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Small rectangular patch antenna

K.M. Luk, R. Chair and K.F. Lee

A novel folded rectangular patch antenna is designed andmeasured. Compared with a conventional patch antenna with thesame surface area, the resonant frequency is reduced by 37%. Thecross-polarisationevel is - 20dB.

Zntroduction: Microstrip patch antennas have the attractive fea-tures of low profile and light weight, and can be made conformalto mounting structures. In the low microwave frequency range,

however, their sizes may be too large for practical applications.Several techniques have been proposed to reduce the size of theconventional half-wave patch. One approach uses expensive andheavy high dielectric constant material [l ]. Another approach useseither a shorting wall [2] or a shorting pin [3]. A short ing wallleads to the quarter-wave patch while a shorting pin near the feedcan reduce the patch size even further. However, in both cases, thecross-polarisation level is quite high. In this Letter, a novel smallfolded patch antenna is proposed. The antenna is simple in struc-ture and has low cross-polarisation. Compared to a conventionalpatch antenna with the same surface area, the resonant frequencyis reduced substantially.

top view

side view

a b C plane

L I W I di I d2 I h i I hz 1 I1 I 12 i W i I W 2 I w3 I W4 1 W5 I Wg

31 I 51 I 2 0 15 I 2 I 3 I 1 5 I 2 3 I 3 0 11151 9.51 5 I 1 5 I 11

(unit:")

Fig. 1 Structure ofpatches and antenna

W = 31mm,L = 51mm, d , = 20mm, d, = 15"h , = 2mm, h , = 3mm, w 1 9.5mm, w,= 5mm1, = 10mm, l2= 15mm, l3= 15mm, 4 = 23"

1 C I

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

frequency, GHz a

Fig. 2 S WR against frequency

(i) single layer patch;& = 2.65GHz; BW= 1.23%(ii) folded-patch configuration 1;fo = 2.1GHz;BW = 2.03%

(iii) folded-patch configuration2; f0

= 1.66GHz; BW = 3.16%

Antenna structure: The geometries of the antennas are shown inFig. 1.  Fig. l a presents a conventional patch antenna with lengthL = 5 1mm and W = 31mm. This antenna excited in TMIomode isused as a reference. Fig. lb shows a folded patch antenna, desig-nated as folded-patch configuration 1, which is made of a coppersheet of length 85.5" and width 31mm, while Fig. ICshows asecond folded patch antenna, designated as folded-patch configu-ration 2, which is made of a copper sheet of length 111mm andwidth 31" The antennas are fed by coaxial feeds. Although allthe three antennas have the same length in the top view, it isfound that the resonant lengths of the folded patches are effec-

tively longer than the length of the conventional patch. Thedimensions of each patch antenna are chosen after a series ofextensive measurements, and are close to the m i n i " resonantfrequency for each type of folded patch.

6

5

$ 4

.-e8 3

2

1

n"1.4 1.6 1.8 2.0 2.2 2.4

frequency, GHz mB

Fig. 3 Measuved gain- onfiguration 1configuration 2

0 0

180' 180'

Fig. 4 Folded-patch configuration 1 radiation patter n

a E-planeb H-plane- o-polarisation

10dB/div

a b

cross-polarisation

O0 0

180' 180'

a b

Fig. 5 Folded-patch configuration 2 radiation patte rn

a E-planeb H-plane- o-polarisation

10dBidivcross-polarisation

Measurements: An HP8510C network analyser and a compactrange with an HP85301Cantenna measurement system was used

to measure the SWR and the radiation pattern of each antenna,respectively. As shown in Fig. 2, the resonant frequency of therectangular patch antenna is decreased when the patch is folded.The resonant frequency for the conventional rectangular patch is2.65GHz. Folded-patch configuration 1 has a resonant frequencyof 2.1 GHz (20.75% decrease) while the folded-patch configuration2 has a resonant frequency of 1.66GHz (37.26% decrease). Thegains of the two configurations are 6.1 and 5.8dBi, respectively(Fig. 3). The thickness of the antenna (Smm), in units of substratewavelength, is -0.056 at 2.1GHz and 0.044 at 1.66GHz. Theradiation pattems of the folded-patch configurations have beenmeasured at 2.1 and 1.66GHz, respectively. As shown in Figs. 4 

and 5, the co-polarisation pattems have maxima in the broadside

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direction. The cross-polarisation maximum is --20dB below theco-polarisation maximum. This is significantly ower than that fora shorted quarter-wave patch or patch with shorting pin.

Discussion: Two novel designs for small rectangular patch anten-nas have been presented. The resonant frequencies of both folded-patch configuration 1 and folded-patch configuration 2 werefound to be decreased compared to the the resonant frequency ofthe conventional patch. This is because the effect of folding thepatches is an increase in the physical length of the patch. Refer-ence to Fig. 2 shows that the bandwidth (SWR = 2) is 2.03% forfolded-patch configuration 1, 3.16% for folded-patch configura-tion 2, and 1.23% for conventional patch. It was found that whenthe height of each layer is increased, the bandwidth of the antennaincreases but the resonant frequency also increases slightly. Com-pared to the patch with a shorting wall or shorting pin, the foldingdesigns result in considerably lower cross-polarisation.

0 EE 1998Electronics Letters Online No: 19981643

K.M. Luk and R. Chair (Department o Electronic Engineering, CityUniversity of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong K ong)

K.F. Lee (Department o Electrical Engineering, University o MissouriColumbia, Engineering Building We st, Columbia, Missouri 6521 , U S A )

20 November 1998

References

1 LO, T.K.: ‘Miniature aperture-coupled microstrip antenna of veryhigh permittivity’, Electron. Le tt. , 1997,33, pp. 9-10

2 PINHAS, s,and

SHTR IKMAN ,

s.: ‘Comparison between computed andmeasured bandwidth of quarter-wave microstrip radiators’, ZEEETrans., 1988,AP-36, (ll), pp. 1615-1616

3 WATERHOUSE, R.B.: ‘Small microstrip patch antenna’, Electron.

Lett. , 1995,31, pp. 604605

Low-thresholdAIGaN-GaN heterostructurefield effect transistors for digita l applications

R. Gaska, J. Deng and M . S . Shur

The fabrication of an AlGaN-GaN based digital inverter circuit isreported, in which a large gain (up to 180) and a noise margin of-0.W have been obtained. The measured temperature coefficient

of the switching voltage of the inverter was -3SmVi”C up to90°C. The simulations predict that such an inverter shouldoperate up to -230°C.

AlGaN-GaN heterostructure field effect transistors (HFETs) havedemonstrated an extremely low gate leakage current and exhibiteddrain current saturation at positive gate bias as high as +6V [l].The low values of the gate leakage point out to the potentialapplications of AlGaN-GaN HFETs in high temperature digitalcircuits such as, for example, the direct coupled field effect transis-tor logic (DCFL) family. Such circuits operate on a single powersupply and, thus, require switching transistors with a positive (orsmall negative) threshold voltage. The first AlGaN-GaN baseddigital circuit was demonstrated by Khan et al. [2].

In this Letter, we report the performance of AlGaN-GaNHFET-based inverter at elevated temperatures.

The low-threshold AI,,zsGa, ,N-GaN HFETS were grown on

sapphire substrates using low-pressureMOCVD [3]. We minimisedthe threshold voltage of the devices by growing nominallyundoped epilayer structures and by depositing a very thin (-lOnmthick) AlGaN barrier with a 25% aluminum molar fraction. Thisallowed us to fabricate HFETs with -75-80% depleted channeleven at zero gate bias (see T1 in Fig. 1). The electron mobility inthese heterostructures was 840cm2Ns and 3000cmZiVs at roomtemperature and a t 77K, respectively. The sheet electron densitywas -5 x l O ’ * ~ m - ~nd did not change with temperature. As seenfrom Fig. 1, the maximum source-drain current of these low-threshold devices was -130mAimm, which is almost an order ofmagnitude lower than in our high power HFETs with doped chan-nel design [l]. The maximum transconductance was 125mSimm

measured at a positive gate bias V ,= 0.25V. A large energy banddiscontinuity in the AlGaNiGaN heterostructure, strong piezoelec-tric effect and a large electron effective mass resulted in a turn-onvoltage for the gate current as high as +2.2V (see [l]).

, . , ‘ I . I . , . ,

140

T200

E150 -EB -Ei

E100 -L3

K

m-

% 50 -

0 -

- 120

. E

. E

-80 $

60 z

- 40

IOl)$

m+-

-c

s. c

- 20

- 0

-1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0

gate bias,V L@?

Fig. 1  Transfer characteristics of two Al, *,Ga, ,5N-GaN HF ET s (TIand T2)used in digital circuit shown in Fig. 2 

source-drain current calculated using following fitting parame-ters fo r T1 and T2, respectively: output conductances 0.013 and 0.01V-I; mobility 0.15m2/Vs; sheet electron density 7.3 X and 9.8 X

1015m-Z;ource and drain resistance 5 Q saturation velocity 1.1 X lo5and 1.5 x 105m/s; knee voltage parameters 5 and 1.9; distance to

buffer layer charge 30nm; threshold voltages -0.6 and -1.13V; gate

length 2 p

The devices exhibited a very low gate leakage, a sharp pinch-offand very high breakdown voltage. The gate leakage current in a4 5 p wide HFET was 200pA at a reverse gate bias of -1V andremained unchanged up to V ,= -10V. For comparison, the maxi-mum drain current in this device was -6.5mA at V , +2.0V. Thus,the on-to-off ratio of this device was of the order of 108.

To increase the breakdown voltage, we fabricated HFETs withoffset gate design [3]. The breakdown voltage was measured in4 5 p wide devices with a source-drain spacing of 5 p , gatelength of 2 p , and source-to-gate and gate-to-drain distances of0.5 and 2 S p , respectively. The irreversible breakdown of thedevices at a gate bias V,= -1 V occurred at a source-drain voltageV, = 330V. The estimated average electric breakdown field is -1.3x 106V/cm. This breakdown field is higher than the previously

reported record value of 1.1 x 1O6Vicm 4] and is close to the max-imum breakdown field estimated in [3 ] .

6

5

4

>9 3

>”

2

1

0 1 ’ ’ I . ’ * ’ . 1 1

Vi”, v jozlizi

0 1 2 3 4

Fig. 2 Characteristics of inverter

Based on switching (Tl) and load (T2) AlGaN-GaN HFETs withthreshold voltages of -0.5V and -1.0V and GaN Schottky barrier

gate fo r level shiftingInset: schematic diagram of digital circuit

diode (D) with turn-on voltage of 1V connected in series with HFET

We used these the low-threshold HFETs to realise a GaN-baseddigital circuit. Fig. 2 shows the measured characteristics of the

ELECTRONICS LETTERS 10th December 1998 Vol. 34 No. 25 2367