Wideband quarter-wave patch antenna with shorting pin

W.C. Mok, R. Chair, K.M. Luk and K.F. Lee

Abstract: The design and radiation characteristics of a wideband quarter-wave patch antenna with shorting pin are prcscnted. By the introduction of a shorting pin to the quarter-wave patch antenna with air substrate thickness of 0.08&, the antenna impedance bandwidth can be increased from 6.4% to 20.7%. Thc far ficld radiation pattem has a maximum gain 4.1 dBi occurring at O = -58" from the broadside direction. Detailed study based on simulation software 1E3D is presented. The results show good agreement with measurements.

1 Introduction

Low profile, lightweight, low fabrication cost and sn~all size are attractive features of microstrip antennas. They have been widdy used in various practical systems. The size of the antenna, however, may cause inconvenience for applications at the lower microwave frequencies. There are several techniques available for reducing the size of a patch antenna, such as the use of a shorting pin [l], rhorting wall [2] and high permittivity dielectric substrate [3]. These techniques, however, have the common disadvantage of reducing the bandwidth of the antenna.

In this paper, the des@ and radiation characteristics of a wideband quarter-wave patch with a shorting pin are presented. It is shown that, contrary to the case for the half- wave patch, the shorting pin in a quarter-wave patch increases the resonant frequency while significantly widen- ing the bandwidth. The impedance bandwidth is increased by a factor of about 2.5 in comparison to the half-wave patch. The size of thc shorted patch is about 18% smaller than the half-wave patch. The achievable impedance bandwidth (SWR<2) is over 20%, for an air substrate 11 of 0.081ia. The E-plane copolansed field is not symmetric due to the presence of the shorting wall. The pattern is tilted away from the broadside direction. Another feature of the radiation patterns is that the cross polarisation level is higher than the copolarisation level in the H-plane. This is due to the radiation of the shorting pin and shorting wall. The maximum gain is 4.1 dBi. The effects of feeding position d, and radius of shorting pin r, are studied. All simulation results are obtained using simulation software package IE3D 8.2.

2 Antenna structure

Fig. 1 shows the proposed patch antenna structure, comprising a patch with shorting pin and shorting wall.

Q IEE, ZW3 IEE Proceeding.? online no. 20030439 doi: IO. IW9/ipmap:ZW30439 P a p r 6nl received 26th luly 2W2 and in rwired form 7th Novrmkr 2W2 W.C. Mok. R. Chair and K.M. Luk arc wth thc Department of Electronic Engineeing, City University of Hong Kong. 83 Tal Chec Avenue. Kowloon. Honp Kong SAR. People's Republic of China K.F. Lee is with the School of Engineeing, University of blississippi. Univrrsily. MS 38677. USA

The coaxial feed excites the antenna in TMlo mode. The shorting pin is aligned between the coaxial feed and the shorting edge of the quarter-wave patch. I t is d,=20mm away from the radiating edge with radius r, = 0.5 mm. The dimensions of the rectangular patch are 30 mm length, L, by 20" width, W. The SOR coaxial probe has a radius r, = 0.5 mm. The distance 4 betwcen the feeding position and the radiating edge is 5". The distance h between the patch and the ground plane is 8". A foam substrate (+= 1.07) is used to support the patch. The ground plane size is 30.5cm by 30.5cm.

, t 1 ' feed shollino Din

30" \ i sholling wall

3 Experimental results

Experimental results including SWR, radiation pattern and gain were obtained. A HPSSIOC network analyser was used to measure the SWR of the antenna. The network analyser, coupled with an HP85301C antenna measurement system, was used to measure the gain and radiation pattem over a compacted range.

Fig. 2 shows the SWR curves of the quarter-wave patch antenna with shorting pin for different antenna thicknesses h. When the thickness h is increased, the resonant frequency decreases from 3.75 GHz to 2.95GHz and the impedance bandwidth increases from 9.07% to 20%. Table 1 sum- marizes the relationship between thickness h, feeding position d/. shorting pin position ds, resonant frequency AI, impedance bandwidth and gain.

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Table 1: Relationship between thickness h, feeding position dr, shorting pin position ds, resanant frequency f , impedance bandwidth and maximum gain of the quarter-wave patch antenna with shorting pin

Patch thickness, mm (in ;*I

Feed position d'. mm

3 (0.039i0)

4 (0.054;d

5 (0.062;+1

6 (0.072;.01

7 (0.077;+1

8 (0.081i01

9 (0.087i01

Shorting pin position d,. mm

8

10

14

16

18

20

22

Resonant frequency f,. GHr

Bandwidtj, % Maximum aain. dBi

3.75

3.88

3.6

3.49

3.17

2.95

2.79

9.07 3.4

12.40 3.5

17.20 3.3

19.20 3.7

20.19 4

20.68 3

20 3.6

Figs. 3a and 36 show, respectively, the radiation patterns in the E and H planes of the quarter-wave patch antenna with shorting pin at the resonant frequencyf;,= 3.88 GHz when h = 4". The copolarisation in the E-plane is almost omnidirectional on the front side of the patch, and the cross polarisation is about -15dB. I n the H-pkane. the 3dB beamwidth is about loo", and the cross polarisation is higher than the copolarisation. In both the E-plane and H - p h e , the back lobe is about -17dB.

0

90 270 90 270

180

Figs. 4 and 5 show the radiation pattems of the quarter- wave patch antenna with shorting pin at the remnant frequencies of fa= 3.49GHz (when . h=hmm) and j;,=2.95GHz (when h=Rmm) respcctively. It can be observed that the radiation pattern does not change substantially with antenna thickness.

IE.5 Proc.~Mic". A~rIcn,,os Propag., Vol. I50 No. I , F&wry 2W3

270 90

Fig. 6 shows the measured gain of the antenna at the corresponding maximum direction with different antenna thicknesses h. With 11 = 4 mm, the maximum gain was 3.4dBi measured at the maximum direction B = -58". The maximum gain. measured at the maximum direction O = -54"; was 3.7dBi with h=hmm, while the maximum gain measured at the maximum direction 0 = -56" was 3 dBi with h = 8 mm.

4 Parametric study

Fig. 7 shows the simulated and measured SWR curves for h=4mm, bmm and Rmin. The IE3D R.2 simulation software package was used for all simulations. The shift in simulated resonant frequency fo is less than 3% in most cases when compared with measurements. In light of the

51

3.0 3.5 4.0 4.5 frequency, GHr

Fig. 8 pin r.,

Simulr,terl (intennrr SWR for dgferent radii of shorting Fig. 6 ne.s.srr h

Measured gain of anretina for dfferwit mfentxi thick-

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1 .U

r m

2.5 3.0 3.5 4.0 4.5 frequency. GHz

Fig, 7 antenna

SWR of horh ,ner,sured wd simulation rrsultr f i w rhr

good agreement, the following parametric study was performed by simulation.

Table 2 shows the effect of the radius of the shorting pin r, on the impedance bandwidth. The antenna thickness h, feeding position dfi radius of feed r, and shorting pin position rs remain unchanged (i.e. h = 6mm, += 3 mm, rJ=0.5mm and dT= 16") while the radius of the shorting pin r, was varied from 0.1 mm to 1.5". The resonant freqnencyfo increased from 3.5 GHz to 3.82Ghz and the maximum bandwidth was 21.6% for r,= 1.3". Fig. 8 shows the simulated SWR of the antenna with different shorting pin radii r,. Matching of the antenna improved as r, was increased from 0.1 mm to 1.5".

Table2 Relationship between shorting pin radius r,, resonant frequency fo and bandwidth

Shorting Resonant Band- pin radius frequency width, Yo rs mm 8, GHr

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

3.5

3.59

3.63 3.69

3.72

3.75

3.79

3.82

111.7

19.9

20.7

20.9

2'1.2

2'1.5

2'1.6

2'1.4

SX

Fig. 9 shows the SWR of the quarter-wave patch antenna for different feed positions. The antenna thickness h and shorting pin position d, were kept constant (i.e. / I = 6mm and d, = I6 mm) when the feed position d, was varied from 14" to 2 m , moving towards the radiating edge. The resonant frequencyf" increased continually. The bandwidth and gain of the antenna were also enhanced when the feed was moved towards the radiating edge. The maximum bandwidth was 20.7% when (if= 4 mm while the maximum gain was 5.6dBi when d/=2nim. Table 3 summarizes the relationship between fecd position dr, resonant frequencyf;,, bandwidth and gain.

l."

2.5 3.0 3.5 4.0 frequency. GHz

Fig. 9 Simulated antentin SWR for dflerentfied positions 1 4

Table 3: Relationship between feed position d,, resonant frequency fo, bandwidth and gain

Feed position Resonant Bandwidth, Gain, dEi d,. mm frequency %

f,, GHz

14 2.71 6.3 3.9 12 2.88 11.7 4.2 10 3.06 14.6 4.4

8 3.24 17.1 4.6

6 3.43 19.3 4.9

4 3.57 20.7 5.2

2 3.66 19.8 5.6

Finally, Table 4 summarizes the effect of shorting pin position 4, on resonant frequency f;, and bandwidth. The antenna thickness h was 6". The feed was placed at

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Table4 Effect of shorting pin position d, on resonant frequency fo and bandwidth

08 t

elevation panern galn display dB,

0

08 L

elevation panein gain display dB,

0

08 L elevation pattern gain display dB,

capoi. in H-plane

- - - - - - - - -

Shorting pin Resonant Bandwidth position d, mm frequency %

f,. GHz

24 22 20

18 16 14 12 10 8 6

3.29 3.4 3.47 3.59 3.63 3.68 3.72 3.7 3.64 3.52

10.2 14.9 17.3 19.9 20.7 20.6 18.9 16.1 12.6 7.1

df = 3mm away from the radiating edge. Both feed radius y and shorting pin radius r, were kept constant. Only the position of the shorting pin d, was changed. A maximum bandwidth of 20.7% was achieved and the resonant frequency .fi was 3.63GHz for cI>= 16". The highest resonant frequency was 3.68 GHz.

5 Discussion

Table 5 compares results for a half-wave patch, a quarter- wave patch and a quarter-wave patch with shorting pin. All results were obtained using software IE3D 8.2. The three antennas were of the same size and thickness, with L = 30 mm. W= 20 mm and h = 6 mm. The feed radius r, and shorting pin radius u , ~ were 0.5". It is observed that the bandwidth of the quarter-wave patch with shorting pin is 20.7%, whereas it is only 8.5% for the half-wave patch and 6.4% for the quarter-wave patch. Introduction of the shorting pin increases the resonant frcquency 1;) from 2.19GHz to 3.63GHz and the gain from 4dB to 5.5dB. Moreover, the size of the quarter-wave patch with shorting pin is about 18% smaller than the half-wave patch. The reason for the impedance bandwidth enhancement is that the quarter- wave patch with shorting pin utilizes the cavity under the patch more efficiently.

Results show that the antenna thickness / I affects the resonant frequency& significantly. When the height changes from 3 mm to 9 mtn, the resonant frequency& reduces from 3.75GHz to 2.79GHz. This behaviour is similar to that of the half-wave patch antenna. There is an increase in crosspolarised field in the H-plane, contributed by the shorting pin and shorting wall. This is a special character- istic of the quarter-wave patch antenna. On the other hand, it is observed that when the radius of the shorting pin r, increases from 0. I m to I .5 mm, the bandwidth increases from 18.7% to 21.4%. Thus, using a thicker shorting pin can increase the bandwidth slightly. The feed position df also critically effects the matching and resonant frequency of the antenna.

The impedance bandwidth of the quarter-wave patch with shorting pin is much wider than the half-wave patch and quarter-wave patch since the quarter-wave patch with shorting pin ciln be modelled as two antennas, one a top- loaded monopole, which includes the pin and patch. and the other ii quarter-wave patch antenna. Fig. I O shows the

GHz,

59

Table 5: Comparison between half-wave patch, quarter-wave patch and quarter-wave patch with shorting pin

Structure Patch thickness Feed position Feed radius Shorting Shorting Resonant Bandwidth, Gain, h, mq- ( x ;.,,I dt mm rt. mm pin position pin radius frequency % dBi

d,. mm r,. mm 6, GHz

Halfwave patch 6 (0.092&) 6 0.5 4.43 8.5 8.5 Quarter-wave patch 6 (0.04511 6 0.5 2.19 6.4 4

Quarter-wave patch 6 (0.O75iQl 4 0.5 16 0.5 3.63 20.7 5.5 with shorting pin

simulated radiation pattems of the proposed antenna at f= 3.3 GHz. 3.63 GHz and 4.05 GHz. The copolansation in the E-plane at f= 3.3GHz, 3.63GHz and 4.05GHz has dips at 0=20"; 25" and 30" respectively. The radiation patterns consist of an end fire mode and a broadside mode, where the end fire mode coma from the top-loaded monopole and the broadside mode from the quarter-wave patch. In the H-plane, the cross polarisation field dominates the radiation. This gives evidence for the existence of a magnetic dipole radiating at the edge of the quarti:r-wave patch since the pattem radiates at 55" instead of 0".

6 Conclusions

A design is proposed for a compact microstrip iintenna based on a shorting wall and shorting pin. Bandwidth enhancement is achieved by using the shorting pin. The maximum bandwidth obtained is 21% at a gain of 4dBi.

Simulated results using the commercial software package 1E3D 8.2 arc in good agreement with measurements.

7 Acknowledgment

The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CityU 1 167/00E), and the Croucher Foundation Senior Research Fellowship.

References

WATERHOUSE. R . B 'Small microntrip patch antenna', El<mmn.

CHAIR. R.. LEE. K.F.. and LUK. K.M.: 'Bindwidth and cross- Lell., 1995, 31, pp. W O 5

Microti. Opi. Tdiriol. Lrll.. 1999. 22. pp. 101-103 LO. T.K.. CHUN-ON. HO. HWANG, Y.. LAM. E.K.W.. and LEE, B.: 'Miniature aoerturesounled micmstrin antenna of YCTY hilrh