Quarter of a Hemispherical Dielectric Resonator: New Geometry Explored to Design a Wideband Monopole -Type Antenna

D. Guha1, Bidisha Gupta2 and Y. M. M. Antar3

1Institute of Radio Physics and Electronics, University of Calcutta, 92 A P C Road, Kolkata 700 009, India, e-mail: dgirpe@yahoo.co.in

2Techno India, EM 4/1, Salt Lake City, Sector-V, Kolkata-700091, India

e-mail: gupta_bidisa@yahoo.co.in

3Department of ECE, Royal Military College of Canada, Kingston, Ontario, K7K 7B4, Canada, e-mail: antar-y@rmc.ca

Abstract

A new shape of the dielectric resonator (DR) is proposed and investigated. The geometry originates from a hemispherical DR by bisecting its one half, which results in a quarter of a hemisphere. This new DR element has been successfully employed in 4-element configuration that generates uniform and highly symmetric monopole like radiation with about 5 dBi peak gain over about 38% bandwidth indicating its suitability for using as indoor/outdoor wireless applications.

1. Introduction The dielectric resonator antenna (DRA) has been playing a significant role in developing compact, low volume, high efficiency planar antennas for microwave and higher frequencies. Different DRA structures developed and used for various applications are discussed in [1, 2]. Increment in the impedance bandwidth has been a major aspect in DRA research in last few years [3, 4]. Another area of focus is to develop and use DRAs for wireless applications [5]. This appears as a challenging area in DRA research. A review on the state-of-art developments is available in [6]. The dielectric ring resonator is a common DRA structure that radiates monopole like field. Composite cylindrical DRAs have been investigated by two of the present authors as broadband variants of low profile monopole type DRAs [7]. They also studied a new geometry of half- hemispherical DRA (h-HDRA) [8] to employ two closely spaced HEM11δ like and TM101 like modes in a two element gap coupled configuration which produces monopole-type radiation over a large bandwidth. Indeed a hemisphere is structurally suitable to radiate uniform oblique radiation surrounding it. In that work, symmetry in radiation fields were achieved using two half-hemispherical units coupled face to face [8]. In this paper, we have exploited eight-fold symmetry of a hemisphere by introducing quarter of a HDRA as shown in Fig. 1. The same principle of exciting two resonant modes in each quarter HDRA thus should improve the characteristics of a four-element composite structure in producing enhanced gain, larger bandwidth and much improved radiation pattern maintaining the 3-D symmetry. The new quarter HDRA geometry, designated as ‘q-HDRA’ has been thoroughly studied theoretically using simulation. Four-element composite DRA structure, which resembles a hemisphere with four symmetric segments, centrally excited by a coaxial probe has been examined as a broadband monopole-type DRA. Some representative results are presented here.

2. The New DR Geometry

Figure 1 shows a q-HDRA geometry, obtained by symmetrically bisecting a hemispherical DRA twice. That produced by single bisection operation, called half-HDRA (h-HDRA) was studied in [8]. A coaxial probe of length l and diameter d is used to excite each element. Two-element and four-element composite structures are shown in Table I, where each element is excited with HEM11δ like and TM101 like modes. The electric fields in each quarter-HDRA due to HEM11δ like mode has both horizontal and vertical components and it is important to note that the

TABLE I

Quarter-HDRA : characteristic of single-element, 2-element and 4-element structure

horizontal components get cancelled leaving the vertical components only in a four-element composite structure. The electric field distribution due to HEM11δ like mode is shown in Fig. 2, which results in monopole-like radiation. The distribution of the electric fields due to TM101 mode is similar as in Fig. 2.

3. Result

To examine the relative change or improvement in the antenna characteristics with respect to the half-hemispherical geometry [8], we have chosen the DRA parameters as in [8] and they are provided in the caption of Fig.3. The dielectric constant of the DR material used is εr = 10. The characteristics of a single q-HDRA have been thoroughly studied. The properties, when it is applied as a 2-elemenet or 4-elemenet composite structure, have also been studied employing simulated data obtained using Ansoft’s HFSS. Some representative results are presented here. The experiments are under process and the measured results will be provided.

Single q -HDRA is easy to excite with the dominant HEM11δ like mode using a coaxial probe and about 11%

q-HDRA configuration

Mode(s) Optimum Bandwidth

(%)

Radiation pattern

Single element q-HDRA

HEM11δ -like 11.0 Broadside

Two element q-HDRA

HEM11δ like, TM101

35.14 Broadside, Oblique

Four element q-HDRA

HEM11δ like, TM101

38.3 Monopole-type

C ross - secti o nal view

DRA - top view a

Fig.1. A single element q-HDRA fed by a coaxial probe.

Fig.2. Electric fields due to HEM11δ like mode in a four-element q-HDRA.

impedance bandwidth (VSWR<2) can be achieved. The return loss characteristics for different probe lengths are shown in Fig. 3. The radiation pattern produced by the dominant HEM11δ like mode in a q-HDRA is shown in Fig. 4.

Table I gives a comprehensive view about the single q-HDRA and its multi-element configurations. A 2-element q-HDRA, shown in Table I can replace an h-HDRA [8] with matching bandwidth enhanced by about 8% and peak gain improved by nearly 7%. Fig. 5 shows the return loss characteristics of a 2-element q-HDRA. Fig. 6 compares its gain values with those of a h-HDRA [8]. The most significant structure is the 4-elemenet q-HDRA which exploits the closely spaced HEM11δ like and TM101 like modes to give about 38% bandwidth which is about 10% larger compared to that of a 2-element h-HDRA as studied in [8]. The return loss values for different probe heights are studied in Fig. 7. Moreover, its radiation property compared to the 2-element h-HDRA shows significant improvement by about 7db near peak gain as indicated in Fig 8. Fig. 9 reveals its 3-D radiation plot revealing uniform monopole type pattern.

4. Conclusion

The new geometry of quarter – hemispherical DRA originated from its 8-fold symmetry provides a unique property of monopole type uniform 3-D radiation pattern over about 38% bandwidth from a 4-element composite structure. Different combinations of this new element can also produce different radiation patterns with polarization diversity. Thus the proposed DR structure and its composite antenna configuration promise for potential indoor/outdoor wireless applications.

2.5 3.0 3.5 4.0 4.5-25

-20

-15

-10

-5

0

S1

1 (dB

)

Frequency (GHz)

l=10mm l=11mm l=12mm l=14mm l=13mm

Fig.3. Effect of variable probe length l on the return loss characteristics of a probe-fed single q-HDRA. a = 20 mm, εr = 10, probe dia = 1.25 mm, ground plane dia = 80 mm.

-180 -135 -90 -45 0 45 90 135 180-20

-15

-10

-5

0

5

10

Gai

n (

dB)

Azimuth (degree)

phi= 0 o

phi=45o

Fig.4.Simulated Gain versus elevation of a single q-HDRA. l =11 mm, other parameters as in Fig. 3.

2.5 3.0 3.5 4.0 4.5-35

-30

-25

-20

-15

-10

-5

0

S11

(dB

)

Frequency (GHz)

l=9mm l=10mm l=12mm l=14mm l=13mm

Fig.5. Effect of variable probe length l on the return loss characteristics of a probe-fed two element q-HDRA. Parameters as in Fig. 3.

-180 -135 -90 -45 0 45 90 135 180-30

-25

-20

-15

-10

-5

0

5

Gai

n (d

B)

Azimuth (degree)

2-el q-hdra, f=3GHz

single h-hdra [8], f=2.9GHz

Fig.6. Radiation characteristics of a 2-element q-HDRA compared with a single h-HDRA [8]. Phi = 90o, other parameters as in Fig. 4.

5. Reference

1. K. M. Luk and K. W. Leung, Dielectric Resonator Antennas, Research Studies Press,1st Edn. 2002. 2. A. Petosa, Dielectric Resonator Antenna Handbook, Artech House Publishers, 1st Edn. 2007. 3. C. S. Deyoung and S. A. Long, “Wideband cylindrical and rectangular dielectric resonator antennas,” IEEE Antennas Wireless Propagat. Lett., vol.5, pp. 426-429, 2006. 4. R. Chair, S. L. S. Yang, A. A. Kishk, L. K. F. Lee and K. M. Luk, “Aperture fed wideband circularly polarized rectangular stair shaped dielectric resonator antenna,” IEEE Trans. Antennas Propagat., Vol. 54, No. 4, pp. 1350-1352, Apr. 2006 5. E. H. Lim and K. W. Leung, “Dual-wideband rectangular dielectric resonator antenna for WLAN communications” Microwave and Optical Technology Letters, Vol. 48, No. 2, pp. 378-380, 2006 6. D. Guha and Y. M. M. Antar, “Novel Designs of Dielectric Resonator Antennas for Wireless Communication“ Int. Conf. Electromagnetics in Advanced Applications, Torino, Italy, September 17-21, 2007. 7. D. Guha and Y. M. M. Antar, “Four-element cylindrical dielectric resonator antenna for wideband monopole-like radiation,” IEEE Trans. Antennas Propagat., Vol. 54, N0. 9, pp. 2657-2662, Sept. 2006. 8. D. Guha and Y. M. M. Antar, “New half-hemispherical dielectric resonator antenna for broadband monopole-type radiation,” IEEE Transactions on Antennas and Propagation Vol. 54, N0. 12, pp. 3621-3628, Dec. 2006.

Fig. 9. 3-D radiation pattern of 4 element q-HDRA at 3.5 GHz

-180 -135 -90 -45 0 45 90 135 180-30

-20

-10

0

Gai

n (d

B)

Azimuth (degree)

q-HDRA, f=3.5GHz

h-HDRA[8], f=3.3GHz

Fig. 8. Radiation property of four- element q–HDRA compared with those of two- element h-HDRA [8]. Phi = 90o, other parameters as in Fig. 7.

2.5 3.0 3.5 4.0 4.5-30

-25

-20

-15

-10

-5

0S

11 (

dB)

Frequency (GHz)

l=10mm l=12mm l=14mm l=13mm

Fig.7. Return loss characteristics of a probe-fed four element q-HDRA with different probe lengths. Parameters as in Fig. 3.