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Bow-tie antennas on high dielectricsubstrates for MMIC and OEIC applicationsat mi ll imetre-wave frequenciesD. Mirsliekar-Syahkal and D. Wake

Indexing terms: MMIC, Antennas, Microstrip antennasA two-element bow-tie dipole antenna and a singleelementbow-tie slot antenna fabricated on a high dielectric constant (&, =10.2)substrate are introduced for applications at rmllunetre-wavefrequencies. The former antenna provides 2GHz bandwidth at35GHz and the latter 1.3GHz at 32.7GHz. With a broadbandmatch, these antennas would yield significantly higherbandwidths.

Introduction; Recent interest in the development of radio-fibre sys-tems at millimetre-wave frequencies [11 has prompted the study ofplanar antennas on GaAs and InP. In particular, the latter sub-strate provides an opportunity for the fabrication of full optoelec-tronic integrated circuits (OEIC).Owing to the high dielectric constant (E, = 12.5) and smallthickness (100 - 500pn) of GaAs and InP, microstrip antennasfabricated on these substrates have extremely narrow bandwidthand exhibit unacceptable losses at millimetre-wave frequencies [2].Possible alternatives are the printed dipole antenna [3] and its dualform the slot antenna. The theoretical performance of the formerantenna has been investigated extensively and reported in [3- 1.A different category of antennas which can offer large band-widths is the biconical antenna [6]. A flat version of the biconicaldipole antenna is the triangular sheet antenna or the bow-tie(dipole) antenna [7]. The length and the flare angle are the mainfactors that govern the bandwidth of these antennas. Althoughthese antennas are not as broadband as the original conical ones,an appropriate design can produce bandwidths greater than thoseof the dipole antennas. Furthermore, owing to their planar shapes,bow-tie antennas can be etched on substrates. This is an advan-tage when developingMICs and monolithic MICs for radio appli-cations. Bow-tie slot antennas are believed to provide the samefeatures as bow-tie dipole antennas, and they are both expected toexhibit less conductor losses than the microstrip antenna. The lat-ter benefit stems from reduced current densities on the antennasand on their back (ground) planes. There are, however, surface-wave losses associated with the bow-tie dipole antennas [4, 51 andparallel plate waveguide mode losses with the conductor backedbow-tie slot antennas. In the former case, the losses can be madenegligible by observing the optimum substrate thickness given by[41

(1)A04o p t IFor a dipole antenna with a ground plane at a position of h 2 LA4=&/4&, or less, eqn. 1 is satisfied automatically.In this work, two bow-tie antennas developed on a hg h dielec-tric constant substrate (q=10.2) for operation at millimetre-wavefrequencies are introduced. One of these structures is a two-ele-ment bow-tie dipole antenna and the second structure is a single-element bow-tie slot antenna. Both antennas are fed by the micro-strip line and have a backplane in order to direct radiation into ahalf-space. The experimental return losses of these antennas arepresented and discussed.

rnm

ground plane

ground plane

1062111Fig. I Structures of two-element antennasa Bow-tie dipoleb Thin dipolec Single-element bow-tie slotLength l is 'Iqf wavelength at operating frequencyThis length may be trimmed for good performancebetween the two strips. This is important, since using a small slotwas found to give rise to a small coupling between the feed andthe coplanar strip line and hence to a low power transfer from themicrostrip line to the antenna.The total antenna length including the gap is -4mm. Consider-ing an effective dielectric constant of 5.6 ( E ~ ~(10.2 + 1)/2), thislength is equivalent to a resonant frequency of 32GHz for a fdl-wavelength antenna. To operate at the same frequency, the half-wavelength antenna has a smaller length and hence an increasedconductor loss. In the E-plane, the full-wavelength bow-tie dipoleis expected to exhibit a narrower beamwidth as compared with itshalf-wavelength counterpart . The two layers of the dielectric inFig. la provide 500pm thickness, or -0.17 hdat 32GHz. Thismeans that the surface wave losses are within the permitted rangeset by eqn. 1.

0-10-20-30a

0;10--20-30-b 30 LO 50

Two-element bow-tie dipole: In Fig. l a the structure of a two-ele-ment bow-tie dipole antenna is shown. The antenna consists oftwo 250 pm Duroid substrates of E, = 10.2. The thickness and thedielectric constant of the substrates are purposely chosen close tothose of GaAs/lnP substrates. The bottom substrate with the con-ductor backing supports a microstrip line which feeds the antenna.This line can be part of an MMIC or OEIC. On the top substrate,the antenna elements are laid. Each element has a 60" flare angleand an approximate input impedance of 150Q [7]. The metallisa-tion is -17pm in thickness.The two elements are connected through a coplanar strip trans-mission line of 75Q which can be designed using data in [8]. Thedimensions of this line were chosen so that a large slot was formed

0-10-20-30C 9621210 frequency,GHz0 50

Fig. 2 Return losses fo r two-element antennasa Bow-tie dipole, marker 1 35 I GHz point 52b Thin dipole, marker 1 30 8GHz point 9c Single-element bow-tie slot, marker 1 32 7GHz point 28The expenmental results (S,,) for the two-element bow-tie

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dipole antenna are shown in Fig. 2a. They are recorded using acalibrated 50GHz HP8510C network analyser.As seen in the Fig-ure, the centre frequency is -35GHz, which is -9% higher than thepredicted frequency. This discrepancy is obviously due to theapproximate nature of calculation for finding the resonant fre-quency. The -1Odb bandwidth is -2GHz, corresponding to 6% .For comparison, the return loss (S,,)of a two-element thin dipoleantenna, Fig. lb, of the same structure as that of the two-elementbow-tie antenna in Fig. l a is given in Fig. 2b. In this case, thebandwidth is -1%0, which is much lower than that of the bow-tieantenna.It should be said that no attempt was made to refine the per-

formance of the bow-tie antenna, and it is believed that the band-width can be made much larger, especially if the substratethickness is chosen slightly thicker than the optimum thickness. Inthis case, the bandwidth increases substantially at the expense ofefficiency [4].A further increase in the bandwidth can be achievedusing thicker metallisation for the antenna and providing a broad-band matching transition between the feed and the antenna. It isinteresting to note that there is an increase in the losses in theantenna system beyond 45GHz. This is mainly caused by radia-tion into surface waves.The approximate radiation pattern of the two-element bow-tieantenna in Fig. l a can be obtained by considering that it can bereplaced by four antennas where two of them are the image of theother two in the ground plane. The radiation pattern of each ele-ment may be approximated by that of a dipole.

Single-element bow-tie slot antenna: In Fig. IC, the structure of asingle-element bow-tie slot antenna fed by a microstrip line isshown. In this antenna, the flare angle is 60, the slot length is2m m and the slot opening at the antenna centre is - 4 0 0 ~ . herest of the specifications are the same as those given for the previ-ous antennas.

Considering that E~~= 5.6, the half-wavelength resonant fre-quency of the antenna should be -32GHz. The measured returnloss, Fig. 2c, indicates that the resonant frequency is -32.7GHzand the -10dB bandwidth -1.3GHz, corresponding to 4% . Fromthe shape of the return loss around the resonant frequency, it canbe deduced that this antenna is capable of providing a much largerbandwidth if a broadband match is provided between the antennaand the feed line. In fact it was observed experimentally that aslight change in the antenna position with respect to the feedcauses the resonant frequency to shift, indicating the availabilityof some extra bandwidth. An increase in the slot flare angle andmetallisation thickness would also increase the bandwidth, butthey are yet to be investigated. The radiation pattern of theantenna is similar to a slot antenna with a conductive backplane.

Apart from the resonant frequency of the antenna, a few otherresonances are seen in the return loss shown in Fig. 2c. These werefound to be caused by the finite size (length and width) of the die-lectrics involved in the antenna structure. The parallel plate modeand any other excited modes radiate at the dielectric boundaries.By a proper matching, energy coupled to these modes can bereduced and hence the antenna efficiency can be increased.Conclusion: Two bow-tie antennas developed on a high dielectricconstant substrate (E! = 10.2) for operation at millimetre-wave fre-quencies have been introduced. One of these structures is a two-element bow-tie dipole antenna and the second structure is asingle-element bow-tie slot antenna. Both antennas are fed by themicrostrip line and have a backplane. The bow-tie dipole antennaexhibited 6% bandwidth at 35GHz and the single-element bow-tieslot had 4% bandwidth at 32.7GHz.With a broadband match,these antennas would yield significantly higher bandwidths. Theseantennas are suitable for MMICS and OEICS transmitter andreceiver units and can be scaled for use at -65GHz where futuremobile and radio-fibre systems are to operate.0 EE 1995Electronics Letters Online No: 19951426 5 October 1995D. Mirshekar-Syahkal (Department o Electronic Systems Engineering,University o Essex, Colchester, Essex, CO4 3SQ, United Kingdom)D. Wake (B T Laboratories, Martlesham He ath, Ipswich IP5 7RE,United Kingdom)

ReferencesWAKE, D., WALKER, N.C. , and SMITH, 1.C.: Zero-bias edge-coupledInGaAs photodiodes in mm-wave radio-fibre systems, Electron.Let t . , 1993, 29 , pp. 1879-1881ROBERTSON, I.D.: A monolithic 35GHz back-face patch antennausing multi-layer techniques. Proc. 23rd European MicrowaveConf., 1993, pp. 348-350RANA, I.E., and ALEXOPOULOS, N.G.: Current distribution and inputimpedance of printed dipoles, ZEEE Trans., 1981, AP-29, pp. 99-105ALEXOPOULOS, N.G , KATEHI, P.B., and RUTLEDGE, D.B.: Substrateoptimisation for integrated circuit antennas, IEEE Trans . , 1983,KATEHI, P.B., and ALEXOPOULOS, N.G.: On the effect of substratethickness and permittivity on printed-circuit dipole properties,IEEE Trans. , 1983,AP-31, pp. 3&39LO, Y.T., and LEE, s.w.: Antenna handbook (Van NostrandReinhold, New York, 1988), Chap. 17BALANIS, CA.: Antenna theory, analysis and design (Harper &Row, New York, 1982)HOFFMANN, R.K.: Handbook of microwave integrated circuits(Artech House, Norwood, 1987)

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Planar dipole arrays wit h equal elementinput impedancesW . Chen, L. Jen and S.M. Zhang

Indexing terms: Antenna arrays, Antennas, Dipole antennasPlanar antenna arrays of unequal dipoles in which all the inputimpedances of the elements are the same are proposed to simplifythe matching networks or to improve the matching condition forbroadside arrays. A method to design this type of array isdescribed. The array produces a given radiation pattern. Theelements are of different lengths and different radii.

Introduction: It is a coi~~noiixpectation in the design of antennaarrays that the elements show similar electrical properties to easethe design of the matching network. In a normal planar array ofantennas of the same configuration the input impedances of theelements are not the same. Furthermore they are quite differentnear edges, especially arround corners. Therefore a precise match-ing network must be designed for every single antenna when highperformances are required. If the input impedances of the elementscan be designed to be the same, the matching will be much simplerthan is usually the case.We have found that by modifying the sizes of the elements ofan array their electrical properties could be adjusted and the H-plane radiation pattern of the array could be maintained [l, 21. Wehave designed a linear array of parallel unequal dipoles in whichthe input powers into elements are in a given distribution [l]. In aprevious Letter we found it possible to design a linear low sidelobearray in which all the input impedances were the same and werepure real [2]. In this Letter we extend that idea to planar arrays.An 8 x 12 planar array is designed as an example. An array whichradiates a low sidelobe pattern is initially synthesised. After modi-fying the lengths and the radii of the dipole elements, we finallyobtain an array which maintains its low sidelobe pattern and inwhich all the input impedances are equal to 75Q and which isquite suitable to be matched with ordinary cable.Method: Consider an N x M planar array in free space. The ele-ments are parallel dipoles of different lengths and radii lying in aplane on a rectangular lattice. There are N rows in the array andM parallel dipoles in each row. It is assumed that the array wouldproduce a low sidelobe radiation pattern with its main beam at thebroadside of the plane. Pocklingtons integral equation for thecurrents and the driving voltages of the elements is applied as~ 3 1

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