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K.S. Alimgeer1, S. Muzahir Abbas1, Raheel M. Hashmi2, H. Zahra1, Z. Qamar1, Shahid A. Khan1

(1 Department of Electrical Engineering, COMSATS Institute of Information Technology, Islamabad, Pakistan

2 Department of Electronics and Information, Politecnico di Milano, Milan, Italy)

MULTIBAND RESONANCE GENERATION BY FEED MANIPULATION IN MICROSTRIP ANTENNA

Microstrip antennas have been in vogue since almost two decades and their popularity is still increasing with progress in manufacturing of miniature communication equipment. In the field of antenna design, a major area of interest has been to design antennas which can simultaneously operate on multiple frequencies to facilitate the operation of a device based on different communication modes. In this paper, a new technique is being presented which helps in achievement of the multi-band characteristics in regular patch antennas by manipulating the feed point location on the patch. The presented approach reveals that variation in number of resonating bands for an antenna can be observed by altering the feed location on the patch. The analysis and research has been carried out to obtain the resonating band outcomes and it endorses the findings presented this research. The focus has been to implement the desired solution by exploiting the tolerance levels of device peculiarities, consistently tracking the needs of area of application and standards of the network under consideration.

Introduction

Microstrip antennas are very popular for low profile applications and are being used to minimize the dimensional constraints in manufacturing of communication equipments [1]. The world is converging towards the era of unified communications and the future holds the implementation of heterogeneous networks to realize the concept of seamless interoperability. In order for a wireless communication equipment to operate with multiple communicating interfaces, it must employ an antenna capable of supporting more than one frequencies of operation [2]. Such antennas are termed under the category of multiband antennas.

Microstrip antennas are preferred for most of the user-end applications because of the ease of operation offered by them. These antennas operate on low-power and offer very low fabrication costs as they are itched over a dielectric substrate on top of a conducting ground plane [1].

Multiband behavior in antennas has been achieved using several complex techniques like slots [3], notched edges [4] and antenna arrays [5] etcetera; however, a different approach is presented in this paper to demonstrate the effect of varying the feed point location from the centre edge-fed patch on either side. Although sophisticated multiband antennas are popularly placed in the category of log-periodic antenna, also termed as fractal antennas; the associated design and manufacturing complexity is, nonetheless, quite high and needs reduction based on the tolerance offered by the application environment. Generally, the application of multiband antennas can be found in networks operating at dual bands for example a Wi-Fi based LAN operating on 2.4 GHz and 5.45 GHz simultaneous switched access, or a GSM network operating on 900/1800 MHz dual band. Moreover, for interoperability of Wi-Fi hotspots in an IEEE 802.16 network, multiband antennas are employed in the enabled equipment to support the communications at all the three available frequency bands i.e. 2.4 GHz and 5.45 GHz for Wi-Fi and 3.4 GHz for the 802.16 based Metropolitan Area Network (MAN) configurations.

The conventional techniques for stimulating the dual band or multiband characteristics in microstrip antennas involve placement of slits in the patch which are of measured length and width and are highly sensitive to the frequency of operation [6]. This sensitivity in the placement of the slots induces complexity in fabrication and a modest variation occurring during the manufacturing of the prototypes causes a serious shift in the operating frequency or the number of operating bands. Contrary to this, the research presented in this paper demonstrates that dual band or multiband characteristics can be inducted in an antenna with a comparatively much simpler methods, although, compromising the performance characteristics compared to their predecessor methods. Moreover,

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the desired number of operating bands can also be adjusted as single, dual or multiband by careful variation of the feed location.

Rest of the paper is organized as follows: Section II presents the background requirements and review of recent works in the area of focus while Section III describes the developed hypothesis and structural approach. Section IV describes the feed variation approach and in Section V, the simulation results and analysis are presented. Section VI concludes the paper while Section VII describes the future work.

Background & Literature

In [7], [8] and [9], some of the most significant achievements have been addressed, emphasizing the need to have dynamic resonance arrays. Multiband arrays are used to analyze and build phased directionality in wireless networks [10], which can in turn prove to be of extreme interest for Quality of Service (QoS) enhancements in communication equipments. In all scenarios involving systematic designs, cost and benefit relationship is an aspect to be considered at the utmost priority in order to maximize the provision of services to masses along with stranding to the standardization constraints. MIMO systems also employ multiband features which are achieved by using highly sophisticated design methods presented in [11] and [12], but in turn, with the torment of high cost and complexity associated with it; thus, planting a hindrance in rapid and low-cost deployment of the designed functionality of equipments.

Ongoing research on multiband antennas further generates interests in the area of reconfiguration of resonating structures in real-time, allowing for variable polarization and system in-concentric operation. These techniques have been discussed deeply in [13], [14] and recommendations clearly suggest a boost in costs and complexity to design such resonating structures. This fact, on the contrary, imparts a not so visible, but long lasting limitation on the developed design, in terms of deployment of this technology and high-cost service provisioning.

Furthermore, excavating into the knowledge base of recent researches regarding multiband resonance, we can surely not ignore heterogeneous network support, which is essential in present day devices [15]. The current focus is not just limited to the design of antennas with operational capabilities as discussed in the preamble, but also to achieve them in such a way that reduces the design and manufacturing complexity along with satisfying the cost optimization constraints [16]. Thereby, allowing a carefully controlled tradeoff among design complexity, performance precision; heterogeneity support and associated implementation costs.

Rationale & Structure Tailoring the achievement of multiband resonance employing the scissors of facts presented

in section II, the aim of this experimental research is to achieve a method which creates a descent in both cost and design complexity. Moreover we wish to exploit the tradeoff relationship between operational accuracy and design complexity. Considering that the applied resonator works in a network which is not exposed to extreme fading for example, indoor communication networks, small wireless LANs or local peer-to-peer Access Point nodes, then slight variations in operational characteristics may be tolerated if they offer shear reduction in development complexity. The extent of acceptance of these tolerances, allow us to design multiband resonators in less sophisticated ways, still fulfilling the constraints of performance characteristics demanded by system peculiarities.

Thus, the major focus of this work is placed on the study of the variation in antenna characteristics by altering the feed locations. In order to develop a hypothesis, we exploit the fundamental properties of antenna design regarding antenna dimensions and feed locations [17], suggesting that a pattern of variations can be detected to occur periodically, with cyclic and controlled shifting of feed. Apparently, the detection of such patterns can help us to predict and to obtain, desired multiple frequencies of operation, and make necessary parametric adjustments. Hence, a simple rectangular patch was chosen to test the hypothesis, presented in [18]. A major reason to choose such a design was its low implementation cost and particularly, strong

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performance characteristics, as demonstrated in [18]. The geometrical specifications of the employed patch antenna are demonstrated by Table I whereas Table II presents the properties of the substrate employed in the simulation environments.

TABLE I. TEST ANTENNA SPECIFICATION [18]

Dimensions Components

Length (mm) Width (mm)

Ground Plane 80.0 50.0

Radiating Patch 27.5 40.0

Transmission Line 26.25 4.0

TABLE II. SIMULATION ENVIRONMENT SPECIFICATIONS [18]

Layer Type Loss Tangent

Thickness (mm) Permittivity

1 Air 0 32 1

2 FR-4 (lossy) 0.019 1.5 4.9

To support the argument, software analysis and simulations were carried out using evaluation versions of AWR Microwave Office (MWO-228, VSS-100) and High Frequency Structure Simulator (HFSS). These software package is quite user friendly and provides highly accurate visualizations of micro strip patch antennas. It must be emphasized here that software analysis has been carried out using two simulators to ensure accuracy of results and endorse the correctness of deductions from the generated outputs.

Feed Manipulation Technique Initially, the patch antenna was fed using a 50 ohms transmission line attached at the central

bottom edge of the patch. Let us refer to this initial location as point (0,0) expressed in units of (x,y) millimeters. Figure 1 shows the trend of shifting and varying the feed points across the feeding edge of the patch.

Fig. 1. Manipulation trend followed for analysis (deviations in mm)

At this point, the antenna exhibited normal behavior with a single operating band at 2.4 GHz having a fractional bandwidth of 140 MHz. After this, the feed location was given a positive shift of 1 mm, rendering the feed point to be (1, 0). This resulted in a dual band behavior with the first resonant band centered at 2.09 GHz and the second one at 2.64 GHz. These two bands possessed an average bandwidth of 85 MHz/each and demonstrated return losses below -20 dB. This trend continues with an increase in the distance of the feed location from the origin with the number of resonant bands fixed at two, depicting minute variations in the return loss.

(0,0) Origin

(+x,y) positive shifting

(-x,y) Negative shifting

Feed Line

Antenna Feeding Edge

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TABLE 3 - DUAL BAND CHARACTERISTICS WITH FEED VARIATION Feed Points Reference to Centre (0,0) (mm)

Left Shift Centre Right Shift -3 -2 -1 0 1 2 3

No. of Bands 1 2 2 1 2 2 2

Frequency (GHz) 2.61 2.08 2.04 2.4 2.0 2.10 2.1

Return Loss (dB) -13 -22 -18 -34.5 -23 -24 -24

Bandwidth (MHz) 50 80 70 150 80 80 90 B

and

I %age

bandwidth 1.92 3.85 3.43 6.25 3.8 3.98 4.2

Frequency (GHz) - 2.63 2.59 - 2.6 2.66 2.6

Return Loss (dB) - -24 -25 - -21 -20 -17

Bandwidth (MHz) - 110 107 - 90 88 90 B

and

II

%age bandwidth - 4.18 4.13 - 3.4 3.31 3.3

Table III summarizes an overview of the dual band resonance characteristics of the antenna based on shifted feed points. It was further observed that when the feed location reached 15 mm from the origin, at point (15,0), the antenna started to exhibit tri-band behaviors. The resonant bands were found to be at 2.1 GHz, 2.55 GHz and 3.05 GHz with an average return loss of -14 dB. The varying placement of the transmission line feeding the patch from the centre of patch towards the far right-end illustrates a regular recurrence in the surfacing of resonating bands. Gradually varying the feed points makes the antenna to resonate, firstly at a single frequency band, then, it demonstrates a dual band behavior which is maintained for some feed locations and finally, as the placement of feed becomes greater than 10 units from centre, tri-band characteristics are exhibited.

Thus the simulation scenario is based over placement of the feed location across various points on the feeding edge of the patch and observing the outcomes based on the number of resonant bands achieved in context to the figures of merit like return loss, fractional bandwidth, and voltage standing wave ratio (VSWR).

Simulation Results Employing on the approach prescribed in Section IV, the simulations were performed to

extract a general conclusive behavior from the test setup. The results obtained by the simulations carried out during this project are presented in this section. In these graphical results, the abscissa represents the frequency of operation in GHz whereas the ordinate represents return loss in decibels (dB).

Single Band Resonance The basic idea is to shift the feeding point from the center point of the radiating patch towards

the left and right edges. The graph in Figure 2 shows the results of the antenna resonance when transmission line feed was placed at the centre point, be it (0,0), as in Figure 1.

The results shown in figure 2 are on the operating frequency at which the antenna was designed. The resonance is quite clear with a return loss of -34.59 dB at 2.4 GHz. The fractional bandwidth, as measured by the software simulation result, is approximately 150 MHz as calculated at VSWR value of 2.0.

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Fig. 2. Magnified snapshot of result at feed location (0,0)

Dual Band Resonance: In the figures below, the points at which dual band behavior is found are plotted against return

loss. The feed location has been varied on the left and right of the central feed point and the results are presented here. Figure 3 shows the dual band behavior of antenna when location of the feed is given a positive shift of 1 mm.

Fig. 3. Shifted feed location results at (1,0)

Note the existence of a third band on the right side of the dual bands which, at present, is not providing acceptable return loss. The dual band average return loss is -22 dB. Figure 4, 5 6 and 7 demonstrate the result of behavioral simulations to observe dual bands by giving positive and negative shifts of 1 mm, 3 mm and 6 mm to the feed.

Fig. 4. Shifted feed location results at (-1,0)

Variations can be seen in the return loss with shifting of the feed locations. Hence, a tradeoff relationship is explicitly observed between multiband resonance and fractional bandwidth. It is highly dependent on the application for which the design is to be employed, the level of toleration to be accommodated regarding the observed inverse relation.

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Fig. 5. Shifted feed location at (3,0)

Fig. 6. Shifted feed location at (-3,0)

Fig. 7. Shifted feed location at (6,0)

Tri-Band Resonance As the feed point is moved further to right or left of the central feed location, the return loss

and VSWR of the resonating bands attains the acceptable threshold needed to overcome operational merits. The results in the following figures show the behavior of the bands to be converging towards tri-band characteristics. Figure 8 and 9 show 3 bands with an average return loss of -13 dB and -12 dB respectively. However, after a certain units of further shifting, the tri-band behavior starts to diminish as the bands move close to -10 dB at certain feed points above 12 mm from centre.

Fig. 8. Shifted feed location at (-12,0)

Fig. 8. Shifted feed location at (-15,0)

Multiband Resonance Investigating the trend described above, the frequency sweep was carried out on a large scale

and it was observed that the antenna starts to show multiband behavior after the feed location is approximately an order of 10 or more units shifted from the feed point origin.

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Fig. 10. Shifted feed location at (-18,0)

There is inconsistent variation in the return losses after a shift of -15 units which results either in a multiband behavior at some points, or a shear single band resonance. Figure 10, 11 and 12 show the results of the extended frequency sweep using the same parametric figures. The multiband behavior is clearly exhibited in these figures.

Fig. 11. Extended frequency sweep with feed at (-15,0)

The simulation results suggest that the hypothesis regarding the sweeping of feed over the edges of the patch to get multiple resonance bands has been successful. Based on the results, we suggest that a complex resonating structure which is supplied with multiple input feed lines can be used as a reconfigurable radiating patch with ability to radiate the signals in more than one spectrum, adjusting according to the communication network characteristics and the environmental conditions. The need to perform this switching may depend on the variation levels of experienced fading, monitored SNRs, vertical handoff capability integration or any other system dependent property. Of course, this spectrum switching is performed and adjusted by implementing back-end support as the radiator itself is only a passive structure acting as the front-end. This argument suggests that the approach may also be further extended to make the radiators exhibit radiations over a series of available polarizations, using feed-line switching across its length and width simultaneously. Figure 12 shows a final extended frequency sweep generating approximately 5 operational bands, measured over RL of -10. In case the application is to be made in an environment which is relatively more consistent, the bands below RL value of -5 may be considered appropriate; thus allowing for a much wider range of operational frequencies with suitably large bandwidths.

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Fig. 12. Extended frequency sweep with feed at (-12,0)

Conclusion

In the paper, a new empirical technique of achieving multiband characteristics in microstrip patch antennas by manipulating the feed location has been presented. The results, after careful interpretation and analysis, suggest that the proposition proved to be successful. Manipulation of feed location in patch antennas does produce a single, dual or multiband resonance in the antenna structure the extent of which is dependent on point of feed, displaced from the central feed location. Hence, an easy to implement technique for tuning the antenna to work on more than one frequency has been introduced. Moreover, the relationship observed between the performance characteristics and the number of resonating bands suggests that there is a tradeoff between the number of resonating bands and the offered return loss and VSWR by the antenna. The greater the number of bands for an antenna, tuned using the feed manipulation method, the more the deviation if return loss and VSWR from their ideally desired values.

Future Work

The trend observed in feed point manipulation suggests that more work is required to explore this area of feed adjustable microstrip antennas. The future work regarding this research includes the behavioral analysis of microstrip antennas fed using angular transmission lines and their shifted versions. Also, the effects of feed manipulation on radiation characteristics of antennas have to be investigated.

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