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Making Millimeter-Wave Technology More Accessible: Part 3

Parent Category: 2022 HFE

By Yonghui Shu and Andrew Laundrie

Ferrite Devices

Ferrite non-reciprocal devices are widely used in microwave and mm-wave systems to provide port isolation or to control signal flow. Isolators are two-port devices that are mainly used for port impedance matching improvement or to guide signals in a certain direction. Circulators can be used for either port isolation or for signal duplexing. Faraday rotation isolators can generally cover full waveguide bandwidths with high levels of isolation. Junction isolators and circulators offer lighter weight and more compact size. Although junction isolators and circulators are typically optimized for specific bandwidths, recent progress has advanced some junction isolators and circulators to cover full waveguide bands in WR-42, WR-34 and WR-28 with somewhat compromised performance degradation, such as slightly higher insertion loss and lower isolation at the band edges. Figures 9A, 9B and 9C show a G-Band Faraday isolator, a W-Band narrow-bandwidth junction circulator, and a Ka-Band full waveguide band junction circulator, respectively.

A number of coaxial ferrite devices are also offered as COTS products. The frequency coverage of ferrite devices is from 8.2 to 220 GHz over specific bandwidths. Recent progress has been made to miniaturize Faraday rotation isolators. Currently, compact and miniature Faraday isolators are offered as COTS products that cover operating frequencies up to 220 GHz. Their smaller size can enhance system integrations. Figure 9D shows a G-Band miniature Faraday isolator that spans 140 to 220 GHz with 4-dB typical insertion loss and better than 20-dB isolation, with a physical length just over one-half inch.

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Figure 9A. A G-Band Faraday isolator covers 140 to 220 GHz

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Figure 9B. A W-Band junction circulator targets 92 to 98 GHz

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Figure 9C. A Ka-Band, full waveguide band circulator spans 26.5 to 40 GHz

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Figure 9D. A G-Band miniature Faraday isolator covers 140 to 220 GHz

Antennas

For any wireless system, antennas have a major impact on overall performance. With advanced signal-processing methods, multiple signal paths can be embraced to achieve greater system capacity and functionality. However, more traditional systems benefit from having antennas with good polarization purity and limited side-lobe responses. Eravant carries many rectangular and circular horn antennas (Figures 10A and 10B). Horn antennas support applications from radar and communication systems to advanced test instrumentation. With phase error corrections, lens-corrected horns provide enhanced performance (Figure 10C). Scalar horns and choke flange horns offer lower side lobes compared to other alternatives. They also provide nearly equal beam widths in their E and H planes, along with high cross-polarization rejection. Selected models are shown in Figures 10D and 10E. Higher gain and narrower beam widths are realized using Gaussian optics antennas (Figure 10F) and Cassegrain antennas (Figure 10G). They can serve in applications operating from several GHz up to 325 GHz.

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Figure 10A. Rectangular Horn Antenna, WR-12 Band

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Figure 10B, Circular Horn Antenna, WR-15 Band

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Figure 10C. Lens Correct Horn Antenna, WR-10 Band

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Figure 10D. Scalar Horn Antenna for WR-28 Band

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Figure 10E. Choke Flange Antenna for WR-12 Band

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Figure 10F. Gaussian Optical Lens Antenna, WR-06 Band

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Figure 10G. Cassegrain Antenna, WR-22 Band

Dual-polarized antennas are often needed in radar and communication systems, test and measurement systems, and antenna ranges. They can be used as duplexing antennas when dealing with separate Transmit and Receive signals, or with circularly polarized signals. Dual polarized antennas are also convenient when performing antenna tests where test signals having either vertical or horizontal linear polarization are both desired. The antenna polarization is selected through switches, eliminating the need to physically rotate the Transmit or Reference antenna. The arrangement avoids time-consuming setup procedures and possible measurement errors caused by misalignment of the source antenna and the DUT. Additionally, circularly polarized signals can be transmitted by simultaneously exciting the Vertical and Horizontal antenna ports using the appropriate offsets for phase and amplitude.

There are two common ways to construct dual polarized antennas. Examples of quad-ridge antennas, and antennas employing ortho-mode transducers (OMTs), are shown in Figures 11A, 11B, 11C, 11D, and 11E.

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Figure 11A. Quad Ridge Horn for the 5G FR2 Band, 6 to 44 GHz

Quad-ridge antennas use tapered-slot or Vivaldi-like antenna apertures. Two such apertures are co-located at right angles to each other to yield separate Vertical and Horizontal polarization ports, usually employing coaxial connectors. Alternatively, a waveguide horn is often paired with an OMT to realize a dual-polarized antenna. The horn antenna may have either a rectangular or circular feed. Examples of standard OMT based dual polarized antennas are shown in Figures 11D and 11E. Other configurations can support separate right-hand and left-hand circular polarizations without requiring additional engineering efforts. OMT-based antennas typically offer higher cross-polarization rejection, higher port isolation, flatter gain, well-defined beam shapes, or lower side lobes when compared with quad-ridge antennas.

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Figure 11B. A quad-ridge horn operates from 4 to 24 GHz

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Figure 11C. Model SAV-0434031428-KF-U5-QR spans 4 to 40 GHz

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Figure 11D. Model SAH-2434231060-328-S1-280-DP operates from 24 to 42 GHz

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Figure 11E. Model SAF-7531141340-110-S1-100-DP covers 75 to 110 GHz

The pros and cons of these antenna types are summarized in Table 1. In general, quad-ridge antennas cover broader operating bandwidths, often more than an octave, such as 2 to 18 GHz or 4 to 24 GHz. They are limited to the lower or middle mm-wave frequency bands due to stringent machining and assembly boundaries. The main drawback of OMT-based antennas is their operating bandwidths, being limited to standard waveguide operating bandwidths generally.

Table 1. Dual Polarized Antenna Comparison

Item Quad-Ridge Antenna OMT-Based Antenna
Antenna Type Circular or rectangular horn All types
Operating Bandwidth Ultra-broad, such as 2 to 18 GHz Waveguide bandwidths in general
Gain Low, 10 to 20 dBi typically Wide range, 10 to 50 dBi
Side Lobe Levels High, 10 to 20 dB Wide range, antenna type dependent
Beam Width Limited range Wide range, antenna type dependent
Cross-Polarization Low, 25 dB typical High, 70 dB typical
Port Isolation Low, 20 dB typical High, 40 dB typical
Port Type Coaxial Waveguide or Coaxial

Because of the performance limitations of quad-ridge dual polarized antennas, OMT based antennas are used in more applications. By selecting among various antenna types, such as the conical horn, pyramid horn, probe antenna, lens-corrected horn, scalar horn, choke flange horn, Gaussian antenna or a dish, a variety of dual polarized antennas can be configured with OMTs. However the antenna port of standard OMTs is configured with square waveguide. To make a connection between an OMT and an antenna with a circular waveguide feed, a mode transition is required. Table 2 summarizes the range of OMT-based antennas to illustrate how various OMT-based dual polarized antennas can be readily configured using standard COTS components.

Table 2. OMT-Based Dual Polarized Antennas Overview

Dual Polarized Antenna Types Features
OMT + Conical Horn (SAC Series) Full waveguide band performance, gain is limited to 25 dBi, high side lobe level, lower cost
OMT + Pyramid Horn (SAR Series) Full waveguide band performance, gain is limited to 25 dBi, high side lobe level, lower cost
OMT + Choke Flange Horn (SAH Series) Full waveguide band performance, broader beamwidth and low gain, low side lobe level, lower cross-polarization, moderate cost
OMT + Scalar Feed Horn (SAF Series) Full waveguide band performance, broader beamwidth and gain up to 17 dBi, low side lobe level, lower cross-polarization, moderate cost
OMT + Lens Corrected Horn (SAL Series) Full waveguide band performance, narrow beamwidth and high gain depending on the dish size selection, low side lobes, moderate cost
OMT + Gaussian Antenna (SAG Series) Full waveguide band performance, narrow beamwidth and high gain depending on the aperture size selection, low side lobes, lower cross-polarization, high cost
OMT + Cassegrain Antenna (SAY Series) Full waveguide band performance, narrow beamwidth and high gain depending on the dish size selected, lower cross-polarization, high cost

OMTs are not only used as key antenna feed elements.   They are also the key duplexing component in many Radar and communication systems that transmit and receive signals with high isolation. Cross-polarized transmit and receive signals enable full-duplex communication through a single antenna with very low coupling between channels. Although other duplexing devices such as Transmit/Receive (T/R) switches and ferrite circulators are commonly employed as duplexers, OMTs often result in the best performance overall in terms of operating bandwidth, insertion loss and isolation. Recognizing this potential, Eravant supplies OMTs with some of the best performance metrics available, including 40-dB port isolation and high levels of cross-polarization suppression. The OMT family covers the frequency range of 7 to 220 GHz in 15 waveguide bands, namely, WR-112 to WR-05. The OMTs support full waveguide band operation, with some covering wider bandwidths for specific applications. Figures 11F and 11G illustrate two popular OMT models for Ka-band 5G mm-wave FR2 applications, and for E-Band last-mile and automotive radar applications. The Ka-band model works beyond the WR-28 waveguide operating bandwidth, covering the frequency range of 23 to 44 GHz. The E-Band OMT spans 60 to 90 GHz to encompass the last-mile frequency bands of 71 to 76 GHz and 81 to 86 GHz.

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Figure 11F, Model SAT-343-28028-S1 OMT for the 23 to 44 GHz band

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Figure 11G. Model SAT-FE-12212-S1 OMT operates from 60 to 90 GHz

Omnidirectional antennas are often found where the landscape requires uniform signal coverage. Offering gains from 2 to 7.5 dBi, COTS passive omnidirectional antennas cover full waveguide bands between 26.5 and 140 GHz. Models SAO-2734030810-28-S1 and SAO-7531140230-10-S1are presented in Figures 12A and 12B, respectively.

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Figure 12A. An omni-directional antenna covers 26.5 to 40 GHz with 10-degree beam width

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Figure 12B. An omni-directional antenna spans 75 to 110 GHz with 30 degree beam width

Finally, planar array antennas are available as fixed-beam microstrip arrays, slotted waveguide arrays, and beamforming arrays. Fixed array antennas cover frequencies up to 75 GHz while slotted waveguide arrays can reach 35 GHz. Additionally, a modular antenna system facilitates the construction of custom arrays for Multiple-Input Multiple-Output (MIMO) antenna systems that exploit multi-path propagation routes for increased channel capacity and improved system reliability. The modular array uses a practical realization to provide flexibility in forming the array size. The basic building block, i.e. the radiation element, is a single patch antenna that offers 6 dBi nominal gain with beam widths of and 50 degrees in the Vertical or E Plane and 95 degrees in the Horizontal or H Plane. A technical paper provides design details, antenna performance, and the mechanical structure for various MIMO antenna configurations [6]. Representative models of array antennas are shown in Figures 13A, 13B and 13C. A 4x16 MIMO array is illustrated in Figure 13D.

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Figure 13A. A fixed beam microstrip antenna array operates at 24.125 GHz

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Figure 13B. A 35-GHz slotted waveguide array antenna offers 27 dBi Gain

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Figure 13C. Model SAM-2832830695-DM-L1-64C is a 28-GHz 64-elements array antenna

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Figure 13D. Assembly illustration for a 4x16 modular phased array antenna

Passive and Interconnection Components

Modern microwave and mm-wave subsystems are configured based on two major transmission line formats, rectangular waveguides and coaxial cables. The waveguide is still the dominant transmission line in the mm-wave bands, while coaxial cables are catching up to reach 110 GHz using 1 mm connectors. Waveguide remains dominant because of its low loss characteristics and higher power handling capacity. Coaxial cable offers mechanical flexibility and lower weight where electrical performance is a less of a concern.

Passive and interconnection components are important elements in mm-wave subsystem design and construction. These components appear in both waveguide and coaxial configurations to include power dividers, directional couplers, magic tees, filters, diplexers, terminations, adapters, and various waveguide components such as bends, twists and straights. As a result, transitions between the two major transmission media are usually unavoidable. Therefore a wide range of waveguide-to-coax adapters is offered to cover the frequency range of 7 to 125 GHz in 13 waveguide bands, WR-112 to WR-08. Coaxial ports range from Type N to 1 mm connectors. Figures 14A and 14B illustrate WR-10 to 1 mm adapters in end-launch and right-angle configurations. Other components such as power dividers, filters, directional couplers, waveguide terminations, and flexible waveguides are shown in Figures 14C, 14D, 14E, 14F, and 14G. Many other adapters covering complete waveguide bands and their coaxial connector counterparts are offered as COTS products.

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Figure 14A, SWC-101F-R1, WR-10 to 1 mm Adapter, End-Launch

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Figure 14B, SWC-101F-R1, WR-10 to 1 mm Adapter, Right-Angle

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Figure 14C. SWP-75311408-10-E2-H, 8 Way Power Divider for 75 to 105 GHz

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Figure 14D. SWF-96312460-12-L1 Lowpass Filter, 60 to 96 GHz

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Figure 14E. SWD-1040H-10-DB W-Band dual directional coupler covers 75 to 110 GHz

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Figure 14F. SWL-1057-S8, W-Band 500-Watt High Power Load

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Figure 14G. SWG-22118-FB-FT-A-G, WR-22 300-mm flexible waveguide section

Millimeter-wave Packaging

Component packing has been a constant challenge to the mm-wave industry. Despite recent advancements in surface-mount packaging technologies [8], connectorized components that contain bare semiconductor die are still playing a major role when electrical performance is the priority. In addition, the final RF interfaces of any microwave or mm-wave system are either coaxial or waveguide, driven by common antenna port configurations. Therefore the waveguide interface is still the preferred and most common solution for packages to achieve the best system performance.

Coaxial connector types, SMA through 1.85 mm, are widely used in the mm-wave industry to cover frequencies up to 67 GHz with reasonably good RF performance. Recent progress has pushed operating frequencies beyond 110 GHz with 1 mm and 0.8 mm connectors. Coaxial connectors offer easy packing adaptations and minimize engineering efforts when glass bead feedthroughs are used. Currently, the highest frequency glass bead available is for 1.85 mm connectors. Therefore, special feedthrough designs are required for packages utilizing 1 mm connectors. In contrast, waveguide-interfaced packages often require completely new engineering designs, as well as a custom designed package that is costly and time consuming to produce.

Following the concept of the coaxial connector, waveguide connectors were developed. The novel devices were recently invented and trademarked as Uni-GuideTM connectors. They are used in exactly the same way as flange-mount two-hole coaxial connectors. Figures 15A, 15B and 15C illustrate their assembly details.

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Figure 15A. A coaxial connector interfaces to a glass-bead feedthrough

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Figure 15B. A waveguide connector interfaces to a glass-bead feedthrough

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Figure 15C. A waveguide connector is used the same way as a coaxial connector

The performance of the waveguide connectors is similar to that of their coaxial counterparts. Figure 16 shows measured results for a WR-28 waveguide connector.

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Figure 16. Measured Performance of Model SUF-2812-480-S1

Currently, standard waveguide connectors cover the WR-28, WR-22 and WR-19 waveguide bands. Figure 17 shows the product family. These connectors have mounting holes that accept 2-56 screws with 0.48-inch separation between the screws, to match a common industry format for coaxial connectors. For other bands, such as WR-42, WR-15 and WR-12, coaxial package designs must be modified to accept the waveguide connectors. This is because the standard 0.48-inch screw separation for coaxial connectors is not compatible with the waveguide or flange dimensions.

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Figure 17. A Photo of Ka, Q and U Band Uni-Guide

The benefits that component manufacturers can now enjoy are,

  1. Flexible package configurations including various connector types and waveguide orientations
  2. Packaged devices with hermetic seals without expensive waveguide window technology
  3. Waveguide interfaced packages without NRE costs and long development cycle times
  4. Fewer standard packages for improved inventory efficiency

For system integrators, Uni-GuideTM connectors can also help to eliminate many interconnection devices, such as coax to waveguide adapters, waveguide twists and bends to make the system more compact and cost-effective. With all of these benefits, the Uni-GuideTM product family, from the device packaging and system integration points of view, not only helps to make mm-wave technology more accessible and affordable, but also reduces NRE costs and product development cycle times [12].

Millimeter-wave Testing

Another obstacle facing users of mm-wave technology is testing. Thanks to the maturity of the microwave industry’s continuous development, the operating frequency of the most advanced microwave test equipment routinely covers 10 MHz to 50 GHz. Some options extend to 67 GHz and beyond. However, equipment operating higher than 67 GHz becomes much more expensive and less available. An alternative is to add frequency extenders to existing lower-frequency test equipment. A range of frequency extenders is available to extend microwave test equipment to 67 GHz and higher. This approach is typically used with microwave synthesizers, sweepers, spectrum analyzers, noise figure meters, and vector network analyzers (VNAs) to reach 50 GHz and above. Example frequency extender models are presented in Figures 18A, 18B, 18C and 18D. To support VNA testing, high-quality and cost-effective waveguide calibration kits are also offered for frequencies up to 220 GHz. A G-Band calibration kit is shown in Figure 19.

Common challenges facing mm-wave and THz measurement are,

  1. Misalignment
  2. Waveguide cocking
  3. Screw insertion and adjustment
  4. Waveguide bending and flange deformation
  5. Time-consuming procedures

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Figure 18A. An E-Band X4 frequency extender works with a 26.5 GHz synthesizer to generate signals from 60 to 90 GHz

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Figure 18B. A W-Band harmonic mixer extends a 26.5-GHz spectrum analyzer to 110 GHz

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Figure 18C. A D-Band Noise Figure and Gain test set operates up to 170 GHz

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Figure 18D. A G-Band VNA extender works with a 20-GHz VNA to reach 220 GHz

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Figure 19. A G-Band calibration kit covers 170 to 220 GHz

Among all microwave and mm-wave tests and measurements, 2-port network scattering parameters are the most demanding. To improve the quality, accessibility, and productivity of VNA testing, a number of innovative tools have been developed.

The waveguide Quick Connect tool improves the speed and accuracy of making temporary mm-wave waveguide connections. Two models work with common waveguide flanges used at frequencies from 33 GHz to 1 THz (Figures 20A and 20B). The benefits of the Quick Connect tools include:

  1. Eliminates DUT surface scratching
  2. Improves waveguide connection efficiency
  3. Avoids waveguide cocking
  4. Accommodates tight spacing between test equipment and DUTs
  5. Forms secure waveguide connections during tests

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Figure 20A. Quick Connect tools join waveguide flanges without using captive screws

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Figure 20B. Quick Connect tools securely join waveguide flanges for rapid testing

           

Perhaps one of the most impactful mm-wave test system improvements is the recent development of a more effective contactless waveguide flange [8]. At Eravant, the Proxi-GuideTM contactless waveguide flange and the Wave-GlideTM rail positioning system were invented to greatly simplify and streamline the operation of VNA frequency extenders. The contactless flange is a waveguide section with a novel flange design that forms an RF choke when connected to another waveguide flange. The design eliminates the requirement for perfect mechanical contact between flanges. The rail positioning system allows repeatable positioning of mm-wave https://www.eravant.com/products/vector-network-analyzer-extendersVNA frequency extenders. They perform easy alignment and rapid connections between the DUT and a test setup that employs contactless flanges. The contactless flange and the rail system are shown in Figures 21A and 21B, respectively.

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Figure 21A. Contactless Flange

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Figure 21B. Millimeterwave VNA Extender with Contactless Flange and Rail System

The Proxi-FlangeTM contactless flange and Wave-GlideTM rail system offer the following benefits:

  1. Allows more reliable and accurate test system calibration using an easy and rapid procedure
  2. Eliminates calibration and testing errors caused by waveguide cocking to produce more accurate and reliable DUT test data
  3. Helps hardware manufacturers deliver better quality DUT products without any wear on the flange threads or scratches on flange surfaces
  4. Eliminates the need for highly skilled operators or technicians
  5. Allows connections in tight spaces by eliminating screw insertion
  6. Preserves test equipment accuracy and extends test equipment lifespan
  7. Supports the testing of DUTs with non-standard flanges
  8. Releases mechanical alignment stress, increases productivity, and guarantees consistent DUT test results
  9. When used with computer-controlled actuators they can support fully automated high-volume production testing

Virtually all mm-wave test ports can be equipped with contactless flanges to realize faster and more reliable tests and measurement results. [14]

Conclusions:

Today, mm-wave COTS products offer total product solutions to engineers, researchers, and developers to construct and demonstrate future technologies and applications. Further, these products can also help to reduce the cost and cycle time of future system development and implementation. Novel waveguide connectors offer a new way to package waveguide-interfaced components and subsystems, As a result, many custom designed packages are avoided, saving development time and cost. A contactless flange and rail system, along with many frequency extender products, have the potential to greatly enhance mm-wave test and measurement systems. All this has made mm-wave technologies more accessible and more affordable for new applications.

Acknowledgement

The authors would like to acknowledge Dr. Fathy for his ideas in organizing this presentation. His comments, suggestions, and encouragements during the preparation of this manuscript are greatly appreciated.

References

[1] P. Bhartia and I. J. Bahl, “Millimeter Wave Engineering and Applications,” John Wiley & Sons, New York, 1984. ISBN: 0-471-87083-8.

[2] James. C. Wiltse, “History of Millimeter and Submillimeter Waves,” IEEE Transitions on Microwave Theory and Techniques, Vol. MTT-32, pp. 1118-1127, September 1984. DOI: 10.1109/TMTT.1984.1132823

[3] Nicholas C. Currie and Charles E. Brown, “Principles and Applications of Millimeter-wave Radar,” Artech House, Boston, 198. ISBN: 0-89006-202-1.

[4] Aritra Banerjee et al, “Millimeter-Wave Transceivers for Wireless Communication, Radar, and Sensing: (Invited Paper),” IEEE, Custom Integrated Circuits Conference, Austin, TX, April 2019, pp. DOI: 10.1109/CICC.2019.8780147.

[5] Ismail Nasr et al., "A Highly Integrated 60 GHz 6-Channel Transceiver With Antenna in Package for Smart Sensing and Short-Range Communications," IEEE Journal of Solid-State Circuits, vol. 51, no. 9, pp. 2066-2076, Sept. 2016.

[6] Taiyun. Chi et al., "17.7 A packaged 90-to-300GHz transmitter and 115-to-325GHz coherent receiver in CMOS for full-band continuous-wave mm-wave hyperspectral imaging," IEEE International Solid-State Circuits Conference, pp. 304-305, 2017. DOI:10.1109/ISSCC.2017.7870382

[7] John C. Mahon, Michael Clark and Peter Katzin, “A Surface Mount 45 to 90 GHz Low Noise Amplifier Using Novel Hot-via Interconnection,” 2018 IEEE/MTT-S International Microwave Symposium Digest, Philadelphia, PA, June 2018, pp. 293 to 296. DOI: 10.1109/MWSYM.2018.8439302

[8] Nathan Seongheon Jeong et al, “A recent development of antenna-in-package for 5G millimeter-wave applications (Invited paper),”2018 IEEE 19th Wireless and Microwave Technology Conference (WAMICON), Sand Key, FL, April 2018. DOI: 10.1109/WAMICON.2018.8363905

[9] Biswa P. S. Sahoo et al, “Enabling Millimeter-Wave 5G Networks for Massive IoT Applications: A Closer Look at the Issues Impacting Millimeter-Waves in Consumer Devices Under the 5G Framework,” IEEE Consumer Electronics Magazine, Volume: 8, Issue: 1, Jan. 2019. DOI: 10.1109/MCE.2018.2868111

[10] Mohamed Sayed, “Millimeter Wave Tests and Instrumentation.” 65th ARFTG Conference Digest, Long Beach, CA, June 2005. DOI:10.1109/ARFTGS.2005.1500563

[11] Masahiro Horibe et al, “Improvement of offset short calibration technique in waveguide VNA measurement at millimeter and sub-millimeter wave frequency,”29th Conference on Precision Electromagnetic Measurements (CPEM 2014), Rio de Janeiro, Brazil, August 2014.DOI: 10.1109/CPEM.2014.6898404

[12] Yonghui Shu, “Practical Waveguide Connector Uni-GuideTM,” IEEE, CLASTECH, Los Angeles, CA, Nov. 1, 2019.

[13] Latha Christie et al, “Design and comparison of Waveguide Windows,” 6th International Conference on Advances in Computing & Communications, ICACC 2016, Cochin, India, pp243-250, Sept. 2016.

[14] Cornelius Mayaka, Yonghui Shu, Dhanraj Doshi, “Robust Contactless Waveguide Flange for Fast Measurements,” IEEE, MTT-S International Microwave Symposium Digest, Atlanta, GA, June 2021.

[15] Charles Oleson and Anthony Denning, “Millimeter Wave Vector Analysis Calibration and Measurement Problems Caused by Common Waveguide Irregularities.” 56th ARFTG Conference Digest, June 2000.

[16] Andy Fung, et at, "Two-Port Vector Network Analyzer Measurements in the 218–344- and 356–500-GHz Frequency Bands", Microwave Theory and Techniques IEEE Transactions on, vol. 54, no. 12, pp. 4507-4512, 2006.

[17] E. Pucci and P.-S. Kildal, “Contactless Non-leaking Waveguide Flange Realized by Bed of Nails for Millimeter Wave Applications,” 6th European Conf. on Antennas and Propagations., Prague, pp. 3533-3536, 2012.

[18] Lingyun Ren et al, “Modular and Scalable Millimeter-Wave Patch Array Antenna for 5G MIMO and Beamforming,” 50th European Microwave Conference (EuMC), Jan. 2021, pp 336 - 339. DOI: 10.23919/EuMC48046.2021.9338130

 

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