Microstrip line-fed L-strip patch antenna

C.L.Mak, K.M.Luk and K.F.Lee

Abstract: A wideband electromagnetic-coupled single-layer microstrip patch antenna is studied experimentally. With dielectric and foam substrates of total thickness - 0.13&, a rectangular patch antenna with impedance bandwidth (SWR s 2) of 49% and 3dB gain bandwidth of 54% are obtained. It has an average gain of 6.5dBi and stable radiation patterns across the passband. Furthermore, with the employment of a stacked parasitic patch, a 58% impedance and gain bandwidth is achieved with similar radiation patterns and a higher average gain. A notable structure in the feeding design is that an inverted L-shaped strip is connected to the end of the microstrip line and no matching network is required.

1 Introduction

Microstrip patches are one of the popular types of anten- nas. It is, however, well known that the major weakness of such an antenna is its inherently narrow bandwidth, typi- cally a few percent. Techniques have been proposed in the literature for broadening the bandwidth such as the use of a thick substrate [l] and a U-slot patch [2, 31. It was found in [2, 31 that, with the utilisation of a U-shaped slot for a probe-fed patch, the typical handwidth attained is - 30%. Another method is to employ the aperture coupling tech- nique proposed by Pozar in 1985 [4], which, however, exhibits a relatively high backlobe radiation. Patch anten- nas employing such feeding techniques have typically 22% bandwidth for the single-layer case [5] and 37% bandwidth for the stacked dual-patch case [6]. A feeding method lhat does not have the high hacklobe drawback is the proximity coupling technique introduced by Pozar and Kaufman in 1987 [7]. With an impedance matching stub connected to the feed line, 13% bandwidth was achieved. Recently a sin- gle-layer proximity-coupled patch antenna with U-slot is proposed [XI, with 20% bandwidth. An impedance match- ing network connected to the end of the feed line is also required.

Recently, a new broadhanding technique, in the form of an Lshaped probe, was applied to a coaxial-fed microstrip antenna [9], resulting in 35% handwidth. In this paper, L strip patch antennas fed by microstrip lines are investi- gated. Both single-patch and dual-patch cases are studied, with the objective of achieving broadband characteristic.

2 Antenna structures

Single-patch antenna The basic geometry of the antenna is shown in Fig. 1. A rectangular patch (PI), designed to operate at fo =

0 IEE, 1999 IEE Proceedhgf a h e no. 19990569 DO? 10.1049/ipmap:19990569 Paper f i t received 8th December 1998 and in revised form 7th May 1999 C.L. Mak and K.M. Luk are with the D e m e n t of Electronic En&eaing, city University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hang Kong SAR, Peoples’ Republic of China K.F. Lee is with the Depamnent of Electncal Engineering, University of Mis- SouriCalUmbia, Enghering Building West, Columbia, Missouri, U S A

282

4.5GHz, with width W = 30- (0.454) and length L = 25- (0.3754) is supported by a foam layer of thickness H = 8.3- (0.1245&). A 508 microstrip feed line is etched on a dielectric substrate (er = 2.33) of thickness h = 1.6mm (0.0244) which is located symmetrically with respect to the patch. The patch is electromagnetically cou- pled by an L-shaped strip, which is connected to the end of the microstrip line. The vertical and horizontal lengths of this Lstrip are a = 5mm (0.075&) and b = lOmm (0.15&), respectively. The horizontal portion of the strip is sup- ported by another foam material. The vertical portion is located under the patch at a distance S = lmm (0.015&) from the edge of P1 as shown in Fig. 1. The width of the L-shaped strip is selected to be equal to the width of the 508 line. The antenna is excited in the TM,, mode.

W

~ ~~

perspective view

a

tWP1) dieiectric e (s,=2.33)

Y

ground plane foam side view

h - Fig. 1 Bmicgeomehy ofmimom$line$?dpatch m r e m n Pelspective view; b Side view

Stacked dual-patch antenna The side view of the dual-patch antenna designed to oper- ate at fo == 4.5GHz is shown in Fig. 2. A parasitic patch (P2), with width Wp = 40mm (0.64) and length Lp = L = 25mm (0.375&), is added above the lower patch (PI). P1 and P2 are respectively at heights of H = 5mm (0.075&) and Hp = 8.3mm (0.12454) above the same dielectric sub-

IEE Proc.-Microw. Antennas Propog., Vol. 146. No. 4, August I999

5

f 2 -15

-20 1 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

(I) x__ 6

frequency, GHZ B

7

6

10

5

9 0 5 .: 5

4 $ 0

-10 3

-15 2

-20 1 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 8.5

frequency, GHz

b Fig.3 parch and dual- rch mlfovws a Singlqxtch: g",ua.l.patch (i) mpolarisstion gain; (ii) crosspolarisation: (iii) SWR

Capolariration gain, crosspolarisation gain and SWR culws ofsinglc

3 Measurements

The standing wave ratio (SWR) and gain (copolarisation gain and crosspolarisation gain) of both antennas are meas- ured, respectively, by an HP8510C network analyser and a compact range with an HF'85310C antenna measurement system. As shown in Fig. 3a, the single-patch antenna has a bandwidth of 49% (SWR s 2) and a 3dB gain bandwidth of 54%. From Fig. 3b, the dual-patch antenna has 58% of both impedance bandwidth and 3dB gain bandwidth. Both antennas have copolarisation gain > 6dBi across most of the passband. Moreover, Fig. 3 also shows that the level of crosspolarisation gain (in the H-plane and at particular directions) is relatively high close to the upper end of the

IEE Proe.-Microw Antennas Propa&. Yo!. 146, No. 4, August 1999

passband. Figs. 4-7 show the radiation patterns at 3.5, 4 and 4.5GHz of the single-patch and dual-patch antennas. It can be observed that the patterns are symmetrical about the broadside direction and attain - -20dB bacMobe radia- tion. It is found that the crosspolarisations of both anten- nas are below 20dB in all directions except at @around 45" in the H-plane, with a level of -15dB average for the single- patch case. A similar level of crosspolarisation was obsetlied from the probe-fed U-slot patch antenna [3] and L-probe-fed patch antenna [9]. The radiation patterns are stable across the passbands of both antennas.

0 dB 0 dB

H-plane E-plane a

H-plans E-plane

b Fig.4 Whfionpaffernr of skgie-pafch mtem af 3.5 rmd4GHz a 3,5GHz, b 4GHz - copolarisation ...,..,.. crasspalaisation

0 dB 0 dB

\ I /

H-plane E-plane Fig.5 Rad~rwnpafrem of sin icpafch rmtenm ai 4.5GHz

~ copolansation ... .. .. crosspofirisation

For the antennas with other values of H or Hp, the impedance bandwidth changes very little; however, the gain is highly sensitive to these parameters. For instance, in the single-patch case, with the value of H varying from 5 to 9mm (0.075h to 0.135&), it is possible to attain an imped- ance bandwidth (SWR s 2) > 44%. However, the average gain will be below 6.5dBi when H = 9mm and 7mm. It is necessary to mention that when varying the values of H or Hp, we also need to alter the parameters a, b and s to have a wideband performance.

283

0 dB 0 dB

H-plane a

0 dB 0 dB

H-plane E-Plane

b Fig.6 R ~ ~ f i o n p l l e m of dualpieh miem ai 3.5 mvl4GHz a 3.5GHz, b 4GHz

0 dB 0 dB

H-plane E-plane W i a l i o n p t l e m of&-plch mimm ai 4,SGHz Fig. 7

4 Discussion

The broadband performance of the proposed antennas is basically achieved by the idea of employing a thick sub- strate [l]. In our design, a foam of thickness of - 0.12& is used to support the radiating patch. Without the proposed L-strip, it is difficult to couple the energy from the micros- trip l i e to the patch as the separation between them is too large. Therefore a step, which is designated as an L-strip, is introduced at the end of the line so that the spacing between the patch and the feed line can be reduced. In addition, the Lstrip-feed method can further enhance the bandwidth. The horizontal part of the Lstrip of length b < &I4 incorporated with the patch provides a capacitance to suppress the inductance introduced by the vertical part of the Lstrip. The whole structure of the L-strip acts as a

series L C resonant element, which is connected in series with the parallel R-LC resonant element of the patch. The resonant frequency of the former has to be close to that of the TMol mode of the patch for wideband performance. For the traditional probe-fed patch antenna, the probe only provides an inductance, which degrades the bandwidth per- formance of the patch antenna. As mentioned before, the crosspolarisation is quite high in some directions, especially at the upper part of the passband. However, it is found that this shortcoming can be overcome in array design [lo], in which the feed probes are connected to the microstrip l i e underneath the ground plane via small holes. In other words, the radiating patches and the feed line are not in the same side of the ground plane. Moreover, experiments show that the width of the horizontal portion of the Lstrip does not have much effect on enhancing the bandwidth.

5 Conclusion

A single-layer microstrip-he-fed patch antenna of 49% impedance bandwidth (SWR 5 2) has been proposed. With the same total thickness and feeding method, a stacked dual-patch antenna with 58% impedance bandwidth has also been found. Both antennas have stable radiation pat- terns and an average gain > 6dBi across the passband. The remarkable feature of the antenna is that a small step is introduced at the end of the feed line. We describe this as an L-shaped strip connected to the end of the feed line. No slots on the patch and matching network along the feed line are required to enhance the bandwidth. Moreover the noncontact structure facilitates the fabrication of antenna arrays.

6 Acknowledgment

The work is supported by the CERG project, Hong Kong SAR, 9040210.

7 References

1 HALL, P.S.: ‘Probe cornmuation in thick microstrio oatches’. Elec-

4 P0Z.k. D.M.: ‘M&trio antenna aoertureauoled to a microstrio-

8 MAK, C.L., LUK, K.M., and LEE, K.F.: ‘Proximity-coupled U-slot patch antenna’, Elcciron. Lett., 1998,34, pp. 715-716

9 LUK. K.M.. MAK. C.L.. CHOW, Y.L.. endLEE. K.F.: ‘Broadband microstrip patch antenna’; Elect?& h i t . ; 1998, 34,’pp. 1442-1443

10 MAK,C.L., LUK, K.M., TONG,K.F., CHOW,Y.L., and LEE, K.F.: ‘A novel broadband rectangular microstrip antenna’. Pro- ceedings of 1998 Asia-Pacific Microwave conference, Y o k o h a , Japan, 1998, Vol. 2, pp. 1031-1033

284 IEE Pros.-Mierow. Antennas Props. . Vol. I46, No. 4, Augusl I999