Design of a 7-Element Electronically Steerable Passive Array Radiator (ESPAR) Antenna for the ISM Band

Felicito S. Caluyo, Alejandro H. Ballado Jr.

Mapúa Institute of Technology, Manila, 1002, Philippines

Abstract — Developments in the field of wireless

communications resulted in the creation and approval of an unlicensed use of the ISM band. The utilization of the said frequency band encountered limitations on power and antenna gain to minimize possible interference. Antenna design is an important consideration in the implementation of any wireless system such as those operating in the ISM band. A 7-element ESPAR was designed, fabricated and tested in a typical laboratory set-up using a generator and receiver assembly. Antenna characteristics such as gain, HPBW, SWR and return loss were measured. The results show that the designed antenna exhibit directivity and ample interference rejection capability.

Index Terms —ESPAR, beam steering, SIR, antenna.

I. INTRODUCTION

Recent developments in the field of wireless communications has demanded for new design techniques in coming up with transmitting and receiving antennas. Antennas implemented for application in the Industrial, Scientific and Medical (ISM) band, specifically, in mobile communications and wireless local area network (WLAN), should possess directivity to cover a desirable area when radiating and provide ample rejection of unwanted interfering signals when receiving. It is therefore paramount to design antennas capable of forming specific radiation patterns, where the main lobe gain can be increased while minimizing radiations on unwanted directions and consequently improving the overall signal-to-interference ratio (SIR). The improvement in the SIR will address signal error problems commonly encountered in some wireless system.

The ESPAR antenna is an appropriate device that offers versatility through its beam steering with simple structures and low implementation cost [1] [2]. Owing to ESPAR antenna’s effectivity in suppressing interference through its beam/null steering capabilities, a configuration of 2.4/5.2 GHz dual-band ESPAR antenna was proposed by Shibata and Furuhi [3] in 2005. Dimousious, Tsitouri, Panagiotou and Capsalis [2] designed a multipurpose tri-band ESPAR antenna with steerable-beam-pattern for maximum directionality at the frequencies of 1.8, 1.9 and 2.4 GHz. The ESPAR antenna can concentrate its maximum radiation toward the intended receiver and steer its radiation nulls toward the interfering signals [4]. As such, signal-to-interference ratio is greatly improved. The quality of performance of a given wireless system, such as the carrier-to-noise ratio (C/N) and energy per bit to noise

density ratio (Eb/No), is dependent on the signal level arriving at the receiver and the degree of its noise rejection capabilities. However, the signal level at the receiver and noise/interference rejection is a function of the designed antenna. Several studies were made about the ESPAR antenna’s ability to focus the radiation in a desired direction and steer its null to avoid interference, but a single ESPAR employed for the ISM band has not been extensively studied. It is therefore necessary to investigate an ESPAR antenna for the said frequency band, to realize its potential for ISM applications, including but not limited to, mobile communications, wireless LAN, ad-hoc wireless networks, etc. The goal of this study is to design a 7-element Electronically Steerable Passive Array Radiator (ESPAR) antenna and to analyze its performance for the ISM band previously stated. The measurement of the resulting antenna radiation characteristics such as the gain, standing wave ratio (SWR), radiation pattern, half-power beamwidth (HPBW) and return loss will also be performed.

II. ANTENNA DESIGN CONSIDERATIONS

A. Antenna Basics and ESPAR A number of wireless applications require radiation

characteristics that may not be achievable through the use of a single antenna element. In the course of designing the antenna, a combination of radiating elements in an electrical and geometrical arrangement may result in the desired radiation characteristics. The arrangement of the antenna array may be completed such that the radiation from the elements adds up to give a radiation maximum in a particular direction or directions, minimum in others, or otherwise as desired [5].

Typically the radiation pattern of a single element antenna is relatively wide, and each element provides low values of directivity. However, in most wireless communications systems it is necessary to design antennas with very high gains to meet the system demand. This is normally accomplished by increasing the electrical dimension of the antenna. Increasing the dimensions of a single element antenna often leads to more directive characteristics. Another way to extend the dimensions of the antenna without necessarily increasing the size of the individual elements is to form an assembly of radiating elements in an electrical and geometrical configuration. This antenna, formed by multielements, is referred to as an array. The ESPAR antenna falls under this

category, where in most cases the elements of an array are identical. The use of identical elements is not necessary, but it is often convenient, simpler, and more practical compared to arrays of varying elements. The individual elements of an array may be of any structure, such as wires, aperture, or patch, to provide very directive patterns. It is also necessary that the fields from the elements of the array interfere constructively in the desired direction and interfere destructively in the remaining space [5]. B. ESPAR Main Beam Steering

The ESPAR [1] antenna is one of the latest improvements in antenna design that offers promising solutions to problems encountered in various wireless system. The ESPAR antenna consists of a single active radiator and several parasitic (non-driven) elements loaded at the base with reactances, normally comprising of varactor diode and inductor [6]-[9], as shown in Fig. 1 [4]. The radiation pattern is controlled electronically through the variable reactors, and the reactance can be varied between positive and negative values by varying the supply voltage to the varactor diode [1]. In the basic configuration of a 7-element ESPAR antenna, the active monopole element is located at the center of the circular ground plane while the other elements, the parasitic monopole elements, are placed at the same spacing and encircles the central element.

Fig. 1. ESPAR Antenna Structure [4]

By electronically controlling the loading reactances of the parasitic elements thereby altering their electrical lengths, the main beams and the nulls can be formed and steered throughout the azimuth [1-2] [8] [10].

III. ESPAR PROTOTYPE

The ESPAR antenna configuration consists of a single active monopole element surrounded by six equidistant, parasitic, monopole elements of constant radius from the center, refer to Figure 1. Each parasitic element is base loaded

with some variable reactance, while the entire array ideally rests upon a conductive ground plane. The reactive loads alter the antenna currents, influencing its radiation characteristics. Thus control of the reactive loading allows radiation beam and null steering. Varactor diodes have been typically suggested for the reactances, however, variable capacitor was employed in the prototype. The only additional components on the parasitic elements are the RF chokes to isolate the parasitic element’s microwave signal from the dc control lines [4].

The ESPAR antenna elements were calculated using 1500 MHz as the test frequency. For the active monopole element and the six parasitic monopole elements the dimension was 0.25λ. The ground plane was cut from a double sided PCB with a radius of 0.5λ, while the ground skirt with a height of 0.25λ was made from a copper sheet having a length of 0.597 meter.

A. Reactance for the Parasitic Monopole Elements The loading circuit for the parasitic monopole elements was

fabricated using variable capacitors and coils. The biasing to the six loading circuits was provided by a variable dc source, B1. The six parasitic monopole elements were terminated with adjustable reactance, comprising of the tuning capacitors C1-C6, and coils L1-L2, as shown in Fig. 2. Signals supplied to the center feeding port, the active monopole element, excite the passive monopoles.

Fig. 2. Loading circuit diagram used for the parasitic elements. B. Construction of the ESPAR Antenna

The active monopole element together with the six parasitic elements were mounted on top of the fabricated ground plane. The loading circuit, comprising of a variable capacitor and a coil, was connected at the base of the parasitic elements. The active monopole was terminated at the base with an SMA connector. For the ground plane assembly, the 0.095-meter circular double-sided PCB was solder-joined at the edge with the ground skirt. An existing solid metal ground structure, in the form of a double sided PCB, was employed. The active

monopole element was fed with an SMA panel mount connector, affixed to the bottom of the ground plane, while the parasitic monopole elements were base-loaded with combinations of variable capacitor and coil to provide the necessary reactance.

IV. MEASUREMENT AND ANALYSIS OF RESULTS

The ESPAR antenna response was plotted using AntennaLab [11]. The ESPAR radiation pattern polar plot in the azimuth direction is shown in Fig. 3, while a sample radiation pattern plot in rectangular coordinates is shown in Fig. 4.

Fig. 3. Radiation pattern of the ESPAR antenna in the azimuth

direction.

ESPAR radiation polar plot revealed that beam focusing was achieved and that the direction at which the main lobe was directed was arbitrarily toward 125 degrees. Two minor lobes were exhibited in different directions while null signal is also present. Through proper adjustment of the loading circuits at the parasitic monopole elements, the beam/null can be steered in a desired direction.

Gain was measured to be 12.6 dB. The direction of maximum radiation was toward 125 degrees, while two minor lobes were present. The Front-to-Back ratio was calculated using the values of the signal level at 125 degrees and 305 degrees, which turned out to be 8.8 dB. Schlub, Lu, and Ohira proposed an ESPAR at 2.4 GHz that produced a gain of 8.08 dB. The resulting 12.6 dB gain of the designed 7-element ESPAR at the test frequency of 1500 MHz was higher than the maximum gain obtained in [4].

Fig. 4. Radiation pattern of the ESPAR Antenna in rectangular

coordinates.

The half-power beamwidth was measured through the polar plot acquired where the beamwidth corresponds to the angular separation between the half-power points (-3 dB points) in the main lobe of the ESPAR radiation pattern. The measured beamwidth was 80 degrees. This clearly indicates that the ESPAR was able to concentrate its radiation into a small angle, thereby providing high gain in the direction of the major lobe.

Fig. 5. Plot of the ESPAR Antenna Return Loss vs. Frequency.

The Return Loss Function of the software used measured the ratio of forward to reverse power over the frequency range of 1200MHz to 1800 MHz. This provided an indication of how good the matching between the transmission line and the antenna feed point was. The data was displayed as a plot of reverse power in dB against frequency. The forward power reference was written above the top of the plots and this value was used as the 0dB line. The reverse power plot was then referenced to this line. The SWR can be calculated from the return loss and a scale showing a measure of the SWR is also displayed [11]. Fig. 5 shows the return loss vs. frequency plot of the ESPAR antenna.

Measured return loss for the ESPAR antenna at the operating frequency of 1500 MHz was -10 dB, while the SWR was 1.9. Although the resulting SWR was low, an SWR < 2,

further reduction can still be introduced by incorporating antenna matching device. Through the use of impedance matching device inserted between the ESPAR antenna and the generator transmission line, the SWR can be significantly lowered to a ratio close to 1:1. This in turn would decrease the return loss.

V. CONCLUSION

A 7-element ESPAR was fabricated and tested at a frequency of 1500 MHz. The performance analysis of the designed ESPAR antenna showed that a gain of 12.6 dB is achievable while allowing an SWR of less than 2. Directionality in the radiation pattern of the ESPAR allows coverage for specific locations while avoiding interferences, at the same time providing significant improvement in the signal level at the intended receiver. Finally, the proposed ESPAR antenna is applicable for extensive use in the ISM band, not only because of its simplicity and lower construction cost but more importantly, because of its ability to steer its main beam into a desired direction and position its null in the direction of unwanted interferences.

REFERENCES

[1] K. Gyoda and T. Ohira, “Design of Electronically Steerable Passive Array Radiator (ESPAR) Antennas”, IEEE Antennas and Propagation Society International Symposium, vol. 2, pp. 922-925, July 2000.

[2] T. D. Dimousios, C. I. Tsitouri, S. C. Panagiotou and C. N. Capsalis, “Design and Optimization of a Multipurpose Tri-band Electronically Steerable Parasitic Array Radiator (ESPAR) Antenna with Steerable-Beam-Pattern for Maximum Directionality at the Frequencies of 1.8, 1.9 and 2.4 GHz with the Aid of Genetic Algorithms,” 2008 Loughborough Antennas & Propagation Conference, 2008, pp. 253-256.

[3] O. Shibata and T. Furuhi, “Dual-band ESPAR antenna for wireless LAN applications,” IEEE Antennas and Propagation Society International Symposium, 2005, vol. 2B, pp.605-608.

[4] R. Schlub, J. Lu and T. Ohira, “Seven-Element Ground Skirt Monopole ESPAR Antenna Design From a Genetic Algorithm and the Finite Element Method,” IEEE Transactions on Antennas and Propagation, vol. 51, no. 11, November 2003, pp. 3033-3039.

[5] C. A. Balanis, Antenna Theory Analysis and Design, 2nd ed., New York: Wiley, 1997.

[6] M. Taromaru and T. Ohira, “Electronically Steerable Parasitic Array Radiator Antenna – Principle, Control Theory and its Applications,” ATR Wave Engineering Laboratories, Kyoto, 2005.

[7] R. F. Harrington, “Reactively controlled directive arrays”, IEEE Transaction on Antennas Propagaion, vol.AP-26, no.3, May 1978.

[8] T. Ohira and K. Iigusa, “Electronically Steerable Parasitic Array Radiator Antenna”, Electronics and Communications in Japan, Part2, vol. 87, no. 10, pp. 25-45, 2004.

[9] C. Sun, A. Hirata, T. Ohira, and N. C. Karmakar, “Fast beamforming of electrically steerable parasitic array radiator antennas: theory and experiment,” IEEE Transaction on Antennas Propagaion, vol.52, no.7, pp. 1819-1832, July 2004.

[10] C. Sun, N. C. Karmakar and T. Ohira, “Experimental Studies of Radiation Pattern of Electronically Steerable Passive Array Radiator Smart Antenna”, IEEE Antennas and Propagation Society International Symposium, vol. 3, pp. 884-887, June 2003.

[11] AntennaLab Manual, Tutor’s Manual (57200-USB-OT Ed05 022006), Feedback Instruments Limited, 2006.