cm3) is lower than that of ZnAl2O4

(4.58 g/cm3) [16, 34].

3.2 Morphological observation

AFM images of three-dimensional Zn(1-x)MgxAl2O4

measured at room temperature are shown in Fig. 4. The

scanning size was 1 lm 9 1 lm. The AFM images reveal

the formation of a uniform surface with valleys as the Mg

content increased. The valleys disappeared at x = 0.30

(Fig. 4g). The roughness average (rms) of ZnAl2O4 was

approximately 37.8 nm (Fig. 4a). The rms decreased

from 14.7 to 2.0 nm because of increasing Mg content,

which results in the formation of a solid solution of

0.00 0.05 0.10 0.15 0.20 0.25 0.30

16

18

20

22

Lattice Parameter Crystallite Size

Concentration (x)

Cry

stal

lite

Siz

e (n

m)

8.0108.015

8.0208.025

8.0308.0358.040

8.0458.050

8.0558.0608.065

8.0708.075

8.0808.085

Lattice Param

eter (A)

Fig. 2 Crystallite size and lattice parameter of Zn(1-x)MgxAl2O4

with different Mg-content

0.00 0.05 0.10 0.15 0.20 0.25 0.30

4.660

4.665

4.670

4.675

4.680

4.685

4.690

4.695

4.700

4.705

App

aren

t Den

sity

(g/

cm3 )

Mg Concentration (x)

Fig. 3 The apparent densities of Zn(1-x)MgxAl2O4 thin films

prepared by sol–gel method

Table 1 Structural parameters of Zn(1-x)MgxAl2O4 thin films

Mg-content (x) D (nm) a (A) hkl Apparent

density (g/cm3)

0.00 21.44 8.048 311 4.67

0.05 19.17 8.043 311 4.68

0.10 18.49 8.037 311 4.69

0.15 18.25 8.043 311 4.67

0.20 17.60 8.048 311 4.68

0.25 17.09 8.037 311 4.69

0.30 16.33 8.032 311 4.70

432 J Sol-Gel Sci Technol (2014) 69:429–440

123

(Mg/Zn)Al2O4 and increases in ceramic density and er [35].

The doped samples (Fig. 4b–g) had smoother surface areas

compared with ZnAl2O4 (Fig. 4a) because of decreased

grain and crystallite sizes. The decrease in rms and grain

size data (Fig. 4h) occurred in parallel with the decrease in

crystallite size in the XRD data.

3.3 Field-emission scanning electron microscopy

(FESEM)

SEM images of the Zn(1-x)MgxAl2O4 ceramics annealed at

700 �C for 1 h are shown in Fig. 5. In general, all of the

films were well annealed, and the average grain sizes

Fig. 4 AFM images (a–g); and graph of crystallite size and lattice parameter of Zn(1-x)MgxAl2O4 with different Mg-content

J Sol-Gel Sci Technol (2014) 69:429–440 433

123

decreased with increasing x. For x = 0.05, 0.10 (Fig. 5b,

c), the ceramics contained two types of grains (bright

grains and dark grains), indicating the coexistence of two

types of crystal phases. Morphological study results

showed spherical particles (dark grains) in the undoped

sample (Fig. 5a); these particles increased in size to form

nanoflake structures (bright grains) when ZnAl2O4 was

doped with Mg at early stages (Fig. 5b, c). The nano-

structure of MgAl2O4 consisted of small flake-shaped

particles. The flake-shaped structures of MgAl2O4

increased whereas the sphere-like grain sizes of ZnAl2O4

decreased with increasing Mg concentration. This decrease

in grain size occurred in parallel with the decrease in

crystallite size according to the XRD data (Fig. 2) and

grain size according to the AFM data (Fig. 4h).

As x increased, the average grain size of the Zn(1-x)

MgxAl2O4 thin films decreased from 69 nm (x = 0.00) to

43 nm (x = 0.30) because of the smaller grain size of Mg2?

(0.65 nm). All samples in the present study showed

decreases in average grain size, which indicates the

replacement of Zn2? ions with different amounts of Mg2? by

substitution. The decrease in ceramic grain size consider-

ably degraded the quality factor (Q 9 f) because of the

formation of pores [9, 27, 36]. This phenomenon occurred at

x = 0.15 (Fig. 5d) and x = 0.20 (Fig. 5e). However, at

x = 0.05 and x = 0.10, the number of pores decreased

because of minor Mg addition compared with the undoped

sample (Fig. 4a). Decreases in the number of pores, which

may improve Q 9 f, are observed in Fig. 5f, g. The decrease

in the number of pores at x = 0.05, 0.10, 0.25, 0.30 is due to

the increasing number of flake-shaped particles and uniform

and compact grain size, both of which improve the Q 9 f.

3.4 UV–Vis analysis

The absorption spectra of Zn(1-x)MgxAl2O4 particles with

different compositions of x = 0.00, 0.05, 0.10, 0.15, 0.20,

0.25, 0.30 are measured in the wavelength range of 200–

800 nm. The energy band gap (Eg) can be determined by

plotting the curve of (ahm)2 against the photon energy, hm,

and extraploting the linear portion of the curve to the hmaxis. The Eg for the films can be calculated using the fol-

lowing equation [4, 37]:

/ ¼ A

hmhm� Eg

� �1=2 ð3Þ

where a is the absorption coefficient and A is a constant

depending on the type of transition. Equation (2), for any

x = 0.00 x = 0.05 x = 0.10

x= 0.15 x =0.20 x = 0.25

x = 0.30

Fig. 5 SEM images of Zn(1-x)MgxAl2O4 with different Mg-content

434 J Sol-Gel Sci Technol (2014) 69:429–440

123

0.00 0.05 0.10 0.15 0.20 0.25 0.30

3.8

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

Ban

d G

ap (

eV)

Mg Concentration (x)

(h) x = 0.30 (g) x = 0.30

(a) x = 0.00 (b) x = 0.05

(c) x = 0.10 (d) x = 0.15

(e) x = 0.20 (f) x = 0.25

Fig. 6 The optical band gap (a–g) and graph of variation band gap (h) of CaxZn(1-x)Al2O4 thin films

J Sol-Gel Sci Technol (2014) 69:429–440 435

123

energy, can be rearranged and written in the following

form:

/hmð Þ2¼ A2 hm� Eg

� �ð4Þ

The relationship between (ah)2 and hv is presented in

Fig. 6, where the Eg value of ZnAl2O4 is 3.84 eV (Fig. 6a)

and that of Zn(1-x)MgxAl2O4 increases linearly from 4.0 to

5.3 eV (Fig. 6h). The Eg of ZnAl2O4 obtained in this work

is similar to the results of Wan Jalal et al. [3, 4]. This work

further reports the band gap of Zn(1-x)MgxAl2O4 for the

first time. Zn(1-x)MgxAl2O4 Eg values of over 3.84 eV may

have been influenced by the effect of the Mg band gap

(from 5.36 eV [2] to 7.8 eV [6]). The Eg may also increase

because of decreases in intermediary energy levels within

the optical band gap.

The Eg values of ZnAl2O4 thin films doped with Mg are

also related to the form of the solid solution, as discussed

using the XRD data. Samples with a smaller crystallite size

and with the form of a solid solution without any defects

show higher Eg values [38]. Thus, the Eg of films with Mg

content is associated with the contributions of intermediary

energy levels of both ZnAl2O4 and Mg phases (Fig. 6b–g).

We suggest that the ideal Eg of (Mg/Zn)Al2O4 ranges from

3.84 to 5.3 eV. Wan Jalal et al. [4] reported that decreasing

the Eg can improve dielectric properties, which is suitable

for generating patch antennas.

3.5 FTIR analysis

FTIR analysis of the Mg-doped ZnAl2O4 thin films was

performed in the wave number region from 400 to

4,000 cm-1. The absorption spectra of the Zn(1-x)MgxAl2O4

thin films are shown in Fig. 7. The broad absorption band

centered at 3,400–3,852 cm-1 is assigned to the stretching

vibrations of O–H groups brought about by the water content

of the crystallites. The band at 2,332 cm-1 can be attributed

to the presence of oxygen bonds in the fcc crystal lattice of

oxygen atoms. This peak is observed in all spinel compounds

and can be treated as a characteristic feature of spinel-

structured crystallites [4]. The intensity of the peak at

2,332 cm-1 decreased as the amount of Mg increased, which

indicates Mg occupation of tetrahedral (AlO3) sites. The

absorption band at around 1,919 cm-1 corresponds to O–H

bending vibrations attributed to free water molecules [39].

The spectra of all of the samples showed a peak at

1,333 cm-1 attributed to Al–O stretching vibrations [1].

Peaks in the range of 400–900 cm-1 are attributed to

metal–oxygen, aluminum–oxygen, and metal–oxygen–alu-

minum stretching [1, 33]. The band attributed to (Zn/

Mg)Al2O4 spinel appeared at approximately 648 cm-1.

According to Kim et al. [40], band characteristics of the

spinel structure are well observed at approximately

469 cm-1. These bands correspond to the bending modes of

ZnO, MgO, and AlO6, all of which form the (Zn/Mg)Al2O4

spinel thin films [41].

3.6 Dielectric constant

The dielectric constant, eris consists of real permittivity or

relative dielectric constant which is generally called

dielectric constant of the test material and imaginary per-

mittivity or dielectric loss. The er is calculated from the

capacitance value, which can be obtained using the fol-

lowing equation:

er ¼ Cd=eoA ð5Þ

where C is the capacitance of the capacitor formed by

inserting a sample between two metal plates, d is the

Fig. 7 FTIR spectra of

Zn(1-x)MgxAl2O4 thin films.

(a) x = 0.00, (b) x = 0.05,

(c) x = 0.10, (d) x = 0.15,

(e) x = 0.20 and (f) x = 0.30

436 J Sol-Gel Sci Technol (2014) 69:429–440

123

thickness of the sample, eo is the permittivity of free space,

and A is the cross sectional area of the sample.

The relative permittivity er of the samples as a func-

tion of the frequency of the applied LCR spectrometer

ranging from 20 Hz to 1 MHz at room temperature is

shown in Fig. 8a. The er of the samples decreased with

increasing frequency, which can be explained by Koop’s

phenomenological theory [42] and the Maxwell–Wagner

model of interfacial polarization [43]. The er of the

Zn(1-x)MgxAl2O4 ceramics as a function of Mg content is

shown in Fig. 8b.

The er (*8.53) of the ZnAl2O4 ceramics was similar to

that found by Surendran et al. [16]. The erof samples

annealed at 700 �C increased slightly with increasing Mg

at x = 0.05, 0.10 but decreased slowly at x = 0.15, 0.20

and further increases in x. The er value increased again up

to 9.35 at x = 0.25, 0.30. This phenomenon is related to the

porous morphology of films with high Mg content, as

shown in Fig. 5d, e. The variation in er (Fig. 8) was con-

sistent with that of the apparent density (Fig. 3).

The increase in dielectric constant may be expected

because the er of MgO (er * 9.80) [44] or MgAl2O4

(er * 8.75) [17] is slightly higher than that of ZnAl2O4.

The combination of ZnAl2O4 and Mg can increase the er.

Higher er values of doped samples indicate that the ion

polarizability (a) of ZnAl2O4 is higher than that of the

Mg2?. Thus, the er of ZnAl2O4 doped with Mg depends

on the of cations and oxygen at microwave frequency

[35]. The er value increased as the amount of Mg and

total polarizability increased. Normally, the change in er

values follows the logarithmic mixing rule [45] as

follows:

ln er ¼ v1 ln er1 þ v2 ln er2 ð6Þ

where er1 and er2 are the dielectric constants of phases with

volumes v1 and v2, respectively.

3.7 Antenna design and GPS performance

GPS patch antennas were fabricated using the

Zn(1-x)MgxAl2O4 ceramics. The design of patch antennas

involves computations of their patch dimensions. The

dimensions of the patch depend on the er values of

Zn(1-x)MgxAl2O4 ceramics and FTO substrates. The sizes

of GPS patch antennas were calculated based on micro-

wave theory using the following equation [46]:

W ¼c

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2= er þ 1ð Þ2

q

2f0

ð7Þ

Normally, the patch width (W) has a minor effect on the

resonant frequency (fo). The patch length (L) determines

the resonant frequency (fo) and can be regarded as a critical

parameter in the design because of the inherent narrow

bandwidth of the patch. The design for L is given by the

following equation [46];

L ¼ C

2foffiffiffiffiffiffiffieeffp � 2DL ð8Þ

where C is the speed of light in free space (vacuum) or

velocity of electromagnetic wave and er is the relative

permittivity of the dielectric material. The effective relative

er (eeff) is provided in terms of the microstrip patch lines

between air and the er by the following equation [46]:

eeff ¼er þ 1

2þ er � 1

2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ 10h/wp ð9Þ

where h is the height of the patch. The additional line

length (DL) on either end of the patch length attributed to

the effect of fringing fields is given by the equation [46]:

DL ¼ 0:412heeff þ 0:3ð Þ w

hþ 0:262

� �

eeff � 0:258ð Þ whþ 0:813

� � ð10Þ

4

5

6

7

8

9

10

Die

lect

ric

Con

stan

t, r

Log frequency (f )

x=0.00 x=0.05 x=0.10 x=0.15 x=0.20 x=0.25 x=0.30

2 3 4 5 6 0.00 0.05 0.10 0.15 0.20 0.25 0.308.0

8.2

8.4

8.6

8.8

9.0

9.2

9.4

9.6

Mg content (x)

Die

lect

ric

Con

stan

t, r

(a) (b)

ε ε

Fig. 8 Variation of dielectric constant of Zn(1-x)MgxAl2O4 with applied frequency

J Sol-Gel Sci Technol (2014) 69:429–440 437

123

The patch length (L) can be calculated as;

L ¼ Leff�2DL ð11Þ

Using er = 8.53 (x = 0.00) and 9.35 (x = 0.30), the

antenna size decreased from 12.5 to 10.8 cm2. The surface

of the patch antenna and ground plane was coated with

200 nm thick silver. Both surfaces were coated to enhance

ohmic contact using the magnetron sputtering technique.

An SMA coaxial connector was attached to the antenna

model by soldering to accommodate an input impedance of

50 ohms. Photographs of the fabricated GPS patch

antennas and measured return losses are shown in Fig. 9.

Fig. 9 Photograph (a) fabricated GPS antenna of GPS patch antenna; and return loss measurement b ZnAl2O4; c Zn0.95Mg0.05Al2O4;

d Zn0.90Mg0.10Al2O4; e Zn0.85 Mg0.15Al2O4; f Zn0.80Mg0.20Al2O4; g Zn0.75Mg0.25Al2O4 and h Zn0.70Mg0.30Al2O4

438 J Sol-Gel Sci Technol (2014) 69:429–440

123

The return loss characteristics of the fabricated antennas

were measured using a PNA series network analyzer

(E8358A, Agilent) at room temperature. Measurements of

all seven antennas were carried out Measurements of all

seven antennas were carried out over the frequency range

of 1.0–2.0 GHz with a frequency step size of 20 MHz to

match the resonant frequency (1.575 GHz); results are

plotted in Fig. 9b–h.

The measured result shows that the all antennas was

achieved the return loss (\-10 dB) and impedance band-

width (90–255 MHz), to accommodate an efficient antenna

feed for GPS application. Among these antennas, the

Zn0.70Mg0.30Al2O4 shows an excellent combination

between return loss (-20.0 dB), small in sizes (10.8 cm2)

and wide bandwidth (255 MHz). Sotoudeh et al. [47]

reported, the antenna with a bandwidth greater than the

minimum GPS specification ([2.046 MHz) ensures full

functionality. Thus, we concluded that the magnesium

(Mg) is an effective dopant material with ZnAl2O4 to

improve the performances of GPS patch antenna.

4 Conclusions

Zn(1-x)MgxAl2O4 microwave dielectric ceramic thin films

were successfully synthesized through the sol–gel method at

low temperature sinterability (700 �C in air for 1 h). This

method was suitable for synthesizing ZnAl2O4:Mg and

yielded a single-phased nanostructure showing high quality

and satisfactory crystallinity of spinel structure. A Zn(1-x)

MgxAl2O4 solid solution with a spinel structure was

observed in the ZnAl2O4 and MgAl2O4 system. Mg was

successfully doped in the ZnAl2O4 lattice and emerged as

the same phase even at higher Mg contents. The nano-

structure of (Mg/Zn)Al2O4 corresponded to the standard

pattern of fcc with a dominant peak of (311). Addition of Mg

decreased the crystallite size, grain size, a, and surface

morphology of the films. Patch antennas with various

dimensions were fabricated to operate at 1.5754 GHz (L1

band). The performance of GPS patch antennas were dem-

onstrated by experimental methods and measured using a

PNA series network analyzer. The return loss of the antennas

improved from -16.6 to -20.0 dB, and the devices oper-

ated at a frequency of 1.570 GHz. Measured bandwidths in

the range of 90–255 MHz were further obtained. The GPS

patch antenna with the best performance was obtained using

the Zn0.70Mg0.30Al2O4 specimen. Addition of Mg improved

the performance of the GPS patch antennas.

Acknowledgments This project was performed in the Photonic

Technology Laboratory, Institute of Microengineering and Nano-

electronics (IMEN), Universiti Kebangsaan Malaysia, Malaysia.

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