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  1. This is the first report of a distributed amplifier (DA) realized through monolithic integration of transistors with a substrate-integrated waveguide (SIW). The DA uses a steppedimpedance microstrip line as the input divider like in conventional DAs, but uses a low-loss, high-power-capacity SIW as the output combiner. The input signal is distributed to four GaN high-electron mobility transistors (HEMTs) evenly in magnitude but with the phase successively delayed by 90° at the fundamental frequency. The HEMTs are separated by a half wavelength at the second harmonic frequency in the SIW, so that their outputs are combined coherently at the SIW output. To overcome the limited speed of the GaN HEMTs, they are driven nonlinearly to generate second harmonics, and their fundamental outputs are suppressed with the SIW acting as a high-pass filter. The measured characteristics of the DA agree with that simulated at the small-signal level, but exceeds that simulated at the large-signal level. For example, under an input of 68 GHz and 10 dBm, the output at 136 GHz is 24-dB above the fundamental. Under an input of 68 GHz and 20 dBm, the output at 136 GHz is 14 dBm, with a conversion loss of 6 dB and a power consumption of 882 mW. This proof-of-principle demonstration opens the path to improving the gain, power and efficiency of DAs with higher-performance transistors and drive circuits. Although the demonstration is through monolithic integration, the approach is applicable to heterogeneous integration with the SIW and transistors fabricated on separate chips. 
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    Free, publicly-accessible full text available December 5, 2024
  2. F-band substrate-integrated waveguides (SIWs) are designed, fabricated, and characterized on a SiC wafer, along with SIW-based filters, impedance standards, and transitions to grounded coplanar waveguides (GCPW). The GCPW-SIW transitions not only facilitate wafer probing, but also double as resonators to form a 3-pole band-pass filter together with an SIW resonator. The resulted filter exhibits a 1.5-dB insertion loss at 115 GHz with a 34-dB return loss and a 19-GHz (16%) 3-dB bandwidth. The size of the filter is only 63% of previous filters comprising three SIW resonators. These results show the feasibility for monolithic integration of high-quality filters with high-efficiency antennas and amplifiers in a single-chip RF frontend above 110 GHz, which is particularly advantageous for 6G wireless communications and next-generation automobile radars. 
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    Free, publicly-accessible full text available December 5, 2024
  3. It is well established that access to social supports is essential for engineering students’ persistence and yet access to supports varies across groups. Understanding the differential supports inherent in students’ social networks and then working to provide additional needed supports can help the field of engineering education become more inclusive of all students. Our work contributes to this effort by examing the reliability and fairness of a social capital instrument, the Undergraduate Supports Survey (USS). We examined the extent to which two scales were reliable across ability levels (level of social capital), gender groups and year-in-school. We conducted two item response theory (IRT) models using a graded response model and performed differential item functioning (DIF) tests to detect item differences in gender and year-in-school. Our results indicate that most items have acceptable to good item discrimination and difficulty. DIF analysis shows that multiple items report DIF across gender groups in the Expressive Support scale in favor of women and nonbinary engineering students. DIF analysis shows that year-in-school has little to no effect on items, with only one DIF item. Therefore, engineering educators can use the USS confidently to examine expressive and instrumental social capital in undergraduates across year-in-school. Our work can be used by the engineering education research community to identify and address differences in students’ access to support. We recommend that the engineering education community works to be explicit in their expressive and instrumental support. Future work will explore the measurement invariance in Expressive Support items across gender. 
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    Free, publicly-accessible full text available October 1, 2024
  4. As 6G wireless communications push the operation frequency above 110 GHz, it is critical to have low-loss interconnects that can be accurately tested. To this end, D-band (110 GHz to 170 GHz) substrate-integrated waveguides (SIWs) are designed on a 100-μm-thick SiC substrate. The fabricated SIWs are probed on-wafer in a single sweep from 70 kHz to 220 GHz with their input/output transitioned to grounded coplanar waveguides (GCPWs). From CPW-probed scattering parameters, two-tier calibration is used to de-embed the SIW-GCPW transitions and to extract the intrinsic SIW characteristics. In general, the record low loss measured agrees with that obtained from finite-element full-wave electromagnetic simulation. For example, across the D band, the average insertion loss is approximately 0.2 dB/mm, which is several times better than that of coplanar or microstrip transmission lines fabricated on the same substrate. A 3-pole filter exhibits a 1-dB insertion loss at 135 GHz with 20-dB selectivity and 11% bandwidth, which is order-of-magnitude better than typical on-chip filters. These results underscore the potential of using SIWs to interconnect transistors, filters, antennas, and other circuit elements on the same monolithically integrated chip. 
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    Free, publicly-accessible full text available September 3, 2024
  5. At-temperature calibration is not only inconvenient, but also complicated by the temperature dependence of impedance standards. This paper examines the validity of a room-temperature calibration for on-wafer measurements from 70 kHz to 220 GHz, from 25 °C to 125 °C, and up to 48 h. The results indicate that the room-temperature calibration is applicable up to 125 °C provided errors up to 0.5 dB in magnitude and 5° in phase are tolerable. Consistent with previous reports up to 110 GHz, the present errors are mainly caused by the time-dependent system drift instead of the temperature dependence of impedance standards. For unknown reasons, the system proven to be stable at room temperature drifts significantly at elevated temperatures. This makes elevated-temperature measurements challenging because presently it takes approximately three hours for the system to stabilize at a new temperature. Therefore, in the near future, efforts should be concentrated on stabilizing the system faster rather than correcting for the temperature dependence of impedance standards. 
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    Free, publicly-accessible full text available June 16, 2024
  6. This work-in-progress (WIP) paper aims to elucidate how students have developed professional skills since the onset of the COVID-19 pandemic and who are the people who have provided skill development opportunities. Because of the way social distancing affected engineering education during the pandemic, developing professional skills may have been a challenge for engineering students. While online courses and virtual meetings allowed students to remain in contact with faculty and each other, the opportunities to continue having deep relationships (i.e., strong ties) were sparse. Our paper presents an early look at findings from the qualitative phase of an explanatory mixed methods study conducted with 1,234 undergraduates from 13 schools in the US. Our ongoing thematic qualitative analysis reveals that the changes that accompanied social distancing and periods of emergency remote teaching caused by COVID-19 have reinforced different opportunities to develop professional skills than prior to the pandemic. While some students expressed they had fewer opportunities to develop professional skills, participants also identified opportunities to (1) hone written communication skills when inperson discussions were reduced and (2) leverage knowledge from family members to continue developing professionally. Our next steps include finishing the qualitative analysis phase of the project and mixing the qualitative and quantitative data to develop overarching findings that the engineering education community can use to understand how students’ professional skills develop and how to promote that development even during times of educational disruption. 
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    Free, publicly-accessible full text available July 1, 2024
  7. The purpose of our poster presentation is two-fold: 1) to provide an overview of our NSF project, Pandemic Impact: Undergraduates’ Social Capital and Engineering Professional Skills, and 2) to report our progress and preliminary quantitative findings. We hope to discuss our project and preliminary results with fellow engineering educators and receive feedback. The COVID-19 pandemic has impacted engineering education in multiple ways that will continue to be felt for years to come. One of the less understood ways the pandemic has continued to leave a residue on engineering education is how social distancing and online courses altered students’ professional development. Of particular concern are students who were either new to the institution or started their college education during the pandemic. These students have potentially limited opportunities to establish social relationships at their educational institutions compared to students who already developed such relationships when the pandemic-induced online learning took place. The differences in students’ social relationships can have other, more profound impacts on their undergraduate engineering experiences. Research has shown that students’ social relationships provide them with connections to resources and supports essential for navigating an engineering program and help them obtain more opportunities to practice non-technical professional skills [1], [2]. Although social distancing measures diminished and students returned primarily to in-person, the pandemic has altered the development of engineering students in ways not understood. In particular, understanding the nature of students’ social interactions on campus and the types of opportunities for professional development is essential so that instructors and campus staff can respond to the developmental needs of students. As a result, the overarching research question for our project is: How do engineering undergraduates leverage relationships (operationalized as social capital) to gain opportunities to develop professional skills? 
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    Free, publicly-accessible full text available July 1, 2024
  8. Abstract

    We present a spectroscopic analysis of Eridanus IV (Eri IV) and Centaurus I (Cen I), two ultrafaint dwarf galaxies of the Milky Way. Using IMACS/Magellan spectroscopy, we identify 28 member stars of Eri IV and 34 member stars of Cen I. For Eri IV, we measure a systemic velocity ofvsys=31.51.2+1.3kms1, and velocity dispersionσv=6.10.9+1.2kms1. Additionally, we measure the metallicities of 16 member stars of Eri IV. We find a metallicity of[Fe/H]=2.870.07+0.08, and resolve a dispersion ofσ[Fe/H]=0.20 ± 0.09. The mean metallicity is marginally lower than all other known ultrafaint dwarf galaxies, making it one of the most metal-poor galaxies discovered thus far. Eri IV also has a somewhat unusual right-skewed metallicity distribution. For Cen I, we find a velocityvsys= 44.9 ± 0.8 km s−1, and velocity dispersionσv=4.20.5+0.6kms1. We measure the metallicities of 27 member stars of Cen I, and find a mean metallicity [Fe/H] = −2.57 ± 0.08, and metallicity dispersionσ[Fe/H]=0.380.05+0.07. We calculate the systemic proper motion, orbit, and the astrophysical J-factor for each system, the latter of which indicates that Eri IV is a good target for indirect dark matter detection. We also find no strong evidence for tidal stripping of Cen I or Eri IV. Overall, our measurements confirm that Eri IV and Cen I are dark-matter-dominated galaxies with properties largely consistent with other known ultrafaint dwarf galaxies. The low metallicity, right-skewed metallicity distribution, and high J-factor make Eri IV an especially interesting candidate for further follow-up.

     
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