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Chihyun Cho,Ikmo Park,Hosung Choo IEEE 2010 IEEE transactions on antennas and propagation Vol.58 No.9
<P>We propose a modified Wheeler cap method to accurately measure the radiation efficiency of patch antennas with multiple resonances by modeling their impedance as a high-order circuit model. The radiation efficiency is obtained from the power consumption ratio between the radiation and loss conductances using a circuit model we developed. Our technique is validated by measuring the efficiencies of a circularly polarized microstrip patch antenna and a triple-band microstrip patch antenna. The measurement results are in close agreement with those produced by simulations, whereas the Wheeler cap method with a series-(or parallel-) resonant circuit model is shown to be unreliable.</P>
Uncertainty Analysis in EVM Measurement Using a Monte Carlo Simulation
Chihyun Cho,Joo-Gwang Lee,Jeong-Hwan Kim,Dae-Chan Kim Institute of Electrical and Electronics Engineers 2015 IEEE Transactions on Instrumentation and Measureme Vol. No.
<P>This paper proposes an error vector magnitude (EVM) measurement method for a wideband code-division multiple-access source using a real-time oscilloscope. The EVM values are extracted from the measured waveform using a signal processing that finds the appropriate carrier phase and symbol timing. Measurement uncertainty is also evaluated based on a Monte Carlo simulation, where the errors from the real-time oscilloscope and the signal processing are taken into account. The measured EVM of a source at 900 MHz is (0.2586 ± 0.0040)%, (0.2617 ± 0.0060)%, and (0.2543 ± 0.0078)% at 95% confidence level when the real-time oscilloscope has a bandwidth of 2, 4, and 20 GHz, respectively.</P>
Measurement of Analog Modulation Index with a Calibrated Radio Frequency Attenuator
Chihyun Cho,Hyunji Koo,Jae-Yong Kwon,Joo-Gwang Lee,Tae-Weon Kang 한국전자파학회JEES 2021 Journal of Electromagnetic Engineering and Science Vol.21 No.2
This paper presents a method for the accurate and traceable measurement of the analog modulation index. A calibrated step attenuator was used as the main apparatus because it has a higher dynamic range and lower uncertainty than a spectrum analyzer or an oscilloscope. In amplitude modulation (AM), the modulation index is obtained from the amplitude difference between the carrier and the first sideband, as in the conventional method. The resolution and calibration uncertainties of the step attenuator were propagated to the measurement uncertainty of the modulation index. The uncertainty produced by the impedance mismatch and repeatability was also included. For frequency modulation (FM) and phase modulation (PM), the modulation index, β, was estimated (with the step attenuator) from the spectrum of each sideband through the nonlinear fitting of the Bessel function. Thus, the uncertainty of the fitting process was added to the uncertainty of the measurement. The three modulations, AM, FM, and PM, exhibited an expanded uncertainty (approximately 95% confidence level, k = 2) of 0.372% for 50% nominal depth of the AM, 88.8 Hz for the peak frequency deviation of 10 kHz, and 0.88 mrad for a 0.1 radian modulation index, respectively.
Calibration of Time-Interleaved Errors in Digital Real-Time Oscilloscopes
Cho, Chihyun,Lee, Joo Gwang,Hale, Paul D.,Jargon, Jeffrey A.,Jeavons, Peter,Schlager, John B.,Dienstfrey, Andrew Professional Technical Group on Microwace Theory a 2016 IEEE Transactions on Microwave Theory and Techniqu Vol. No.
<P>A channel mismatch calibration method is proposed for use in time-interleaved digital real-time oscilloscope (DRTO) applications. Linear equations are derived using Fourier transforms of the separated signals from each of the time-interleaved analog-to-digital converters (TIADCs). Thus the errors in the TIADCs can be easily calibrated by inversion of a matrix, as opposed to most previous work where additional filters are employed. The calibration accuracy of the proposed method is limited only by the noise produced after the TIADC circuitry, while other methods depend on the filter design. A transfer function measurement method is then proposed for application to commercially available DRTOs. Two-tone signals are measured using DRTOs from various suppliers to validate the proposed method. The occurrence of signals at spurious frequencies is considerably reduced, as demonstrated by the calibrated results.</P>
Improved Wheeler Cap Method Based on an Equivalent High-Order Circuit Model
Chihyun Cho,Jin-Seob Kang,Hosung Choo Institute of Electrical and Electronics Engineers 2014 IEEE Transactions on Antennas and Propagation Vol. No.
<P>The conventional Wheeler cap method usually produces accurate radiation efficiency of small antennas when the antennas under test (AUTs) operate as a simple series or a parallel RLC resonance circuit. However, this method often gives unreliable radiation efficiency if the AUT has a complicated operating principle such as circular polarization (CP), multiple resonances, or broad-band properties. In this paper, we propose an improved Wheeler cap method based on the equivalent high-order circuit model including transformers to provide accurate radiation efficiency, although the AUT does not operate as a simple resonance circuit. For building equivalent high-order circuit models, a method for estimating the initial values of a genetic algorithm (GA) is also proposed, which effectively reduces the searching space and improves the convergence of the optimization. To verify the proposed method, we measure the radiation efficiency of a CP microstrip antenna, a UHF RFID tag antenna, and a triple-resonance microstrip antenna.</P>
Design of a Circularly Polarized Tag Antenna for Increased Reading Range
Chihyun Cho,Ikmo Park,Hosung Choo IEEE 2009 IEEE transactions on antennas and propagation Vol.57 No.10
<P>We introduce a novel circularly polarized tag antenna, consisting of a truncated patch, a shorting plate, and a ground plane, to increase the reading range while remaining in compliance with EIRP regulations. The reading range of the proposed tag is twice that of linearly polarized tags, due to the decreased polarization mismatch between the reader and tag antennas. An additional parasitic patch is loaded onto the structure to boost the reading range in the UHF RFID band. As a result, an average reading range of 8 m is achieved between 860 MHz and 960 MHz, compared to a range of about 3 m with conventional dipole tags.</P>
Broadband electrically small antenna using two electromagnetically coupled radiators
Cho, Chihyun,Park, Ikmo,Choo, Hosung Wiley Subscription Services, Inc., A Wiley Company 2010 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS Vol.52 No.6
<P>We propose an electrically small antenna that achieves broad bandwidth with small antenna size. This antenna is composed of a hemispherical disk-loaded monopole and a hemispherically wound helix to maximize the disk loading effect and the electromagnetic coupling between the two radiating elements in a given space. The fabricated antenna is 10.7 cm in size (kr = 0.449) and exhibits a measured −3 dB bandwidth of about 30% for a center frequency of 200 MHz. The antenna also achieves a radiation efficiency of over 85% and a transmission loss similar to that of a conventional λ/4 monopole, despite the fact that size of the antenna is about 30% that of the λ/4 monopole. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1369–1372, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25164</P>
The Effects of Substrate, Metal-line, and Surface Material on the Performance of RFID Tag Antenna
Chihyun Cho,Hosung Choo,Ikmo Park 한국전자파학회JEES 2007 Journal of Electromagnetic Engineering and Science Vol.7 No.1
We investigated the effects of substrate, metal-line, and surface material on the performance of radio frequency identification (RFID) tag antenna using a tag antenna with a meander line radiator and T-matching network. The results showed that readability of the tag antenna with a thin high-loss substrate could be increased so that it was similar to that of a low-loss substrate if the substrate was very thin. The readability of the tag antenna decreased significantly when the metal line was thinner than the skin depth. The readability of the tag also decreased drastically when the tag was attached to high-permittivity high-loss target objects.
Characterization of a 1 ㎜ (DC to 110 ㎓) Calibration Kit for VNA
Chihyun Cho,Jin-Seob Kang,Joo-Gwang Lee,Hyunji Koo 한국전자파학회JEES 2019 Journal of Electromagnetic Engineering and Science Vol.19 No.4
This paper presents an evaluation method for a 1 mm coaxial calibration kit that can be used from DC to 110 GHz. The analytical model for the calibration kit was revisited and verified by comparing it with the electromagnetic High-Frequency Structure Simulator (HFSS). We also proposed a method to measure or appropriately estimate the physical parameters of the analytic model. This approach calculates the uncertainty based on the physical parameters, so that the uncertainty can be appropriately propagated to different measured quantities based on the covariance between all frequencies, including the real and imaginary parts. To verify the proposed method, a commercially available 1 mm calibration kit was evaluated, and the impedance of a device under test was measured using the evaluated kit. We compared the measured results with those of the National Institute of Standards and Technology (NIST) and confirmed that they agreed well with each other within the uncertainty. Additionally, the multiple reflections caused by the impedance mismatch between the signal source and the instrument was corrected, and its calibrated uncertainty was obtained in the time domain. Thus, the uncertainty of the impedance measurement in the frequency domain was properly propagated to the time domain.
Planar Near-Field RFID Reader Antenna for Item-Level Tagging
Chihyun Cho,Chuyong Lee,Jeongki Ryoo,Hosung Choo IEEE 2011 IEEE antennas and wireless propagation letters Vol.10 No.-
<P>In this letter, we propose a novel UHF planar near-field antenna for the application of RFID item-level tagging. The proposed antenna was designed to have a strong and uniform H<SUB>z</SUB>-field over a broad antenna aperture to identify the various items with stable reading performance. To obtain a strong near H<SUB>z</SUB>-field, two coupled patches are employed along with a microstrip-line feed, resulting in average H<SUB>z</SUB> of -15 dBA/m on antenna aperture (30 × 30 × 10 cm<SUP>3</SUP>). We also measured the reading range, and it confirmed that the proposed antenna is suitable for a commercial RFID smart-shelf application.</P>