Jihun Choi

Extremely Small Two-Element Monopole Antenna for HF Band Applications

This paper presents a novel antenna architecture to achieve an extremely small form factor for HF band applications. The approach is based on manipulating the topology of a short monopole antenna without utilizing a high index material. A new architecture incorporating two radiating elements is configured, which allows significant gain enhancement. It is shown that such architecture can render a miniaturized HF antenna on air substrate having lateral and height dimensions as small as 0.0115λ0 × 0.0115λ0 × 0.0038λ0 (150 mm× mm × 50 mm for operation at 22.9 MHz). It is found that the measured gain of such architecture can be as high as - 18.1 dBi, which is 16.7 dB higher than a reference inverted-F antenna realized on a high index material (εR = 10.2) having exactly the same dimensions. The proposed antenna architecture is composed of two in-phase radiating vertical elements connected to two inductors between which a capacitive top load is connected to achieve the desired resonant condition. The two vertical elements act effectively as a monopole having increased height. It is also shown that the gain of the antenna can be increased monotonically by increasing the quality factor (Q) of the phase shifter. High Q air-core inductors that can be accommodated in electrically small monopole antenna are designed and incorporated in the phase shifter to achieve a gain value of - 17.9 dBi. Details about the proposed design approach, simulation, and measurement results are discussed.

Measurements and Physics-Based Analysis of Co-Located Antenna Pattern Diversity System

This paper investigates the advantages offered by radiation pattern diversity using a new physics-based analysis that takes into account the complex radiation patterns of the transmit(Tx) and receive(Rx) diversity antennas in conjunction with an accurate deterministic, coherent, and polarization preserving propagation model for a complex indoor scenario. Unlike techniques that utilize spatial diversity, radiation pattern diversity offers a unique opportunity to achieve compact diversity antenna systems especially with the advent of enabling antenna miniaturization techniques. In this work, a co-located antenna radiation pattern diversity system is proposed and its performance is analyzed using an accurate physics-based diversity analysis technique. The proposed analysis technique utilizes an efficient deterministic propagation model modified by incorporating the complex gain of the Tx and the complex effective heights of the Rx antennas into the rays launched and received by the antennas in a coherent manner. The proposed system is realized and tested in complex indoor scenarios based on which complex correlation coefficients between various channels and the effective diversity gain are computed which are then utilized as a figure of merit for improved channel reliability. For the scenarios considered, it is shown that the apparent diversity gain is at least 9.4 dB based on measured results.

Electrically Small Folded Dipole Antenna for HF and Low-VHF Bands

A novel, highly miniaturized, lightweight antenna operating at the low-VHF band is presented. Earlier studies on an extremely short HF monopole antenna consisting of two in-phase vertical elements face an inevitable issue regarding an unbalanced coaxial cable feed due to the very small ground plane . To resolve this problem, as well as to achieve higher bandwidth, we introduce an electrically small folded dipole version of the same antenna having a fully balanced structure. The overall dimension and total mass of the proposed antenna are 10 ×10 ×15 cm³. ( 0.013λ₀ ×0.013λ₀ ×0.02λ₀ at 40 MHz) and 98 g, respectively. The gain and pattern of the fabricated antenna are measured in an elevated range that is in nearly free-space conditions. Measurements are shown to be in good agreement with the design predictions from simulation.

Performance assessment of lower VHF band for short-range communication and geolocation applications

The focus of this paper is to characterize near-ground wave propagation in the lower very high frequency (VHF) band and to assess advantages that this frequency band offers for reliable short-range low-data rate communications and geolocation applications in highly cluttered environments as compared to conventional systems in the microwave range. With the advent of palm-sized miniaturized VHF antennas, interest in low-power and low-frequency communication links is increasing because (1) channel complexity is far less in this frequency band compared to higher frequencies and (2) significant signal penetration through/over obstacles is possible at this frequency. In this paper, we quantify the excess path loss and small-scale fading at the lower VHF and the 2.4 GHz bands based on short-range measurements in various environments. We consider indoor-to-indoor, outdoor-to-indoor, and non-line-of-sight outdoor measurements and compare the results with measurements at higher frequencies which are used in conventional systems (i.e., 2.4 GHz). Propagation measurements at the lower VHF band are carried out by using an electrically small antenna to assess the possibility of achieving a miniaturized, mobile system for near-ground communication. For each measurement scenario considered, path loss and small-scale fading are characterized after calibrating the differences in the systems used for measurements at different frequencies, including variations in antenna performance.

A compact, low-power, low-VHF radio for mobile and wireless communication applications

A cost-effective, low-power, ZigBee-based compact radio system tuned for operation at low VHF band is proposed for mobile communications in complex environments. Existing off-the-shelf radios at this band are bulky, usually operate at high power levels (a few watts), and require relatively large antennas to enable long range communications. To remedy these shortcomings, we develop a small-form-factor low-VHF radio that leverages the ZigBee technology with a frequency conversion technique. A bi-directional frequency converter for the radio is designed to translate ZigBee signals into low-VHF carriers and optimized to minimize power consumption, and simultaneously maximize system gain and sensitivity. This radio also comes with a newly developed miniaturized antenna [1] (≤ 0.02λ0 in height and lateral dimensions at 40 MHz) having much higher efficiency compared to antennas of similar sizes. This antenna allows the compact radio system to operate at lower power and on small robotic platforms needed for military or rescue missions.

Highly miniaturized low-VHF folded dipole antenna for compact, mobile communication applications

For short range communications in the complex environments such as caves, urban and indoor settings, the main concern is to achieve reliable communication over wireless links since large number of scatterers in the communication channel at the conventional microwave radio bands lead to excessive signal attenuation and fast fading. One solution to mitigate such adverse effects is to use radios at much lower frequencies (e.g. HF and Low-VHF) since the signal can penetrate significantly through manmade and natural obstacles while reflection, scattering, and diffraction are far less than those observed at higher frequencies. In order to realize compact, mobile communication systems operating at the low frequencies, however, several challenges, especially in terms of antenna size, are encountered. One issue pertains to design of antennas with small form factors that can provide moderate radiation efficiency and bandwidth. It is noted that antennas with lower gain (efficiency) are tolerable as such drop in gain can easily be more than compensated for by the reduction in the path loss.

Performance analysis of a common aperture antenna diversity system

Summary form only given. An aspect of wireless communication that is of paramount interest is reliable connectivity unhampered by signal fading caused by scatterers such as walls, buildings and other obstacles. For power limited ad hoc networks in complex environments, various phenomenon including multipath, diffraction from sharp corners contribute to the fading and distortion of electromagnetic waves that severely limit the coverage and reliability. Because of the difference in path length among the various signal components and in the absence of a direct signal, the received electric field will have uneven spatial distribution and significant fluctuations. This phenomenon is called fast fading and results in intermittent signal drop-offs causing the communication channel to be unreliable. A viable approach to mitigate fast fading is the use of antenna diversity systems. Antenna Diversity Systems, when used in multipath environments such as indoor and urban environments, enable improvement in signal-to-noise-ratio (SNR) without increasing the transmit power. A thorough and accurate analysis of a diversity system requires three main components: the Tx and Rx antenna radiation patterns (phase and amplitude), the multipath coherent propagation model and calculation of figures of merit for the performance of the diversity system including complex and envelop correlation coefficients and diversity gain. Existing simulation based diversity analysis techniques essentially model the multipath channel using a stochastic model such as Rayleigh and Rician distributions. While these probabilistic models provide generalized approximations to the indoor channel, such models do not accurately capture all the propagation mechanisms such as angle of arrival and polarization, and hence diversity analysis techniques based on such models inherently lack the information needed to assess the true performance of a given diversity system. In this work, we discuss a new diversity system analysis approach that takes into account the complex radiation pattern of the Tx and Rx diversity antennas and make use of an accurate deterministic, coherent, and polarization preserving propagation model for a complex indoor scenario. The new physics-based pattern diversity analysis approach will be introduced. The propagation modeling with specific focus on near-ground antennas will also be discussed. Measurement results in indoor scenarios utilizing a compact, co-located radiation pattern diversity antenna system will be utilized to assess the performance of the diversity system via complex correlation coefficients and diversity gain.

A Non-Foster matched dipole for a low-vhf mobile transmitter system

A major bottleneck of low frequency communication applications is relatively low-data-rate transmissions due to narrow bandwidth with miniaturized antennas. To increase the bandwidth of small antennas, we propose a non-Foster matched antenna designed for a low-VHF transmitter system. A non-Foster element realized by a single negative impedance converter is applied to a passive-matched miniature dipole at 40 MHz. A rigorous stability analysis based on system sensitivity is performed via circuit simulation and measurement. A power transmission coefficient with a reference antenna in an indoor setting along with impedance bandwidth of the non-Foster matched dipole is measured and compared with those of the passive-matched one. The results show 3dB power bandwidth is improved by a factor of two with active matching and more than 80 % of the bandwidth has a power efficiency advantage, compared to passive matching.

Pattern and Gain Characterization Using Nonintrusive Very-Near-Field Electro-Optical Measurements over Arbitrary Closed Surfaces

A nonintrusive near-field measurement technique for 3-D radiation pattern and gain characterization of antennas is presented. The method is of particular interest for low-frequency antennas for which anechoic chambers cannot be developed and far-field measurements are rather cumbersome. Nonintrusive, broadband measurements are performed using an extremely small all-dielectric electro-optical probe to measure the tangential electric fields of an antenna under test (AUT) at a very-near surface enclosing the antenna. Far-field radiation is computed from a new near-field to far-field transformation formulation using only the tangential components of the electric field over an arbitrary surface. This procedure employs reciprocity theorem and the excited electric current on the surface of a perfect electric conductor enclosure having the same geometry as the scanned surface and illuminated by a plane wave. In this way, a full spherical radiation pattern and gain of the AUT are easily computed without expensive computation and truncation errors. To demonstrate the proposed approach, a miniaturized low very high frequency antenna operating at 40 MHz with dimensions

HF/VHF antenna characterization from very-near-field measurements over arbitrary closed surfaces

0.013{ \lambda }_{0} {\times } 0.013{\lambda }_{0} \times 0.02 {\lambda }_{0}