Kihun Chang

A 3-D Luneburg Lens Antenna Fabricated by Polymer Jetting Rapid Prototyping

In this work, we designed, built, and tested a low-gain 20 dBi Luneburg Lens antenna using a rapid prototyping machine as a proof of concept demonstrator. The required continuously varying relative permittivity profile was implemented by changing the size of plastic blocks centered on the junctions of a plastic rod space frame. A 12-cm ( 4λ0 at 10 GHz) diameter lens is designed to work at X-band. The effective permittivity of the unit cell is calculated by effective medium theory and simulated by full-wave finite-element simulations. The fabrication is implemented by a polymer jetting rapid prototyping method. In the measurement, the lens antenna is fed by an X-band waveguide. The measured gain of the antenna at X-band is from 17.3 to 20.3 dB. The measured half-power beam width is from 19° to 12.7° while the side lobes are about 25 dB below the main peak. Good agreement between simulation and experimental results is obtained.

A study on the UWB antenna with band-rejection characteristic

This paper proposes a novel ultra-wideband antenna that has the omni-directional pattern in azimuth and the band-rejection characteristic of the 5.15/spl sim/5.825 GHz band limited by IEEE 802.11a and HIPERLAN/2. The proposed antenna is the circular monopole type with two slits and fed by a CPW line, and it satisfies the UWB band, 3.1/spl sim/10.6 GHz for VSWR < 2.

Equivalent circuit modeling of active frequency selective surfaces

In this paper, active frequency selective surfaces (FSS) with squared aperture with loading are described. Active FSS elements using switched PIN diodes are discussed. The unit cell consists of a square aperture element with a metal island loading and one PIN diode placed at the upper gap considering the vertical polarization. The electromagnetic properties of the active FSS structure are changed by applying a dc bias to the substrate, and estimated by the equivalent circuit model of FSS structure and PIN diode. This active FSS design enables transmission to be switched on or off at 2.3 GHz, providing high transmission when the diodes are in off state and high isolation when the diodes are on. Equivalent circuit in the structure is investigated by analyzing transmission and reflection spectra. Measurements on active FSS are compared with numerical calculation based on the scattering matrix formalism. The origin of experimentally observed frequency responses will be scrutinized.

Active Microwave Metamaterials Incorporating Ideal Gain Devices

Incorporation of active devices/media such as transistors for microwave and gain media for optics may be very attractive for enabling desired low loss and broadband metamaterials. Such metamaterials can even have gain which may very well lead to new and exciting physical phenomena. We investigate microwave composite right/left-handed transmission lines (CRLH-TL) incorporating ideal gain devices such as constant negative resistance. With realistic lumped element values, we have shown that the negative phase constant of this kind of transmission lines is maintained (i.e., left-handedness kept) while gain can be obtained (negative attenuation constant of transmission line) simultaneously. Possible implementation and challenging issues of the proposed active CRLH-TL are also discussed.

Ultra-wide band spiral shaped small antenna for the biomedical telemetry

In this paper, the ultra-wide band spiral shaped small antenna for the implantable micro systems is proposed. Among these implantable systems, the capsule endoscope system operates inside the human body. The human body tissues which have effects on the antenna propagation characteristics are considered. To transmit the diagnostic real-time image data of high resolution at high speed, the system with larger bandwidth is needed. Thus, the bandwidth enhancement of the antenna is needed. To encase the system in the capsule module and to obtain ultra-wide band characteristics, the small sized spiral antenna is designed. The proposed antenna has the bandwidth of 400-510 MHz for VSWR<2 and the fractional bandwidth of 24.1 %. The antenna has the omni-directional radiation pattern and the size of 10.5 mm in diameter and 3 mm in height. Therefore, it is apparent that the proposed antenna provides the ultra-wideband characteristics at the biomedical frequency band and can be utilized to the UWB systems for the biomedical telemetry.

Microwave gain medium with negative refractive index

Artificial effective media are attractive because of the fantastic applications they may enable, such as super lensing and electromagnetic invisibility. However, the inevitable loss due to their strongly dispersive nature is one of the fundamental challenges preventing such applications from becoming a reality. In this study, we demonstrate an effective gain medium based on negative resistance, to overcompensate the loss of a conventional passive metamaterial, meanwhile keeping its original negative-index property. Energy conservation-based theory, full-wave simulation and experimental measurement show that a fabricated sample consisting of conventional sub-wavelength building blocks with embedded microwave tunnel diodes exhibits a band-limited Lorentzian dispersion simultaneously with a negative refractive index and a net gain. Our work provides experimental evidence to the assertion that a stable net gain in negative-index gain medium is achievable, proposing a potential solution for the critical challenge current metamateiral technology faces in practical applications.

Ultra wideband tapered slot antenna with band-stop characteristic

This paper proposes a novel ultra wideband (UWB) tapered slot antenna (TSA) to achieve both enhanced impedance bandwidth for UWB RF front-ends and band-stop characteristics in the 5 GHz WLAN band, limited by IEEE802.11a and HIPERLAN/2. To obtain these two characteristics of wideband and band-stop in the TSA, a broadband microstrip-slotline transition with multi-arm stubs and /spl lambda//4 short stubs are used respectively. From the measured results, it can be shown that a wide bandwidth of 1.99 octaves from 2.8 to 11.09 GHz for a VSWR<2 is obtained while the 5.0 /spl sim/5.93 GHz band is rejected.

Novel slot antennas for harmonic suppression

In this paper, novel microstrip-fed slot antennas with simple structure are proposed to achieve suppression of harmonics. To obtain this operation, conductor lines connected with ground plane in rectangular and meander type slot antennas are used, which have wide-bandstop characteristic. From the measured input impedances and radiation patterns of the proposed antennas, the second and third harmonics are effectively suppressed.

The helical antenna for the capsule endoscope

In this paper, a helical antenna for a capsule endoscope is designed. To encase the system in a small capsule module and to obtain omni-directional radiation pattern for detecting the transmitted signal independent of the transmitter positions, and to obtain wide bandwidth, a small sized normal mode helical antenna whose end is connected to the ground is designed. The diameter and the height of the helix are 8 mm and 5.6 mm, respectively. Measured results show that the antenna has 32 MHz bandwidth for VSWR<2, and an omni-directional radiation pattern.

An X-band Luneburg Lens antenna fabricated by rapid prototyping technology

In this paper, a 3-D Luneburg Lens has been designed, fabricated and characterized. Refractive index control of the lens is based on the mixing ratio of air voids and a polymer. The 12 cm (4λ0 at 10 GHz) diameter lens is designed to work at X-band. The effective permittivity of the unit cell is estimated by effective medium theory and calculated by the finite-element simulation software Ansoft HFSS. Fabrication is implemented by a polymer jetting rapid prototyping method. In the measurement, the lens antenna is fed by an X-band waveguide. The half-power beam width of the antenna is 14 degrees and no obvious side lobe is found in the measurement above the noise floor.