1310 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 45, NO. 8, AUGUST 1997

Microstrip Antennas on SynthesizedLow Dielectric-Constant Substrates

Gildas P. Gauthier,Student Member, IEEE, Alan Courtay, and Gabriel M. Rebeiz,Fellow, IEEE

Abstract— Micromachining techniques using closely spacedholes have been used underneath a microstrip antenna on a highdielectric-constant substrate(�r = 10:8) to synthesize a localizedlow dielectric-constant environment (�r = 2:3). The measuredradiation efficiency of a microstrip antenna on a micromachined635-�m thick �r = 10:8 Duroid 6010 substrate increased from48 � 3% to 73 � 3% at 12.8–13.0 GHz (including 3.3-cm feedline losses). We believe that this technique can be applied tomillimeter-wave antennas (microstrip, dipoles, slots, etc.) on sili-con and GaAs substrates to result in relatively wideband (3–6%)monolithic microwave integrated circuits (MMIC) active antennamodules for phased-arrays and collision-avoidance systems.

Index Terms—Microstrip antennas.

I. INTRODUCTION

Microstrip antennas on high dielectric-constant substrates(GaAs, Silicon) suffer from very narrow bandwidths (less than0.5%), low efficiency, and poor radiation patterns due to thetriggering of unwanted surface waves. Several methods wererecently used to overcome this problem, including drilling awell designed cavity underneath the antenna [1], complete orpartial etching of the substrate underneath the antenna [2]–[4],and etching few holes underneath the antenna to disturb theformation of substrate modes [5], [6].

We have taken a rather new approach. The idea is toetch or drill a series of very closely spaced holesunderneath and around the microstrip antenna and to controlthe effective dielectric constant with the choice of the diameterand spacing of the holes. The period of the holes must be smallcompared to a wavelength so that quasi-static techniques resultin a good estimate of the synthesized dielectric constant. Thehole diameter is around 0.1 or 300 m at 30 GHz and 100

m at 90 GHz, which is compatible with state-of-the-art via-hole formation available in monolithic microwave integratedcircuits (MMIC) processing. The via-hole approach and theassociated localized reduction of the dielectric constant is alsoapplicable to a variety of planar antennas and antenna arrayssuch as microstrip dipoles, slot antennas, and low-gain Vivaldiantennas. In this paper, we have demonstrated the principleon 4- and 13-GHz microstrip antennas on thick dielectricsubstrates with high .

Manuscript received March 25, 1996; revised April 17, 1997. This workwas supported by the U.S. Army Research Office under Contract #DAAL04-94-G-0137 and by Daimler-Benz AG Research Center, Ulm, Germany.

The authors are with the Electrical Engineering and Computer ScienceDepartment, The University of Michigan, Ann Arbor, MI 48109 USA.

Publisher Item Identifier S 0018-926X(97)05602-0.

II. QUASI-STATIC MEASUREMENTS OF

THE EFFECTIVE DIELECTRIC CONSTANT

To prove the quasi-static approach on closely spaced holes,a number of microstrip antennas on a dielectric substratewith different hole densities have been fabricated. The holesare drilled using a numerically controlled machine (NCM)underneath the microstrip antenna and extend at least 3.5from the edge of the antenna in all directions whereisthe substrate height. The dimensions of the antenna andholes are given in Table I. The substrate is a Duroid 6010

, mm (50 mil). The resonant frequencyof the microstrip antennas of length and width and

, respectively, was measured with a network analyzer. Thesynthesized relative dielectric constant was derivedusing [7]

(1)

hence

(2)

with

(3)

and

(4)

The resulting synthesized dielectric constant was com-pared with the value of obtained using a 1-kHzcapacitance measurement technique (Philips PM6303 RCLmeter) between the two metal layers (the microstrip antennaand the ground plane)

(5)

where is the measured capacitance. In all cases, the feedline was a 50- microstrip line on the samesubstrate. Both methods (resonant frequency and capacitancemeasurements) resulted in very similar synthesized dielectricconstant , which is proportional to the volume of thesubstrate removed underneath the microstrip antenna.is then given by

(6)

(7)

0018–926X/97$10.00 1997 IEEE

GAUTHIER et al.: MICROSTRIP ANTENNAS ON SYNTHESIZED LOW DIELECTRIC-CONSTANT SUBSTRATES 1311

TABLE ICOMPARISON BETWEEN THE TWO METHODS TO MEASURE DIELECTRIC

CONSTANTS FORDIFFERENT MACHINED HOLE DENSITIES. THE LAST

COLUMN INDICATES MEASUREMENTS WITH NO DRILLED HOLES.

IN ALL CASES: THE SUBSTRATE HEIGHT IS 1.27 mm (50 ml)

where is the proportion of volume removed, the holediameter, and the center to center spacing in a triangularlattice grid. These results are summarized in Table I. Theresonance method is accurate for low microwave frequenciesand depends on a good reference-plane calibration, which canbecome a limiting factor at high microwave frequencies.

III. RADIATION EFFICIENCY MEASUREMENTS

A. Description of Method

The radiometric technique is an accurate method to mea-sure radiation efficiencies of planar antennas [8], [9]. Theantenna is connected to a calibrated low-noise RF system ofmeasured gain and measured noise temperature. Thesystem parameters are determined using a standardcoaxial hot/cold load measurement method. The gainandtemperature of the RF system with the microstrip antennaare measured using a hot/cold load (hemispherical black bodyabsorber Ecosorb) placed in front of the antenna to cover allits radiation pattern. The gain and temperature arecalculated using

(8)

(9)

(10)

where K and K. and are themeasured RF powers with the hot and cold load, respectively.

is the final resolution bandwidth of the RF system andisthe Boltzman constant. The antenna loss is then

(11)

which is a negative number. The hot/cold load measurementis referenced to the RF antenna connector (Fig. 1). Thisincludes the loss of the coaxial connector, the feed line ohmicand radiation losses (including the matching network), theimpedance mismatch loss to 50, the microstrip antennacavity losses (dielectric and ohmic), and the loss due to

Fig. 1. Microwave test fixture used at 12–13 GHz.

substrate modes excited by the feed line and the microstripantenna.

B. RF Chain Design and Calibration

A detailed view of the RF chain is shown in Fig. 2. Thelow-noise amplifier gain is 21 dB and is well matched to 50

between 12 and 13.5 GHz. The RF bandpass filter has a 3-dB 12.5–13.5 GHz passband insertion loss. The intermediatefrequency (IF) is fixed at 1.5 GHz. Since the local oscillator(LO) is at 14.1–14.6 GHz, the lower sideband (12.6–13.1 GHz)is attenuated by 3 dB and the upper sideband is rejected bymore than 40 dB (Fig. 2). This results in a true single sideband(SSB) measurement. The conversion loss of the mixer isaround 6 dB over the 12.6–13.1 bandwidth. The intermediatefrequency is chosen to be 1.5 GHz and is fixed by the YIGfilter. The final resolution bandwidth of the IF chain is 30MHz. A typical calibrated RF system has a gain of 920.1 dB and a noise temperature of 250 3 K. The poweris averaged over this bandwidth and so is the efficiency andthe impedance mismtach. However, 30 MHz is well within thebandwidth of the microstrip antenna and the averaging erroris minimal.

C. Measurement Procedure

A 12.5 to 13.5 GHz microstrip antenna has been fabri-cated on mm (25 mil) Duroid 6010substrate. The hole design was on a triangular lattice forcompact packing, with a hole diameter of 0.6 mm and a holecenter-to-center spacing of 0.7 mm. The hole area is 1.31.6 cm and extended approximately 4.5from the edge ofthe antenna. A large volume (around 80%) of the materialwas removed and the resulting using the quasi-staticcapacitance measurement described above was 2.3. Therefore,an antenna dimension of 0.70 0.92 cm is chosen for 12.8-GHz resonance. The holes were machined with a numericallycontrolled milling machine (Fig. 3).

1312 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 45, NO. 8, AUGUST 1997

Fig. 2. The RF chain at 12.5–13.5 GHz.

To compare the performance of this antenna we havefabricated other microstrip antennas on ,mm with dimensions of 0.74 0.91 cm and on

, mm with dimensions of 0.330.44 cm. All antennas were designed to resonate around

12.8–13.0 GHz and were matched to approximately 50using an in-line transformer. All antennas were etchedon a 7 7 cm dielectric substrate (3.0 square) and placedon top of a 10 10 cm ground plane (4.3 square), as inFig. 1. We have also measured the efficiency of a pyramidalhorn antenna in the Ku band to validate the experiment. Theexpected radiation efficiency for such a horn is very high (983%) since it has minimal ohmic losses and it is well matchedto 50 .

The calculated resonant impedances for the antenna onand are and

, respectively [10]. The measured input impedance ofall three antennas is shown in Fig. 4. It is seen that theantenna on synthesized has a very wide 10-dB bandwidth of 4.6%. The antenna on showsa 10-dB bandwidth of 1.8% which is, as expected, widerthan the 0.8% 10-dB bandwidth of the antenna.Preliminary calculations indicate that the improved bandwidthof 4.6% for the synthesized dielectric antenna results from aninteraction between the microstrip antenna metal cavity andthe cavity formed around the antenna between the unifromdielectric and the synthesized dielectric

. We are currently working on an FEM model of thisinteraction.

The efficiency of all three antennas was measured using theradiometric method described above. It is seen in Fig. 5 andTable II that the measured radiation efficiency improved from48 3% for to 70 3% for synthesizedand that the synthesized dielectric antenna yields similar valuesas the antenna. In Fig. 5, the results include feed-line losses (ohmic, dielectric and radiation), substrate-mode

Fig. 3. Fabricated microstrip antenna (0.70� 0.92 cm) on a localized low-�rdielectric(�r = 2:3) with a hole region of 1.3� 1.6 cm. The substrate is aDuroid 6010�r = 10:8; h = 0:635 mm.

losses, and mismatch loss to 50. The bell-shaped curveswith a peak at 12.8–13.0 GHz are mainly due to mismatchloss and show the bandwidth of the microstrip antennas to a50- load.

Fig. 5 and Table II also show the calculated radiation ef-ficiency at resonance for the antenna at 12.9 GHzand the antenna at 12.8 GHz. The values weredetermined using Pozar’s microstrip program [10] and donot include connector loss, mismatch loss, or feed-line loss.However, they do include the microstrip antenna ohmic anddielectric cavity losses and the substrate mode losses. Ascan be seen, a very good agreement is achieved with the

measurement and the small difference (8%) isexplained by connector and feed-line losses (50-microstripline on -mm thick substrate). For the caseof the antenna, there is a 19% difference betweenthe calculated and measured values. It could be explained bythe matching network loss, acting as a high-cavity and

GAUTHIER et al.: MICROSTRIP ANTENNAS ON SYNTHESIZED LOW DIELECTRIC-CONSTANT SUBSTRATES 1313

Fig. 4. Measured input impedances of microstrip antennas on�r = 2:2,synthesized�r = 2:3, and �r = 10:8.

Fig. 5. Measured radiation efficiencies of microstrip antennas (includingmismatch, connector, and feed-line losses) on�r = 2:2(�), synthesized�r = 2:3(+), �r = 10:8(�), and horn antenna(�). The calculated valuesfor �r = 2:2 and for �r = 10:8 do not include mismatch, connector, andfeed-line losses.

TABLE IIMEASURED AND CALCULATED RADIATION EFFICIENCIES

AT RESONANCE FORDIFFERENT SUBSTRATES

the radiation loss of the feed line (50-microstrip line on-mm thick or substrate). By

modeling this feed line with HP Momentum [11], we haveestimated a loss of 0.20 0.05 dB/cm at 12.8–13.0 GHz.The line is 3.3-cm long and results in an feed-line loss ofaround 0.7 dB. Taking this loss and the connector loss (0.1–0.2

Fig. 6. Radiation patterns of microstrip antennas on�r = 2:2 at 12.9 GHz,synthesized�r = 2:3 at 12.8 GHz, and�r = 10:8 at 12.8 GHz. E-plane,H-plane (-), and cross-polarization patterns (- -) are shown.

dB) into account, we obtain a measured value of 61%, whichagrees well with the calculated radiation efficiency (67%). Thedifference can be partially attributed to the high-matchingnetwork.

It is important to note that the synthesized antennafeed line is on a substrate (the holes are localizedaround the antenna only). If the feed-line loss is deimbeddedfrom the measured value of 73%, a very good agreement withthe calculated value is obtained: a radiation efficiency of 87%,the same as the antenna (Table II).

IV. RADIATION PATTERNS

The measured patterns of all three antennas at their respec-tive resonant frequencies are shown in Fig. 6. Note that theantennas are all on a 10-cm square holder and suffer fromfinite ground-plane edge effects (Fig. 1). It is seen that the

antenna on a 0.635-mm thick substratesuffers from a large ripple and high cross-polarization levelson broadside due to substrate modes and edge radiation in theE-plane. The synthesized dielectric antenna showsa small ripple mainly due to radiation from the feed lines butthe cross-polarization levels are quite low. As expected, the

antenna exhibits good radiation patterns and lowcross-polarization levels.

V. CONCLUSION

A clear improvement of the radiation efficiency and patternsof microstrip antennas on high dielectric-constant substrateshas been demonstrated, resulting from a local reduction of thesubstrate dielectric constant underneath and around the antennausing a series of closely spaced holes. In this case, the holeregion extends to at least 3.5(or more) around the antenna.As expected, the synthesized low-antenna behaves almostidentically as the uniform low- antenna. Future work willconcentrate on determining the performance of an antennaversus hole region extension. Also, the possibility of using

1314 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 45, NO. 8, AUGUST 1997

different hole configurations to result in localized regions withdifferent dielectric constants underneath or around the antennafor high efficiency designs will be investigated.

ACKNOWLEDGMENT

The authors would like to thank Dr. R. Bradley, NationalRadio-Astronomy Observatory (NRAO), Charlottesville, VA,for lending us the very low-noise 12–18-GHz amplifier andD. Pozar, Univ. of Massachussets, Amherst, for the theoreticalradiation efficiency calculations. We thank the Rogers Corpo-ration, Chandler, AZ, for the generous donation of the Duroidsubstrates.

REFERENCES

[1] D. R. Jackson, J. T. Williams, and A. K. Bhattacharyya, “Microstrippatch designs that do not excite surface waves,”IEEE Trans. AntennasPropagat., vol. 41, pp. 1026–1037, Aug. 1993.

[2] W. Y. Ali-Ahmad, W. L. Bishop, and T. W. Growe, “An 86–106 GHzquasi-integrated low noise Schottky receiver,”IEEE Trans. MicrowaveTheory Tech., vol. 41, pp. 558–564, Apr. 1993.

[3] M. Stotz, G. Gottwald, and H. Haspeklo, “Planar millimeter waveantennas using SiNx-membranes on GaAs,”IEEE Trans. MicrowaveTheory Tech., vol. 44, pp. 1593–1595, Sept. 1996.

[4] R. F. Drayton, “The development and characterization of self-packagesusing micromachining techniques for high frequency circuit applica-tions,” Ph.D. dissertation, Univ. Michigan, Ann Arbor, MI, 1995, ch.5, pp. 102–123.

[5] M. J. Vaughan, K. Y. Hur, and R. C. Compton, “Improvement ofmicrostrip patch antenna radiation patterns,”IEEE Trans. AntennasPropagat., vol. 42, pp. 882–885, June 1994.

[6] G. V. Eleftheriades, “Analysis and design of integrated-circuit hornantennas for millimeter and submillimeter-wave applications,” Ph.D.dissertation, Univ. Michigan, Ann Arbor, MI, 1993, ch. 7, pp. 151–178.

[7] I. J. Bahl and P. Bhartia,Microstrip Antennas. Norwood, MA: Artech,1980, ch. 2.

[8] D. M. Pozar and B. Kaufaman, “Comparison of three methods forthe measurement of printed antenna efficiency,”IEEE Trans. AntennasPropagat., vol. 36, pp. 136–139, Jan. 1988.

[9] D. M. Pozar,Microwave Engineering. Reading, MA: Addison-Wesley,1990, ch. 12.

[10] D. M. Pozar, PCAAD: Personal Computer Aided Antenna Design,version 2.1.

[11] HP-EEsof, Inc., Westlake Village, CA 91362.

Gildas P. Gauthier (S’96) was born in Angers, France, in 1969. Hereceived the Dipl. (telecommunications engineer) and DEA degrees fromENST Bretagne, Brest, France, in 1992 and 1993, respectively. He is currentlyworking toward the Ph.D. degree at the University of Michigan, Ann Arbor.

From 1992 to 1994, he worked with IRAM, Granada, Spain, on millimeter-wave SIS receivers. His interests are in microwave/millimeter-wave antennas,receivers, and communication systems.

Alan Courtay , photograph and biography not available at the time ofpublication.

Gabriel M. Rebeiz (S’86–M’88–SM’93–F’97), for photograph and biogra-phy, see p. 1444 of the November 1996 issue of this TRANSACTIONS.