Choon Sae Lee

RCS of a coated circular waveguide terminated by a perfect conductor

The radar cross section (RCS) of a circular waveguide terminated by a perfect electric conductor is calculated by the geometrical theory of diffraction (GTD) for the rim diffraction and by a physical optics approximation for the interior irradiation. The interior irradiation is generally more than 10 dB higher than the rim diffraction for a/\lambda \geq 1 ( a is the waveguide radius, \lambda is the free-space wavelength). At low frequencies ( a/\lambda \sim 1 ), the interior irradiation can be significantly reduced over a broad range of incident angle if the interior waveguide wall is coated with a thin layer (1 percent of the radius) of lossy magnetic material. Our theoretical prediction is confirmed by measurements. At higher frequencies ( a/\lambda \sim 3 ), a thin layer of coating is effective for the case of near axial incidence, provided that a good transition of the TE_{11} mode near the waveguide opening to the HE_{11} mode inside the waveguide is made. A thicker layer of coating is required for the RCS reduction over wider incident angle.

Planar dual-band microstrip antenna

A novel dual-band microstrip antenna is introduced. The entire structure is planar and can be easily mass-produced by using printed-circuit technology. The antenna consists of multiple layers with metallic strips placed at one of the dielectric interfaces under the radiating patch. The resonant frequencies can be selected over a wide frequency range, and the impedance matching at those two frequencies is relatively simple. The physically intuitive cavity model is used for the analysis. A relatively simple design scheme is included. >

Planar circularly polarized microstrip antenna with a single feed

A novel circularly polarized (CP) microstrip antenna is introduced. The antenna is fed with a single coaxial probe and the structure is planar. The CP bandwidth is much larger than available single-probe microstrip antennas and the CP radiation quality is excellent over the entire upper hemisphere.

Planar, single-feed, circularly polarized microstrip antenna with enhanced bandwidth

The most common circularly polarized (CP) microstrip antennas are made by using a splitting network that feeds the two adjacent sides of a square patch with signals of equal magnitude and 90/spl deg/ phase difference. This splitting network becomes cumbersome especially in large arrays. Single-feed CP microstrip antennas have been investigated by a number of authors eliminating the need for a complex splitting network. Unfortunately these antennas have very narrow CP bandwidth. In the proposed antenna, a double layer microstrip is used. The two orthogonal modes occupy different cavities and the 90/spl deg/ phase difference between the modes is achieved by coupling holes between the two resonant cavities. The coupling hole concept was first introduced by Bethe (1944), and further refined by Collins (1966). In an earlier paper by Lee et al. (1996) each cavity had two side walls in order to suppress any unwanted modes. Unfortunately, side walls make fabrication difficult, especially in array structures. In this paper the side walls are eliminated and the CP bandwidth is substantially increased. In addition, the CP axial ratio does not degrade near the horizon.

Circularly polarized microstrip antenna with a single feed

A novel circularly polarized (CP) microstrip antenna with a single coaxial feed is introduced. The antenna consists of two layers, each radiating fields which at the zenith are perpendicular to each other with 90/spl deg/ phase difference. The CP bandwidth is substantially higher than that of the previously reported CP microstrip antennas with two nearly degenerate modes. In principle, a perfect CP radiation is feasible without the input impedance mismatch. The CP design procedure is independent of the feed location.

Planar double-layer leaky-wave microstrip antenna

This paper presents a double-layer leaky-wave microstrip antenna that is entirely planar for easy fabrication. The cavity model and the full-wave spectral domain method are used to analyze the proposed structure. The results from these two approaches agree well with experimental data. The planar structure makes the leaky-wave antenna convenient for implementation in a monolithic microwave integrated circuit (MMIC) environment. Also, due to the flexibility of the microstrip structure, the geometrical shape can be modified for other desirable radiation patterns.

A simple circular-polarized antenna: circular waveguide horn coated with lossy magnetic material

It is shown that a circular waveguide horn coated with a lossy material in its interior wall can be used as an alternative to a corrugated waveguide for radiating a circularly polarized (CP) field. To achieve good CP radiation, the diameter of the structure must be larger than the free-space wavelength, and the coating material must be sufficiently lossy and magnetic. The device is cheaper and lighter in weight than the corrugated one. >

Impedance matching of a dual-frequency microstrip antenna with an air gap

A microstrip antenna has been proposed to operate at dual frequencies. The microstrip is a single layer patch with nonradiating edges closed with a conducting foil. Resonant frequencies are altered by varying the air gap under the patch. The separation of the resonant frequencies can be nearly zero and has no upper limit in principle. The input impedance is easily matched by shifting the air gap. The radiation patterns are not affected by modification for dual-frequency operation. >

Antenna Efficiency Measurements in a Reverberation Chamber Without the Need for a Reference Antenna

This letter proposes a reverberation chamber approach for antenna efficiency measurement that does not require a reference antenna, that allows the antenna efficiency to be determined over a potentially wide frequency, range and which is applicable to electrically large antennas, thus demonstrating clear advantages over currently employed techniques for antenna efficiency measurement. This letter outlines the proposed method and presents results for a monopole antenna that demonstrate the efficiency can be obtained with reasonable accuracy over a range of frequencies.

Tapered leaky-wave ultrawide-band microstrip antenna

Previous wideband or ultrawideband antennas have been bulky such as horn, helical and log periodic antennas. Conventional microstrip antennas are planar, conformal and inexpensive but have narrow frequency bandwidths. It is extremely desirable to have antennas with all the advantages of microstrip antennas and large bandwidths. The proposed wideband planar leaky-wave microstrip antenna provides such a need.