Single-feed dual-frequency rectangular microstrip antenna with a n-shaped slot

H.-M.Chen

Abstract: A novel slotted rectangular microstrip antenna for dual-frequency operation is proposed. The microstrip antenna is fed by a single probe feed and has a n-shaped slot embedded close to the patch’s radiating edge. The two operating frequencies have the same polarisation planes and similar broadside radiation characteristics. By varying the lengths and the spacing of the two arms of the n-shaped slot, the proposed antenna can have a tunable frequency ratio ranging from about 1.12 to 1.64. Simple semi-empirical formulas of the two operating frequencies have also been obtained. Details of the antenna design and experimental results are presented and discussed.

1 Introduction

Recently, several dual-frequency designs of single-feed slot- ted rectangular microstrip antennas have been reported [ 1-71. The two operating frequencies of such dual-frequency slot-loaded rectangular patches are of the same polarisation planes and similar broadside radiation patterns. In such dual-frequency designs, different tunable frequency-ratio ranges for the two operation frequencies are also obtained. For the case of a pair of narrow slots etched close to and parallel to the patch’s radiating edges, the tunable fre- quency-ratio range is typically within 1.G2.0 [l-31. This frequency ratio limits such designs for applications where a lower frequency ratio is required. To obtain a frequency ratio less than 1.6, the structure of such a slot-loaded microstrip antenna must be modified: two resonating microstrip stubs can be printed on a back substrate and connected to the rectangular patch through two vias in the ground plane [l]. This arrangement permits a frequency ratio ranging from 1.2 to 3, but the structure of such a design becomes complicated and the antenna’s total dimen- sions also increase. The related design that has been reported uses a rectangular patch with a pair of U-shaped slots operated at dual-band [4]. The frequency ratio of the two frequencies can vary from about 1.43 to 1.60, which is lower than that of the rectangular patch [l] with two nar- row slots close to and parallel to their radiating edges. Other designs of placing a pair of properly bent slots [5], a pair of step slots [6] or a pair of comb-shaped slots [7] close to the non-radiating edges of a rectangular microstrip antenna, provide tunable frequency ratios within ranges of about 1.29-1.60 [5], 1.23-1.63 [6] and 1.07-1.29 [7], respec- tively. In such designs, there is a smaller frequency ratio than that of slot-loaded patches [ 1 4 . The disadvantages of these designs [5-71 with the etched slots are more compli- cated and sensitive.

0 IEE, 2001 ZEE Proceedings online no. 20010224 DOf 10.1049/ip-map:20010224 Paper frst received 26th April and in revised form 26th October 2ooO The author is with the Department of Physics, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan 307, Republic of China

This paper shows that by loading a n-shaped slot close to the radiating edge with a single probe feed located along the centreline of the rectangular patch (Fig. l), a new dual-fre- quency operation of the antenna can easily be obtained. The two operating frequencies of the proposed antenna are also found to have the same polarisation planes and similar broadside radiation characteristics, and the frequency ratio of the two frequencies is obtained ranging from 1.12 to 1.64, which is smaller than that of the conventional slotted designs [l-31. This can make the present proposed antenna more suitable for dual-frequency applications where a lower frequency ratio is required. Details of the antenna design and experimental results are presented and discussed.

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Fig. 1 shupeil slot for duul-jkquency operation

Geometry o j single+ed rectcrrzgdur microstriy cinteimu ivitlz CI z-

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Fig. 1 shows the geometry of the n-shaped slotted micros- trip antenna. The rectangular patch has a dimension of L x W, and is printed on a substrate of thickness 11 and relative permittivity E,.. The n-shaped slot is etched on the rectangular patch close to and parallel to one of the radiat- ing edges of the patch. The distances of the embedded slot from the radiating and non-radiating edges are denoted I and w, respectively. The spacing between one arm of the

Various arm positions of the n-shaped slot

IEE Proc.-Microw. Airtentins Propug., Vol. 148, No. I , Fehrunry 2001 60

nshaped slot and the x-axis is W,. The lengths of the arms on the 7c-shaped slot are L,. The widths of all the n-shaped slots are the same, denoted as d (1" in this study). Good impedance matching of the two operating frequencies can be obtained by using a single probe feed at a position xll away from the patch centre.

By loading a pair of narrow slots close to the radiating edges of a retaiigular microstrip patch [I, 21, dual-frequency operation of the microstrip antenna can be obtained. The resonant frequency associated with the modified TM mode is denoted as f i 0 and written as a simple empirical formula. It is first found that by embedding only one slot (without arms of the n-shaped slot) close to the radiating edge of the rectangular patch, the first mode V;o) is excited at 1746MHz. The first mode is well in agreement with the calculated result of the simple formula in [I], which is reso- nant at 1742MHz.

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Fig. 2 Mecisurd return 1o.csjbr the i~roposed rtnteni~ti with various tirtii posi- tion cf the z-shaped slot E,. = 4.4. I? = I.bmm, L = 40mm, W = 30in111, d = I = iii = I mii?, L,y = l8mm - W, = 5mm, ~ W, = 6mm, ~ ~ ~ W, = 7iiim, - ~ ~ ~ W, = Xii im,

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~ measured ~~-~ siiiiulatcd

Metisured ond simultrted monant ,frequencies fj nix1 j j cincl j iquency

When embedding a pair of slots parallel to the non-radi- ating edges of the one slot-loading patch, called mhaped slot loaded antenna, a new resonant mode between the TMlo and TM, modes of the one slotted rectangular patch antenna can be excited. Typical proposed antennas were implemented and tried. Fig. 2 shows the measured return loss for the proposed antenna with various arm positions (JVJ. The corresponding dual-frequency data are listed in Table 1 for comparison. Note that the frequency,fi is the resonant frequency of the perturbed TM,, mode andft is

IEB Proc.-Micwiv. Anle)ii?n.~ Propig. , V d 148, N o . I , FeDrcior)~ 2001

for the new resonant mode excited between the TMlo and TM20 modes. It is first found that the two excited resonant modes have good impedance matching for various arm positions. The first resonant frequency V;) decreases rapidly with increasing ann position ( W,); the second resonant fre- quency (fJ is, however, very slightly affected by the varia- tion of lV5, which results in an increase in the frequency ratio. The experimental and simulated results (by 1E3D software) against the arm position are presented in Fig. 3. The different effects of the slot loading on the first two res- onant frequencies make a tunable frequency ratio V;lfi) in the range of about 1.12 to 1.27. It is worth noting that the narrow bandwidth at the two frequencies is due to the small substrate thickness. In Table 1, it is found that the bandwidth of f 2 , determined from lOdB return loss, is wider than that off,. This is owing to the fact that the sub- strate is thicker with the frequency increasing.

Table 1: Dual-frequency performances of the proposed antenna with various arm positions

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f2J f l Arm position f i , BW, f2 , BW,

MHz, % MHz, % x,,, mm W, (mm)

5 -10.4 1682, 1.52 1885, 1.75 1.12

6 -10.3 1645, 1.52 1877, 1.57 1.14

7 -10.2 1626, 1.45 1878, 1.76 1.16

8 -10.0 1592, 1.35 1877, 1.81 1.18

9 -9.9 1547, 1.26 1870, 1.74 1.21

10 -9.8 1514, 1.12 1867, 1.87 1.23

11 -9.7 1480.1.01 1878.1.81 1.27

Antenna parameters are given in Fig. 2

By using the simulation software IE3D, the excited patch surface current densities for the two operating frequencies of the proposed antennna are also simulated and analysed. The simulated current distributions of the two operating frequencies, f i and& are plotted in Figs. 4a and 6, respec- tively. For the case of f, shown in Fig. 4a, the current dis- tribution of the TM,, mode is strongly perturbed, since the arms of the z-shaped slot are present. Because of the arms of the "ped slot loading, the null of the current distri- bution is close to the two edges of each arm slot. The cur- rents find a resonant condition by circulating around the arm slot, and the current-line path length increases with increasing arm position. To design the two frequencies, simple semi-emperical formulas have been found very usetul. An approximate equation is derived by fitting the experimental data:

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2(2L, + d + kWs + AW)& fl =

k = 0.996, AlV = 2.043 (2) where k and AW are correction factors for the n-shaped slot loading effect. E, is effetive dielectric constant [I].

As for the case offi shown in Fig. 4b, the new resonant mode is slightly affected by changing the arm position. It is seen that the excited current distribution is very similar to that of the TMlo mode of the case without the arm slot. Thus, for the present dual-frequency design, the two oper- ating frequencies are expected to be of the same polarisa- tion planes and similar radiation characteristics. It is also obtained by a curved-fitting method, and an approximate equation can be predicted according to

f2 = 2(38 - L, + kL, + AL)& (3)

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k = 1.037, A L = 0.947 where k and AL are also correction factors of the slot-load- ing effect for the resonant modeji. Eqn. 3 is independent of W,; that is, the resonant mode ji is only slightly affected by the arm position. The calculated results of eqns. 1 and 3 are shown in Fig. 5 for comparison, and the maximum relative differences for f i and& are less than about 0.7% and 0.5%, respectively.

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Fig. 5 mental results& various arm positiow ~ _ _ _ eqn. 1,. measurement ~ eqn. 3, A measurement

Com uriron of evuluuted re,sults of upproxirnate equations und experi-

Note that these approximate equations are based on the experimental results obtained using the FR4 substrate. For the sake of generalisation, the behaviour of different aspect ratios and dielectric constants of the substrate was studied

by simulation analysis. Fig. 6 shows the variation of the two resonant frequencies and ,fi) against Wsl W for dif- ferent values of aspect ratio ( WIL) and different dielectric constants (E,.). In particular, all the curves have 'been obtained by keeping constant the width of the slot (dlL = 0.025) and the substrate thickness (h = 1.6"). As previ- ously mentioned, the curves associated withf2 have a slight variation with respect to Wsl W, but the curves associated withf, have a strong perturbation with respect to arm posi- tion. As expected, the two resonant frequencies increase with decreasing dielectric constant of the substrate for the same aspect ratio (WIL = 0.75). It can also be seen that the two resonant frequencies shift to higher values with decreasing the aspect ratio. This is because the resonant wavelength decreases as the arm position W, decreases. From Fig. 6, it can be found that the curves of f i and f i are parallel. This confirms that the approximate equations can be directly used for a simple and accurate design.

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h = l.bmm, L J L = 0.45, w/W = 0.033, //L = d L = 0.025 (I) W/L = 0.75: E. = 2.2

Resonunt Ji'equencies fi und fi against W,/W for dgerent vu1ue.v of WgL Unu' Er

(5 W/L = 0.60;-& = 4.4 (jii) WIL = 0.75; E,. = 4.4

The radiation patterns of the proposed antenna in the two principal planes are presented in Fig. 7. It is seen that both operating frequencies have the same radiation patterns and polarisation planes, and good cross-polarisa- tion is observed. The on-axis antenna gains of 1.2dBi and 3.0dBi were found for .f; and fi, respectively, with W, = 6". Since the arm slots strongly affect the radiation patterns of the first mode cfi), some gain losses are seen.

3 Various arm lengths of the mhaped slot

To extend the frequency ratio between the two frequencies, the dimensions of L, in Fig. 1 should be varied with fixed arm position (I+',>. Prototypes of this design were also implemented and studied. As for varying the arm length (L,) of the n-shaped slots, two measured frequencies and the frequency ratio are shown in Fig. 8. The corresponding measured dual-frequency performance is also listed in Table 2 for comparison. From the results shown in Tables 1 and 2, the frequency ratio of the proposed antenna can be controlled in a range of about 1.12-1.64, by varying W, and L,,. This obtained frequency-ratio range is lower than that obtained previously [l, 21, which makes the proposed antenna very suitable for dual-frequency applica- tions with a smaller frequency ratio.

From the simulated results of IE3D, it is found that the patch surface current path of the first mode V;) is length- ened and the currents circulate around the two arm slots.

62 I E E Proc.-Microw. Anterinris Propcig., Vol. 148, No. I , Fehruciry 2001

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Table 2: Dual-frequency performances of the proposed antenna with various arm length (L,)

f 2 4 Arm length L, fit BW, fz, BW,

MHz, % MHz, % xp, mm (mm)

22 -7.4 1461,0.96 1806, 1.19 1.24

24 -8.0 1371,0.95 1795, 1.67 1.31

26 -8.6 1285, 1.05 1787, 1.88 1.39

28 -9.0 1218, 1.19 1777, 1.91 1.46

30 -9.6 1146, 1.35 1776, 1.94 1.55

32 -10.4 1086, 1.29 1776, 1.89 1.64

Antenna parameters are given in Fig. 8

This behaviour is shown in Fig. 9a. From the results, it is seen that the resonant frequency f i is significantly decreased as expected. In this case, a resonant condition with the excited patch surface current nulls close to the edges of the two slots is obtained. This resonant condition gives the res- onant frequencyf,, and an approximate equation is derived by fitting the data and written as

( 5 ) f1 = 2(2kL, i- d + ir, + AW)&

k = 0.873, AW = 4.352 (6)

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It is seen that the resonant condition is slightly dependent on the arm length; the excited resonant frequency can be obtained by an approximate equation.

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f 2 = 2(38 - L, + k L , + AL)& IEE Psoc.-Micsow. Antenntis Propag., Vol. 148. No. I, Fcbsuary 2001 63

IC = 1.072, A L = 1.656 ( 8 ) The evaluated results of eqns. 5 and 7 are shown in Fig. 10 for comparison, the maximum relative difference between resonant frequencies is less than about 0.50/0. From the current distributions in Fig. 9, both frequencies have similar broadside radiation patterns and the same polarisation planes, as expected from IE3D simulation results.

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Compari,son of evulnuted results of ripproxirnate equatioiu imd L,, mm

Fig. 0 experimental results,fi)r various arm lengths ~ ~ _ _ eqn. 5 , 0 measurement ~ eqn. 7, A measurement

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4 Conclusions

A novel dual-frequency design of a single-feed rectangular “microstrip antenna with a n-shaped slot has been demon- strated. By varying the arm position and length of the n-shaped slot, the proposed dual-frequency microstrip antenna provides a tunable frequency ratio of range about 1.12 to 1.64, which is suitable for applications with a low frequency ratio. Moreover, a set of simple and semi-empiri- cal approximate equation has been derived for the accurate design of the two frequencies. Also, the two operating fre- quencies have the same polarisation planes and similar broadside radiation.

5 References

1 MACI, S., BIFFI GENTILI, G., PIAZZESI, P., and SALVADOR, C.: ‘Dual-band slot-loaded patch antenna’, IEE Proc., Microw. Anten- ~ L L Y Propug,, 1995, 142, (3), pp. 225-231

2 MACI, S. , and BIFFI GENTILI, G.: ‘Dual-frequency patch anten- nas’, IEEE Anterznus Propug. Mag., 1997, 39, (6), pp. 13-20

3 YAZIDI, M.E., HIMDI, M., and DANIEL, J.P.: ‘Aperture coupled microstrip antenna for dual frequency operation’, Electron. Lett., 1993, 29, (17), pp. 150&1508

4 GUO, Y.X., LUK, K.M., and LEE, K.F.: ‘A dual-band patch antenna with two U-shaped slots’, Microwave Opt. Teclzrzol. Lett.,

5 WONG, K.L., and SZE, J.Y.: ‘Dual-frequency slotted rectangular microstrip antenna’. Electron. Lett., 1998, 34, (14), pp, 1368-1370

6 LU, J.H.: ‘Single-feed dual-frequency rectangular microstrip antenna with a pair of step-slots’, Electron. Lett., 1999, 35, (5), pp. 354-355

7 LU, J.H.: ‘Dual-frequency operation of a single-feed rectangular microstrip antenna with a pair of comb-shaped slots’, iMicrownve Opt. Technol. Lett., 1999, 23, (3), pp. 183-186

2000, 26, (2). pp. 73-75

64 IEE Proc.-Miwoiv. Ailtennm Propug., Vol. 148, No. I , F e f i r u q ~ 2001