# Narrowband-to-Narrowband Frequency Reconfiguration with Harmonic Suppression Using Fractal Dipole Antenna

Authors: S. A. Hamzah; M. Esa; N. N. N. A. Malik; M. K. H. Ismail
Journal: International Journal of Antennas and Propagation
 (2013)

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## Abstract

Harmonic suppressed fractal antenna with switches named TMFDB25 is developed to select desired frequency band from 400 MHz to 3.5 GHz. The radiating element length is changed to tune the operating frequency while the stub is used to eliminate the undesired harmonic frequency. The balun circuit is reduced by 75% from the original size. The antenna is built on a low loss material. It has the ability to select a single frequency out of fifteen different bands and maintain the omnidirectional radiation pattern properties. Furthermore, the antenna is designed, built, and tested. Simulation and measurement results show that the antenna operates well at the specific frequency range. Therefore, the antenna is suitable to be used for switching frequencies in the band of TV, GSM900/1800, 3G, ISM 2.4 GHz, and above.


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## Body

## 1. Introduction

Narrowband-to-narrowband frequency reconfiguration with harmonic suppression is currently preferred to support communication system, particularly wireless application, due to single type of this antenna that can reduce the size of the RF front-end electronic circuits. However, this type of antenna operates at one specific frequency at one time. Besides, the implementation of switch can cause additional loss and thus reduce antenna efficiency. It is noteworthy that high bandwidth tunable reconfigurable antenna has several advantages if used compared to the initial antenna geometry. In contrast, the existence of high frequency mode may interfere the system operation and will require filter circuit at the output. This in turn will raise the complexity of the terminal circuit and the overall size of system. This problem can be overcome by using the state of art of tunable reconfigurable antenna which is able to suppress harmonic frequencies. In addition, its compact size has attracted many antenna designers in recent years.Recently, combining narrowband configuration to tunable narrowband in order to eliminate harmonic frequencies is a new approach. The combination of tunable frequency, harmonic suppression, and size reduction technique is very important and a key to construct the most optimum reconfigurable antenna. The use of fractal antenna can decrease the physical size of the antenna.As far as harmonic suppressed antenna (HSA) with switch is concerned, the harmonic suppressed reconfigurable antennas have been published previously in the literature [1, 2]. The work on this type of antenna has been reported by using a linear dipole and log periodic dipole arrays, respectively. The first idea to create such antenna is presented in [1]. In a reconfigurable linear dipole in [1], 24 switches were used to control the antenna and the stub length. The antenna can change the operating frequency from a narrowband-to-narrowband features as well as suppress the harmonic frequency in the frequency range from 900 MHz to 3.5 GHz. However, this antenna is large due to the antenna itself as well as the tapered balun used. The latter has 28 switches [2]. The antenna can configure the operating frequency, that is, from wideband to narrowband, by switching ON and OFF the radiating elements. Ideal switches are also used to control each pair of dipole arm and to switch the related stub. In addition, the antenna can operate from 1 to 3 GHz and can be changed to other narrowband frequencies with large size.Three antennas, namely, reconfigurable planar inverted-F antenna (RPIFA), reconfigurable ring patch antenna (RRPA), and reconfigurable folded parasitic dipole antenna (RPFDA) with switches have been proposed [3]. These antennas are integrated with 12, 80, and 8 switches, respectively. Through these published papers, the usage of ideal switches to demonstrate reconfigurable antennas is relevant. The features of antennas, that is, switching band, are tabulated and summarized in Table 1.Table 1
Reconfigurable printed antenna with ideal switches.
Reference
Antenna figure
Antenna structure
Switching band
Frequency switching technique
Switch types
Number of switches
[1]
Mirkamali et al. (2006) (University of Birmingham, UK)
Linear dipole
0.9 GHz, 1.05 GHz, 1.205 GHz, 1.460 GHz, 1.750 GHz, 2.05 GHz, and 2.770 GHz
Control effective radiating length
Metal patch (ideal switch)
24
[2]
Mirkamali et al. (2010) (University of Birmingham, UK)
Log periodic dipole
Mode 1: 1–3 GHz,Mode 2: 0.94 GHz, Mode 3: 1.217 GHzMode 4: 1.477 GHz
Control effective radiating  element
Metal patch (ideal switch)
28
[3]
Chamming (2003) (Virginia Polytechnic Institute & State University)
PIFA
1.48 GHz, 1.49 GHz, 1.5 GHz, 1.51 GHz, 1.53 GHz, and 1.51 GHz, and 1.54 GHz
Change the ground plane length
Metal patch (ideal switch)
12
RRPA
1.50 GHz, 1.52 GHz, and 1.53 GHz
Control the ring width
Metal patch (ideal switch)
80
RPFDA
0.94 GHz, 1.01 GHz, 1.03 GHz, 1.04 GHz, 1.05 GHz, and 1.12 GHz
Control the parasitic element length
Metal patch (ideal switch)
8
Proposed in this article
Hamzah et al. (2013) (Universiti Teknologi Malaysia, Malaysia)
Koch dipole
691 MHz, 725 MHz, 734 MHz, 865 MHz, 953 MHz, 987 MHz, 1099 MHz, 1160 MHz, 1190 MHz, 1270 MHz, 1440 MHz, 1650 MHz, 1880 MHz, 2350 MHz, and 3010 MHz
Control effective radiating length
Metal patch (ideal switch)
50Reconfigurable antennas based on fractal shape have attracted many researchers in recent years. The antenna has many advantages such as large bandwidth, multifrequency and can reduce the antenna size. Koch and Hilbert curve as well as a Sierpinski Carpet are among the selected geometries to be studied for the antenna innovation. Many researchers have used these geometries in order to generate reconfigurable antenna that can configure the frequency or radiation pattern while exhibiting size reduction.References [4–11] reported on reconfigurable fractal antennas. A Hilbert curve patch antenna uses six MEMS switches to configure the radiation pattern from 12.4 GHz to 12.65 GHz [4]. Four MEMS switches have been used in a Sierpinski gasket dipole to switch the frequency operation from 14 GHz (band 1) to 8 GHz and 25 GHz (band 2) [6]. Other studies on reconfigurable fractal antennas use Sierpinski gasket, Hilbert curve, and von Koch geometry [5, 7–11]. Three switches located at the radiating element are used to change the operating frequency at 620 MHz, 630 MHz, and 640 MHz [7]. Five states of operating frequencies in Sierpinski gasket’s antenna are demonstrated by controlling the switches condition [8]. Twelve switches have been used to tune 60 GHz and 80 GHz in the Koch patch antenna [9]. Thirty switches have been used in three-dimensional fractal tree antennas to configure operating frequency from 770 MHz to 1570 MHz [10]. Six switches have been employed in Sierpinski gasket antenna to tune the frequency at 2.4 GHz, 5.7 GHz, 9.4 GHz, and 18 GHz [11]. Four switches are used to configure the radiation pattern at 8.4 GHz in a square patch fractal [5]. Some of them are summarized in Table 2.Table 2
Reconfigurable fractal antenna.
Reference
Antenna structure
Number/type of switches
Comments
[10]
Sierpinski gasket
6/MEMS switch
Changing the frequency from 2.4 GHz, 5.7 GHz, and 9.4 GHz to 18 GHz
[6]
Sierpinski gasket
4/MEMS switch
Changing the frequency from 7 GHz, and 8 GHz, 14 GHz to 23 GHz
[9]
Fractal tree
204/copper strip (ideal switch)
Changing the frequency from 600 MHz to 1600 MHz (approximately 20 bands)
[7]
Hilbert curve
2/metal patch (ideal switch)
Changing the frequency from 620 GHz, 627 GHz to 635 GHz
[4]
Hilbert curve
8/metal patch (ideal switch)
Changing the pattern atf = 9.0 GHz
[8]
Koch Patch
18/copper strip (ideal switch)
Changing the pattern at 60 GHz and 80 GHz
[11]
Square fractal loop
4/metal patch (ideal switch)
Changing the pattern atf = 8.4 GHz
Proposed in this paper
TMFDB25
50/copper strip (ideal switch)
Changing the frequency from 600 MHz to 3.5 GHz with suppressed higher order modeOther published works on reconfigurable antenna are available in [12–17] which employed dipole antenna that is integrated with loop and open wire [12], U-Koch slotted monopole antenna [14], bow-tie antenna [15], UWB patch monopole antenna with spiral section [16], cedar-shaped fractal monopole antenna [17], and slotted monopole antenna (combination of square-ring and L-shaped linear) [13], respectively.The work explained in this paper includes the research idea based on “harmonic suppressed frequency reconfigurable antenna” that can be obtained by controlling the radiation element length combined with internal filter. It is named as TMFDB25 antenna [18]. The balun circuit is reduced by 75% of height that made the antenna more practical. Moreover, the current is flowing directly through the terminal to the antenna input. The direction of current flow or current stop from the input terminal can be determined by means of open circuit stub placed at the terminal. An extensive simulation work has been carried out using two commercial software. The measurements are done using semianechoic chamber to validate the technique. The design consideration is explained in Section 2 while all results are reported in Section 3. Finally, Section 4 concludes this paper.


## 2. TMFDB25 Antenna

The geometry of TMFDB25 is shown in Figure1. The antenna is excited using microstrip line with small tapered balun (75% size reduction) by an SMA connector. The antenna consists of a radiating element, stubs and terminal, tapered balun, and switches. Furthermore, the Koch curve is used to minimize the antenna size, while the double-sided structure is selected due to the ability to lock in the harmonic frequency. Figure 1 shows the geometrical layout of the antenna with a gap (slot) which acts as a switch. The slot size of 1.0 mm × 1.5 mm is used to change the radiating element length as well as the stubs. For achieving reconfigurable capability, the antennas need to be able to change its length to resonate at the desired frequencies. In this design the elimination of higher order mode is considered and controlled via the implementation of open circuit stubs as presented in the figure. The antenna dipole arm is connected with 28 switches while 22 switches are used at the stubs to realize the reconfigurable antenna with harmonic suppression capability.Figure 1
TMFDB25 antenna geometry.In this design conceptual, the frequency reconfiguration totally depends on the radiating element length while stubs 1, 2 suppressed their corresponding higher order modes. It can be observed in the figure that the length,ΔL, of the antenna in x-axis is approximately quarter wavelength of 740 MHz. Switches Sa1,Sa2,…,Sa8 and Sb1,Sb2,…,Sb8 located at specific positions are used to switch the operating frequencies, while Sc0Sd0,Sc1Sd1,…,Sc8Sd8 and Se0Sf0 to Se1Sf1 are used to shorten the antenna. The switches Sc and Sd are connected in parallel to join each part of stub 1 while the Se and Sf switches for stub 2. When the antenna operates at 740 MHz, it has two higher order modes operating at 2020 MHz and 3020 MHz. At the moment, all switches are ON (as shown in Figure 1) to allow the antenna radiate at this frequency and at the same time suppress the higher order mode. It should be noted that the stubs have mismatched the antenna at the higher order mode and the concept is extended to other operating frequencies from 400 MHz to 3.5 GHz. The band, switches condition, and the stub length are tabulated in Table 3.Table 3
Switch configuration states for different length of radiating element. Units are in mm.
Length  (x-axis)
Switch conditions (left arm)
Switch conditions (right arm)
MFDB25 Stub 1, Stub 2
Band
137
Sa1 to Sa14 are ON
Sb1 to Sb14 are ON
(37, 24)
Band 1
128
Sa2 to Sa14 are ON
S2b to Sb14 are ON
(36, 23)
Band 2
120
Sa3 to Sa14 are ON
S3b to Sb14 are ON
(34, 0)
Band 3
112
Sa4 to Sa14 are ON
Sb4 to Sb14 are ON
(32, 0)
Band 4
104
Sa5 to Sa14 are ON
Sb5 to Sb14 are ON
(30, 0)
Band 5
96
Sa6 to Sa14 are ON
Sb6 to Sb14 are ON
(27, 0)
Band 6
88
Sa7 to Sa14 are ON
Sb7 to Sb14 are ON
(25, 0)
Band 7
84
Sa8 to Sa14 are ON
Sb8 to Sb14 are ON
(23, 0)
Band 8
76
Sa9 to Sa14 are ON
Sb9 to Sb14 are ON
(22, 0)
Band 9
66
Sa10 to Sa14 are ON
Sb10 to Sb14 are ON
Not used
Band 10
60
Sa11 to Sa14 are ON
Sb11 to Sb14 are ON
Not used
Band 11
52
Sa12 to Sa14 are ON
Sb12 to Sb14 are ON
Not used
Band 12
44
Sa13 to Sa14 are ON
Sb13 to Sb14 are ON
Not used
Band 13
36
Sa14 is ON
Sb14 is ON
Not used
Band 14
28
All switches are OFF
All switches are OFF
Not used
Band 15Besides that, the terminal length, stub width, and stubs location need to be optimized for better performance. These parameters can provide sufficiently small reflections and avoid the appearance of undesired higher order modes. The configuration is proposed since it exhibits omnidirectional radiation pattern with low level of cross-polarization, as well as it can suppress higher order modes, although with a simple structure. The fractal technology applied allows minimization of the antenna size. Open circuit stub has been used to trap the higher order mode that acts as stubfilters. This antenna is simulated using numerical simulations and cross-checked by using the finite element method-  (FEM-) based simulations.The measured results of the return loss of Figure1 are presented in Figure 2. Good agreement is observed between the results obtained using commercial software and measurement. They operate at these particular bands which directly depend on the switches states. The corresponding resonant frequency is also given in the figure.Figure 2
Measured return losses of the TMFDB25 antenna at two conditions: (1) TMFDB25 without stubs and (2) TMFDB25 with stubs 1, 2. In this study,f equals 705 MHz while 1st HM = 1.972 GHz and 2nd HM = 3.048 GHz, and suppressed return losses of 1st HM and 2nd HM equals −1.7429 dB and −1.3321 dB, respectively.


## 3. Results and Discussion

### 3.1. Antenna Optimization-Parametric Study

In this section, three important antenna parameters in Figure1 which are the terminal length, L, distance between stubs,ΔL1-2, and the stub’s widths, ΔW, are studied. By means of varying these parameters in uniform increment, the following data are obtained. The effect of the operating frequency, 1st HM and 2nd HM are observed as shown in the table.Table4 tabulates the simulated resonance frequency for antenna when the antenna terminal length is changed from 10 mm to 22 mm. Results show that the antenna resonates at 740 MHz in the range from 0 to 3.5 GHz with different return losses. When the terminal length is increased, a better return loss is achieved, whereby this action brings the line impedance value to the point closer to the characteristic impedance. The 10 mm length is selected due to its smaller size.Table 4
Effect of terminal length,L, on resonance in Figure 1.
Resonance
L
1, mm
L
2, mm
L
3, mm
L
4, mm
L
5, mm
L
6, mm
L
7, mm
10
12
14
16
18
20
22
f (GHz), dB
0.74, −23
0.74, −24
0.74, −25
0.74, −26
0.74, −27
0.74, −28
0.74, −29
1st HM (GHz), dB
2.02, −1.6
1.98, −3
1.98, −3
1.99,  −3
1.99, −3
1.99, −3
1.99, −3
2nd HM (GHz), dB
2.79, −1.8
2.27, −4
2.28, −4
2.28,  −4
2.28, −3
2.28, −3
2.28, −3Cautious steps are taken to ensure the investigation is valid. The effect of stub location is highlighted in Table5. Actual length of the antenna terminal is approximately 8 mm. The variation starts from 0 mm to 6 mm with an increment of 1 mm each. It is shown that the antenna can maintain the resonant frequency at 740 MHz with excellent return loss of <−20 dB although the stub location is varied. The stub location of 3 mm totally rejected the higher order mode. However, there is harmonic frequency for certain conditions.Table 5
Effect of stubfilter’s length on resonance in Figure1 is location of stub 2 with respect to stub 1.
Resonance
Δ
L
1
-
2, mm
0
1
2
3
4
5
6
f (GHz), dB
0.74, −22
0.74, −29
0.74, −23
0.74, −23
0.74, −27
0.74, −24
0.74, −23
1st HM (GHz), dB
2.36, −31
2, −2
1.99, −3
1.98, −3
1.98, −5
1.96, −7
1.94, −12
2nd HM (GHz), dB
NA
2.36, −13
2.29, −6
2.25, −2
NA
NA
NATable6 tabulates the resonance frequency for antenna when the stub width is changed from 1 mm to 4 mm. The antenna has first resonance at 740 MHz with different return losses for all cases. The width of 3 mm totally rejected the harmonic frequencies and has good return loss of −23 dB for the operating frequency. These optimum parameters have been used in the designing of the TMFDB25 antenna.Table 6
Effect of stubfilter’s width on resonance in Figure1.
Resonance
Δ
W
3, mm
Δ
W
4, mm
Δ
W
5, mm
Δ
W
6, mm
Δ
W
7, mm
Δ
W
8, mm
Δ
W
9, mm
1.0
1.5
2.0
2.5
3.0
3.5
4.0
f (GHz), dB
0.74, −25
0.74, −29
0.74, −36
0.74, −25
0.74, −23
0.74, −19
0.74, −17
1st HM (GHz), dB
1.99, −4
1.99, −4
1.99, −4
1.98, −3
2.02, −1.6
1.99, −3
1.98, −3
2nd HM (GHz), dB
2.35, −13
2.33, −9
2.3, −7
2.27, −5
2.79, −1.8
2.27, −2
2.27, −1.4


### 3.2. TMFDB25 Antenna

Basically, the proposed antenna is constructed by measuring its electrical properties in order to verify the theoretical and simulation results. The selection of this antenna to be developed is based on small size upon optimization and excellent performance. To achieve fifteen-band TMFDB25 antenna, a small strip of copper is used as the substitute for the RF switches. Then, to be a fifteen-band TMFDB25 antenna with suppressed higher order modes, the stubs are connected. They enlarged the number of small strips of copper. The latter is soldered to the radiating element as well as the stub sections at the switch locations to simulate the switch ON states. The copper strips are made from the standard copper tape and are approximately 0.2 mm wide. The tape switches are sufficiently long enough (2 mm) to both span the 1 mm gap separating the radiating element and the stub length. It provides enough metal-to-metal contact for good solder joint. In addition, for good presentation, the switch has been labeled as black colour for ON states and a cross-line (red colour) for OFF states.The corresponding measured results of the return loss are depicted in Figures3 to 5. It should be noted that this research work is very challenging and requires huge amount of measurement period. However, they are successful to be plotted. The huge numbers of switches have been used and hence increase the difficulty to suppress the higher order modes.Figure 3
Simulated and measured return losses at Bands 1, 3, 5, 7, and 9.Then the measured results are compared with the simulation works. It can be seen in Figure3 that the designed antenna can be tuned from band 1 = 691 MHz, band 3 = 725 MHz, band 5 = 865 MHz, band 7 = 987 MHz, and band 9 = 1270 MHz while the simulated are band 1 = 745 MHz, band 3 = 801 MHz, band 5 = 903 MHz, band 7 = 1029 MHz, and band 9 = 1190 MHz at a time.It can also be observed in Figure4 that the antenna can be tuned from bands 2, 4, 6, 8, and 10 with suppressed higher modes. In this case, the measurements are band 2 = 725 MHz, band 4 = 734 MHz, band 6 = 953 MHz, band 8 = 1160 MHz, and band 10 = 1270 MHz while the simulated are band 2 = 766 MHz, band 4 = 843 MHz, band 6 = 969 MHz, band 8 = 1099 MHz, and band 10 = 1270 MHz.Figure 4
Simulated and measured return losses at Bands 2, 4, 6, 8, and 10.Figure 5
Simulated and measured return losses at bands 10, 11, 12, 13, 14, and 15.Then, in Figure5, the antenna tuned the bands from bands 11, 12, 13, 14, and 15 with suppressed higher modes. (Measured: band 11 = 1440 MHz, band 12 = 1650 MHz, band 13 = 1880 MHz, band 14 = 2350 MHz, and band 15 = 3010 MHz while the simulated are band 11 = 1428 MHz, band 12 = 1596 MHz, band 13 = 1879 MHz, band 14 = 2345 MHz, and band 15 = 2922 MHz). The bandwidth covering tunable ranges are obtained as 1659 MHz. The corresponding measured return losses are −12.5 dB (band 1), −17 dB (band 2), −24.5 dB (band 3), −35 dB (band 4), −17 dB (band 5), −20 dB (band 6), −31.4 dB (band 7), −27 dB (band 8), −27.3 dB (band 9), −16.5 dB (band 10), −16.5 dB (band 11), −18 dB (band 12), −19.5 dB (band 13), −18.0 dB (band 14), and −34.5 dB (band 15).The simulation of the operating frequencies (bands 1, 2, and 3 to 15) of the TMFDB25 antenna with stubs has been done using CST code and HFSS code (not shown in this paper due to limited number of pages).Table7 presents the simulated (CST code and HFSS code) and measured return losses and corresponding VSWR performances of the TMFDB25 antenna for each band. The antenna can be tuned to a single frequency at a time as tabulated in the table. The stubs have effectively reduced the input reflections and thus corresponding VSWRs. Channel operating bandwidths is obtained by CST, HFSS, and measurements are 2177 MHz, 2341 MHz, and 2319 MHz, respectively. Good agreement is observed for the measurement and simulation results in terms of operating frequencies, return losses, and VSWRs. The main factor that contributes to the measurement results is a little bit different compared to the simulation data due to the imperfect work during soldering of the copper to join each stub segment.Table 7
Return losses and VSWR results of theoretical predictions versus measurement for TMFDB25 antenna.
Band
Return loss
VSWR
CST
HFSS
Measured
CST
HFSS
Measured
Band 1
745 MHz
−14.4 dB
733 MHz
−25.3784 dB
691 MHz
−12.5 dB
1.468
1.14900
1.5856
Band 2
766 MHz
−16.4 dB
754 MHz
−23.5454 dB
720 MHz
−17.0 dB
1.358
1.12704
1.4373
Band 3
801 MHz
−34.0 dB
796 MHz
−23.5129 dB
725 MHz
−24.5 dB
1.041
1.14301
1.1157
Band 4
843 MHz
−36.8 dB
838 MHz
−23.1433 dB
734 MHz
−35.0 dB
1.030
1.14969
1.0360
Band 5
903 MHz
−39.6 dB
897.5 MHz
−23.6867 dB
865 MHz
−17.0 dB
1.022
1.13998
1.4367
Band 6
969 MHz
−39.5 dB
960.5 MHz
−27.2084 dB
953 MHz
−20.0 dB
1.021
1.09119
1.3403
Band 7
1029 MHz
−31.4 dB
1020 MHz
−29.0275 dB
987 MHz
−15.0 dB
1.055
1.07333
1.4221
Band 8
1099 MHz
−29.1 dB
1104 MHz
−27.9649 dB
1160 MHz
−27.0 dB
1.099
1.08327
1.2220
Band 9
1190 MHz
−27.3 dB
1195 MHz
−26.5608 dB
1250 MHz
−19.5 dB
1.090
1.09860
1.3705
Band 10
1270 MHz
−20.5 dB
1282.5 MHz
−15.8776 dB
1270 MHz
−16.5 dB
1.210
1.38304
1.3993
Band 11
1428 MHz
−23.9 dB
1429.5 MHz
−17.1505 dB
1440 MHz
−16.5 dB
1.136
1.32241
1.3863
Band 12
1596 MHz
−47.0 dB
1597.5 MHz
−22.3475 dB
1650 MHz
−18.0 dB
1.009
1.16524
1.2535
Band 13
1879 MHz
−30.3 dB
1916 MHz
−45.262 dB
1880 MHz
−19.5 dB
1.063
1.01097
1.2723
Band 14
2345 MHz
−32.4 dB
2458.5 MHz
−18.946 dB
2350 MHz
−18.0 dB
1.049
1.25454
1.2737
Band 15
2922 MHz
−34.5 dB
3074 MHz
−18.4166 dB
3010 MHz
−40.0 dB
1.039
1.27271
1.0207In this study, the corresponding measured E-plane and H-plane radiation patterns at the operating frequencies in Figures3 to 5 are presented in Figures 6, 7, 8, 9, 10, and 11, respectively. These figures provide the H-plane (x-z axis) and the E-plane (z-x axis) patterns for bands 1 to 15. In this work, it is found that the pattern behavior for the fifteen bands antenna resembles that of a simple dipole antenna. The patterns are closed to omnidirectional in the H-plane, having a figure-of-eight pattern in the E-plane.Figure 6
Measured E-plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 1 to 5.Figure 7
Measured H-plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 1 to 5.Figure 8
Measured E-plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 6 to 10.Figure 9
Measured H-plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 6 to 10.Figure 10
Measured E-Plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 11 to 15.Figure 11
Measured H-plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 10 to 15.As predicted by the simulations, the radiation patterns of most bands remain nearly constant from one switch state to the next. This situation is desired for the antenna in order to maintain the performance while selecting the operating frequency. This consistency can be clearly seen in the pattern comparison of bands 1 to 15.


## 3.1. Antenna Optimization-Parametric Study

In this section, three important antenna parameters in Figure1 which are the terminal length, L, distance between stubs,ΔL1-2, and the stub’s widths, ΔW, are studied. By means of varying these parameters in uniform increment, the following data are obtained. The effect of the operating frequency, 1st HM and 2nd HM are observed as shown in the table.Table4 tabulates the simulated resonance frequency for antenna when the antenna terminal length is changed from 10 mm to 22 mm. Results show that the antenna resonates at 740 MHz in the range from 0 to 3.5 GHz with different return losses. When the terminal length is increased, a better return loss is achieved, whereby this action brings the line impedance value to the point closer to the characteristic impedance. The 10 mm length is selected due to its smaller size.Table 4
Effect of terminal length,L, on resonance in Figure 1.
Resonance
L
1, mm
L
2, mm
L
3, mm
L
4, mm
L
5, mm
L
6, mm
L
7, mm
10
12
14
16
18
20
22
f (GHz), dB
0.74, −23
0.74, −24
0.74, −25
0.74, −26
0.74, −27
0.74, −28
0.74, −29
1st HM (GHz), dB
2.02, −1.6
1.98, −3
1.98, −3
1.99,  −3
1.99, −3
1.99, −3
1.99, −3
2nd HM (GHz), dB
2.79, −1.8
2.27, −4
2.28, −4
2.28,  −4
2.28, −3
2.28, −3
2.28, −3Cautious steps are taken to ensure the investigation is valid. The effect of stub location is highlighted in Table5. Actual length of the antenna terminal is approximately 8 mm. The variation starts from 0 mm to 6 mm with an increment of 1 mm each. It is shown that the antenna can maintain the resonant frequency at 740 MHz with excellent return loss of <−20 dB although the stub location is varied. The stub location of 3 mm totally rejected the higher order mode. However, there is harmonic frequency for certain conditions.Table 5
Effect of stubfilter’s length on resonance in Figure1 is location of stub 2 with respect to stub 1.
Resonance
Δ
L
1
-
2, mm
0
1
2
3
4
5
6
f (GHz), dB
0.74, −22
0.74, −29
0.74, −23
0.74, −23
0.74, −27
0.74, −24
0.74, −23
1st HM (GHz), dB
2.36, −31
2, −2
1.99, −3
1.98, −3
1.98, −5
1.96, −7
1.94, −12
2nd HM (GHz), dB
NA
2.36, −13
2.29, −6
2.25, −2
NA
NA
NATable6 tabulates the resonance frequency for antenna when the stub width is changed from 1 mm to 4 mm. The antenna has first resonance at 740 MHz with different return losses for all cases. The width of 3 mm totally rejected the harmonic frequencies and has good return loss of −23 dB for the operating frequency. These optimum parameters have been used in the designing of the TMFDB25 antenna.Table 6
Effect of stubfilter’s width on resonance in Figure1.
Resonance
Δ
W
3, mm
Δ
W
4, mm
Δ
W
5, mm
Δ
W
6, mm
Δ
W
7, mm
Δ
W
8, mm
Δ
W
9, mm
1.0
1.5
2.0
2.5
3.0
3.5
4.0
f (GHz), dB
0.74, −25
0.74, −29
0.74, −36
0.74, −25
0.74, −23
0.74, −19
0.74, −17
1st HM (GHz), dB
1.99, −4
1.99, −4
1.99, −4
1.98, −3
2.02, −1.6
1.99, −3
1.98, −3
2nd HM (GHz), dB
2.35, −13
2.33, −9
2.3, −7
2.27, −5
2.79, −1.8
2.27, −2
2.27, −1.4


## 3.2. TMFDB25 Antenna

Basically, the proposed antenna is constructed by measuring its electrical properties in order to verify the theoretical and simulation results. The selection of this antenna to be developed is based on small size upon optimization and excellent performance. To achieve fifteen-band TMFDB25 antenna, a small strip of copper is used as the substitute for the RF switches. Then, to be a fifteen-band TMFDB25 antenna with suppressed higher order modes, the stubs are connected. They enlarged the number of small strips of copper. The latter is soldered to the radiating element as well as the stub sections at the switch locations to simulate the switch ON states. The copper strips are made from the standard copper tape and are approximately 0.2 mm wide. The tape switches are sufficiently long enough (2 mm) to both span the 1 mm gap separating the radiating element and the stub length. It provides enough metal-to-metal contact for good solder joint. In addition, for good presentation, the switch has been labeled as black colour for ON states and a cross-line (red colour) for OFF states.The corresponding measured results of the return loss are depicted in Figures3 to 5. It should be noted that this research work is very challenging and requires huge amount of measurement period. However, they are successful to be plotted. The huge numbers of switches have been used and hence increase the difficulty to suppress the higher order modes.Figure 3
Simulated and measured return losses at Bands 1, 3, 5, 7, and 9.Then the measured results are compared with the simulation works. It can be seen in Figure3 that the designed antenna can be tuned from band 1 = 691 MHz, band 3 = 725 MHz, band 5 = 865 MHz, band 7 = 987 MHz, and band 9 = 1270 MHz while the simulated are band 1 = 745 MHz, band 3 = 801 MHz, band 5 = 903 MHz, band 7 = 1029 MHz, and band 9 = 1190 MHz at a time.It can also be observed in Figure4 that the antenna can be tuned from bands 2, 4, 6, 8, and 10 with suppressed higher modes. In this case, the measurements are band 2 = 725 MHz, band 4 = 734 MHz, band 6 = 953 MHz, band 8 = 1160 MHz, and band 10 = 1270 MHz while the simulated are band 2 = 766 MHz, band 4 = 843 MHz, band 6 = 969 MHz, band 8 = 1099 MHz, and band 10 = 1270 MHz.Figure 4
Simulated and measured return losses at Bands 2, 4, 6, 8, and 10.Figure 5
Simulated and measured return losses at bands 10, 11, 12, 13, 14, and 15.Then, in Figure5, the antenna tuned the bands from bands 11, 12, 13, 14, and 15 with suppressed higher modes. (Measured: band 11 = 1440 MHz, band 12 = 1650 MHz, band 13 = 1880 MHz, band 14 = 2350 MHz, and band 15 = 3010 MHz while the simulated are band 11 = 1428 MHz, band 12 = 1596 MHz, band 13 = 1879 MHz, band 14 = 2345 MHz, and band 15 = 2922 MHz). The bandwidth covering tunable ranges are obtained as 1659 MHz. The corresponding measured return losses are −12.5 dB (band 1), −17 dB (band 2), −24.5 dB (band 3), −35 dB (band 4), −17 dB (band 5), −20 dB (band 6), −31.4 dB (band 7), −27 dB (band 8), −27.3 dB (band 9), −16.5 dB (band 10), −16.5 dB (band 11), −18 dB (band 12), −19.5 dB (band 13), −18.0 dB (band 14), and −34.5 dB (band 15).The simulation of the operating frequencies (bands 1, 2, and 3 to 15) of the TMFDB25 antenna with stubs has been done using CST code and HFSS code (not shown in this paper due to limited number of pages).Table7 presents the simulated (CST code and HFSS code) and measured return losses and corresponding VSWR performances of the TMFDB25 antenna for each band. The antenna can be tuned to a single frequency at a time as tabulated in the table. The stubs have effectively reduced the input reflections and thus corresponding VSWRs. Channel operating bandwidths is obtained by CST, HFSS, and measurements are 2177 MHz, 2341 MHz, and 2319 MHz, respectively. Good agreement is observed for the measurement and simulation results in terms of operating frequencies, return losses, and VSWRs. The main factor that contributes to the measurement results is a little bit different compared to the simulation data due to the imperfect work during soldering of the copper to join each stub segment.Table 7
Return losses and VSWR results of theoretical predictions versus measurement for TMFDB25 antenna.
Band
Return loss
VSWR
CST
HFSS
Measured
CST
HFSS
Measured
Band 1
745 MHz
−14.4 dB
733 MHz
−25.3784 dB
691 MHz
−12.5 dB
1.468
1.14900
1.5856
Band 2
766 MHz
−16.4 dB
754 MHz
−23.5454 dB
720 MHz
−17.0 dB
1.358
1.12704
1.4373
Band 3
801 MHz
−34.0 dB
796 MHz
−23.5129 dB
725 MHz
−24.5 dB
1.041
1.14301
1.1157
Band 4
843 MHz
−36.8 dB
838 MHz
−23.1433 dB
734 MHz
−35.0 dB
1.030
1.14969
1.0360
Band 5
903 MHz
−39.6 dB
897.5 MHz
−23.6867 dB
865 MHz
−17.0 dB
1.022
1.13998
1.4367
Band 6
969 MHz
−39.5 dB
960.5 MHz
−27.2084 dB
953 MHz
−20.0 dB
1.021
1.09119
1.3403
Band 7
1029 MHz
−31.4 dB
1020 MHz
−29.0275 dB
987 MHz
−15.0 dB
1.055
1.07333
1.4221
Band 8
1099 MHz
−29.1 dB
1104 MHz
−27.9649 dB
1160 MHz
−27.0 dB
1.099
1.08327
1.2220
Band 9
1190 MHz
−27.3 dB
1195 MHz
−26.5608 dB
1250 MHz
−19.5 dB
1.090
1.09860
1.3705
Band 10
1270 MHz
−20.5 dB
1282.5 MHz
−15.8776 dB
1270 MHz
−16.5 dB
1.210
1.38304
1.3993
Band 11
1428 MHz
−23.9 dB
1429.5 MHz
−17.1505 dB
1440 MHz
−16.5 dB
1.136
1.32241
1.3863
Band 12
1596 MHz
−47.0 dB
1597.5 MHz
−22.3475 dB
1650 MHz
−18.0 dB
1.009
1.16524
1.2535
Band 13
1879 MHz
−30.3 dB
1916 MHz
−45.262 dB
1880 MHz
−19.5 dB
1.063
1.01097
1.2723
Band 14
2345 MHz
−32.4 dB
2458.5 MHz
−18.946 dB
2350 MHz
−18.0 dB
1.049
1.25454
1.2737
Band 15
2922 MHz
−34.5 dB
3074 MHz
−18.4166 dB
3010 MHz
−40.0 dB
1.039
1.27271
1.0207In this study, the corresponding measured E-plane and H-plane radiation patterns at the operating frequencies in Figures3 to 5 are presented in Figures 6, 7, 8, 9, 10, and 11, respectively. These figures provide the H-plane (x-z axis) and the E-plane (z-x axis) patterns for bands 1 to 15. In this work, it is found that the pattern behavior for the fifteen bands antenna resembles that of a simple dipole antenna. The patterns are closed to omnidirectional in the H-plane, having a figure-of-eight pattern in the E-plane.Figure 6
Measured E-plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 1 to 5.Figure 7
Measured H-plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 1 to 5.Figure 8
Measured E-plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 6 to 10.Figure 9
Measured H-plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 6 to 10.Figure 10
Measured E-Plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 11 to 15.Figure 11
Measured H-plane radiation patterns (co-polar & cross-polar) at operating frequency,f, for TMFDB25 with stubs 1, 2 at bands 10 to 15.As predicted by the simulations, the radiation patterns of most bands remain nearly constant from one switch state to the next. This situation is desired for the antenna in order to maintain the performance while selecting the operating frequency. This consistency can be clearly seen in the pattern comparison of bands 1 to 15.


## 4. Conclusion

TMFDB25 antenna with harmonic suppression capability that is suitable for frequency reconfiguration in the frequency bands of TV (400 MHz–800 MHz), GSM900/1800 MHz, 3G, ISM 2.4 GHz, and above (up to 3.5 GHz) is presented. The TMFDB25 is an improvement on the conventional linear dipole antenna with harmonic suppressed behavior. A total of 50 RF switch locations were incorporated into the design to achieve the desired frequency reconfigurable performance. Single operating frequency out of fifteen bands can be selected by changing the antenna’s length, and the stub is used to suppress the harmonic frequency. The proposed TMFDB25 antenna has a practical size with 51.8% size reduction compared to the reference [1]. The corresponding measured return loss and VSWR are found to be in good agreement with the computed performances. The simulated and measured data demonstrate that the antenna indeed provides the desired wideband frequency reconfiguration. The number of operating frequency can be increased by increasing the number of switches. In addition, an omniradiation pattern is maintained for the whole frequency. The performance verifies the proposed design concept. From the simulation and experimental results, it can be concluded that the proposed antennas have a unique structure compared to the available published “harmonic suppressed reconfigurable antenna” or “reconfigurable fractal antenna.”



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*Source: 834387-2013-09-22.xml*