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

Harmonic suppressed fractal antenna with switches named TMFDB25 is developed to select desired frequency band from 400MHz to 3.5 GHz.The radiating element length is changed to tune the operating frequencywhile 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 andmeasurement 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 2.4GHz, and above.


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 narrowbandto-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.
Reconfigurable 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][5][6][7][8][9][10][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][8][9][10][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.
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.

TMFDB25 Antenna
The geometry of TMFDB25 is shown in Figure 1.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.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, Δ, of the antenna in -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.
Besides 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 crosschecked by using the finite element method-(FEM-) based simulations.
The measured results of the return loss of Figure 1 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.

Antenna Optimization-Parametric Study.
In this section, three important antenna parameters in Figure 1 which are the terminal length, , distance between stubs, Δ 1-2 , and the stub's widths, Δ, 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.Table 4 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.
Cautious steps are taken to ensure the investigation is valid.The effect of stub location is highlighted in Table 5. 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 6 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.

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 fifteenband 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 Figures 3 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.
Then the measured results are compared with the simulation works.It can be seen in Figure 3 that the designed     Table 7 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 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.
In this study, the corresponding measured E-plane and H-plane radiation patterns at the operating frequencies in Figures 3 to 5 are presented in Figures 6, 7, 8, 9, 10, and 11, respectively.These figures provide the H-plane (- axis) and the E-plane (- 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.
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.

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."

Table 1 :
Reconfigurable printed antenna with ideal switches.

Table 3 :
Switch configuration states for different length of radiating element.Units are in mm.

Table 4 :
Effect of terminal length, , on resonance in Figure1.

Table 5 :
Effect of stubfilter's length on resonance in Figure1is location of stub 2 with respect to stub 1.

Table 6 :
Effect of stubfilter's width on resonance in Figure1.

Table 7 :
Return losses and VSWR results of theoretical predictions versus measurement for TMFDB25 antenna.