60 GHz SIWSteerable Antenna Array in LTCC

Bahram Sanadgol ,Sybille Holzwarth ,Peter Uhlig ,Alberto Milano ,and Rafi Popovich

(1.IMST GmbH Carl-Friedrich-Gauss-Str.2,47475 Kamp-Lintfort,Germany;

2.Beam Networks,1 Ehad Ha'am 76248 Rehovot,Israel)

Abstract In this paper,we present a 60 GHz substrate-integrated waveguide fed-steerable low-temperature cofired ceramics array.The antenna is suitable for transmitting and receiving on the 60 GHz wireless personal area network frequency band.The wireless system can be used for HDTV,high-data-rate networking up to 4.5 GBit/s,security and surveillance,and similar applications.

Keyw ords substrate integrated waveguide(SIW);phase shifted injected push-push oscillator(PSIPPO);low temperature co-fired ceramic(LTCC);monolithic microwave integrated chip(MMIC);wireless personal area network(WPAN)

1 Introduction

A pplications of the 60 GHz band for high data-rate services have become more interesting,especially over the past few years.A comprehensive list of multimedia applications using the 60 GHz band can be found in[1]-[3].There is a need for compact,highly efficient front-ends,but designing systems with such front-ends is challenging.This paper willstart with a brief introduction of 60 GHz WPAN applications.The company Beam Networks(BN)is developing a high-performance,low-cost wireless transceiver system at 60 GHz for wireless personalarea network(WPAN)applications.

The antenna design and architecture,which uses the finite difference time domain(FDTD)field solver called Empire,is then described in detail.We analyze active impedance to determine the performance of the scanned antenna array.The antenna for the system comprises four waveguide-fed columns that can be excited with different phases for beam-steering applications.

Far-field measurements of the realized antenna demonstrator are then presented.We show that the measured performance is within the required specifications,and the measurements also confirm that the antenna is low-loss.

2 Transceiver Architecture

Fig.1 shows the transceiver architecture comprising a crystal-locked master transceiver on the right and a slave transceiver on the left.The frequency and phase of the slave transceiver is locked to the master.Each transceiver comprises four channels that interface the antenna in order to form a steerable,focused beam.Areference signalis generated at 15 GHz,and this signal injects and locks the push-push oscillator 15-30 GHz.In this way,coherent signals of the buffer amplifiers are delivered at the output ports of the amplifier for up/down conversion.The phase of each signal in the array is adjusted by tuning the band rejection filter(BRF)of each phase-shifted injected push-push oscillator(PSIPPO)at 30-60 GHz to implement beamforming.

The transceiver can operate in fullduplex or spatial-division duplexing(SDD)mode because of the high isolation that can be achieved with the presented antenna and transceiver design.In Fig.1,two transceivers communicate over a bidirectional link.Both transmit and receive data simultaneously on carriers that use the same frequency.The transmitter and receiver can operate simultaneously as long as any reflected waves receive significant attenuation by the environment.Such SDD operation is typically possible because the channelis highly specular,the antennas are highly directional.

3 Antenna Design and Simulation

In this section,the design and simulation of the substrate integrated waveguide(SIW)array antenna is explained.All modelling and simulations were performed using the 3D field solver called Empire[4],which is based on finite difference time domain(FDTD).

The goals of antenna design are to achieve a bandwidth of 10%in the 60 GHz band,a maximum scan range of±30°,and total antenna gain of 18-20 d Bi.Bandwidth and gain(efficiency)requirements make the choice of the single element more significant.An open waveguide radiator has large bandwidth,low cross polarization,a small front-to-back radiation ratio,and high efficiency.

Loss has always been an important issue in the design of high-efficiency antennas.If designed properly,a waveguide-feeding network could be a low-loss solution compared to a microstrip transmission line.Although microstrip and stripline technologies are very appropriate from an integration perspective,the surface resistance related to these transmission lines increases with the square root of the frequency[5].As a consequence,they are lossy at high microwave frequencies.

Integrating the antenna and feeding network was the next concern in the design process.Waveguide-like structures can be constructed using periodic metallic via posts in a substrate.Realizing an array of such antenna elements and their feeding network requires a relatively thick substrate.Low-temperature co-fired ceramic(LTCC)technology greatly benefits microwave applications and can solve this problem because a many layers,including vias and metalized surfaces in between,can be manufactured.Ceramic substrates as well as gold and silver pastes have excellent physical and electricalproperties.Moreover,materialand processing costs are competitive compared with substrate systems such as HTCC and printed circuit boards for high microwave frequencies[6].

Fig.2 shows the proposed waveguide radiator.This element is designed to have the same radiation properties as an open-ended waveguide,and the dominant propagating mode is TE10.The waveguide walls comprise via fences.Between each layer,metal surfaces(usually gold or silver)connect the vias so that the current along the side walls of the waveguide is not interrupted.In a typical open-ended waveguide,the electric field of the aperture is given by

(1)is a good approximation of the field distribution,but it does not include allthe details for an SIWradiator.In other words,it is not easy to modeland analyse the SIW theoretically.

One of the drawbacks of waveguide radiators is the mismatch to free space.For an SIWantenna element,this mismatch is even worse because of highεr.LTCC materials usually have high permittivity,which makes the SIWradiator suffer from high reflections at the aperture face.To improve the transition from waveguide mode to free-space mode,a substrate with lowerεr must be used or the effective dielectric constant of LTCC must be reduced.The latter can be done by cutting out cavities in the LTCC.The special shape and dimensions of these cavities can help minimize reflection at the antenna interface(Fig.2).

Another known problem with SIWis leakage through the gap between vias.Ideally,the vias are the waveguide walls,and there is no energy loss.However,leakage does exist and is always greater at lower frequencies.This means that denser vias do not necessarily result in better performance[7].If the vias are not compact enough,a band-stop structure can be built in the desired frequency band.The design goal should be to minimize leakage while shifting the stop band to higher frequencies.Of course,LTCC design rules should always be kept in mind.The substrate thickness,via spacing,and via diameter should be carefully chosen.With these in mind,the single element was simulated and optimized with Empire.

The next step was to design a proper feeding network for the array.According to gain requirements,the array was set at 4×4,which should be able to scan in one direction.Each of the four elements in the same column shares a feeding line;therefore,we designed the feeding for one column.Like the radiating elements,the waveguides are realized in the substrate,and via fences are the waveguide walls.The feeding network should be designed in such a way that all antenna elements radiate in phase.Fig.3 shows one array column comprising four active elements that share the same feeding network.The scan specifications require the antenna to be steerable only in one plane(the H-plane here).On the E-plane along one column,the element distance can be increased.This reduces integration complexity and improves array performance.At first,an Eplane T-junction splits the power coming from a standard waveguide WR-15.Using a bend and another E-plane T-junction,each element can be fed with the same phase.There are two passive elements at each column that help improve side-lobe level and reduce the back radiation.These elements are terminated using resistance paste,so the reflection from the elements is minimized,and most of the coupled energy is absorbed in the resistor.A proper interface to the standard waveguide is also needed.Fig.4 shows the electrical field of the feeding network and the active antenna elements for one column.

▲Figure 2.Single-element SIWradiator with cavity for improved matching.

Fig.5 shows the complete array configuration.To optimize the design,the array is simulated,and all four columns are active.The active impedance method guarantees the inclusion of mutualcoupling in the finaldesign[8].This can be very important in the case of a scanning array;optimizing the array in active mode can avoid the need for post-manufacturing tuning.Using the same method,we can analyse how the array input impedance varies with the scan angle.Using this method,the array input impedance has been optimized for an angle off boresight to obtain the best bandwidth for all the scan angles(here,up to 30°).Asimillar array optimized with the active impedance method can be found in[9].

4 Measurement

The antenna was fabricated according to the optimized layout.Fig.6 shows a finished prototype.The complete LTCC tile comprises four columns,and there are only four active elements in each column.The LTCC material used here was Ferro A6-M,withεr around 5.7.The total number of vias for one array is approximately 4300 in all 14 layers.To be able to do the reflection and far-field measurements,a special metal frame was devised.The antenna was fixed in this dedicated test frame and was then fed by the WR-15 standard waveguide from the back through an opening made for WR-15.All the prototypes manufactured in this phase contain only one active column,although the array is optimized for the case where allfour columns are active simultaneously.The reason for only one active column is because the dimensions of a standard waveguide do not allow two standard waveguides beside each other.Four prototypes of the array were manufactured;two had an active outside column,and the other two had an active inside column.

▲Figure 3.One array column consisting of four active elements and two passive elements.The corresponding SIWfeeding network is also shown.

▲Figure 4.Electricalfield of the waveguide feeding network and antenna elements.

▲Figure 5.Final antenna configuration for the array scanning in H-plane.

Figure 6.?Manufactured tile of the four-column array(oneactive column only).

Figure 7.?Reflection coefficientmeasurementand simulation results.

Fig.7 shows return loss against Empire simulation for one prototype.There is approximate agreement between the simulation and measurement over the entire band.The slight difference is usually because of LTCC design tolerances.Although the manufacturing has been done as accurately as possible,the LTCC process is naturally more sensitive to tolerances than other types of PCB manufacturing.Nevertheless,considering the number of vias and layers,LTCC seems to be a robust and reasonable solution.All the manufactured prototypes were measured and show a very good reproducibility,which is crucial for the final series production.

Farfield measurement was done in an anechoic chamber with a special setup and appropriate isolation.Each column was measured separately;that is,all the other columns were terminated passively.By applying the field superposition from each column and the required phase for the desired scan angle,the farfield diagram of the whole array was calculated.The final farfield diagrams for the array steered to±30°are shown in Fig.8.

5 Conclusion

In this paper,the design,fabrication,and measurement of an array antenna for WPAN 60 GHz application was described.The antenna elements were substrate-integrated waveguide radiators and were fed by a SIWfeeding network.The design was realized in LTCC,and the measurements were taken using standard waveguide.There was very good agreement between the simulations and measurements.In the future,this array antenna will be integrated with the MMIC chip to build a commercial WPAN system.

▲Figure 8.Farfield measurements for the scanned array with the proper phase for each column.a)setup,b)results.