Woosang Lee

Full-wave simulation of frequency diverse array antenna using the FDTD method

This paper describes 2-D analysis of a frequency diverse transmit array antenna with a periodic modulated pattern in range using the finite difference time domain methods. The elements of applied linear array are equally spaced and have same phase but operated at different frequencies. The result pattern of frequency diverse array forms range-dependent beam patterns whereas conventional array pattern forms same beam patterns in far field range. This range-dependent beam pattern provides more flexible beam scan options and lessens undesirable interferences such as multipath. The simulation results for radiation patterns of open-ended waveguide (OEG) antenna array are presented. The characteristics of radiation pattern were investigated by the each different simulation for frequency offset change, radiation element space change and array number change.

Broadband Microstrip Reflectarray With Five Parallel Dipole Elements

A broadband microstrip reflectarray with five parallel dipole elements is proposed. Based on the operating mechanisms and the reflection phase characteristics of the five-dipole elements, the elements for the broadband reflectarray are designed. The radiation patterns of the five-dipole element reflectarray with the design frequency of f0 = 15 GHz are measured, and 1-dB gain bandwidth of 33.52% is achieved. The measured peak gain at f0 = 15 GHz is 31.4 dBi, which corresponds to the high aperture efficiency of 64.1%.

Near-Field Pattern of Large Aperture Higher Order Mode Generator Using Backpropagated Fields in Free Space

This paper reports the estimation of the electric field from a large aperture cavity that generates a higher order mode by backpropagation of fields measured in free space. A higher order mode of a TE6,2 mode at 94 GHz is designed and fabricated using a quasi-optical manner for the analysis. The backpropagating field at the aperture is compared with the measured field. The field estimated at the aperture is used to analyze the mode information, such as the amount of mode mixture of two different rotating fields and the phase difference between two modes. This paper provides a quantitative way to analyze the rotating cylindrical modes generated from an oversized cavity.

Design of attenuated horn antenna to receive high power pulse

In this paper, we propose the new attenuated horn antenna configuration to measure high power pulse more than several hundred MW to avoid the RF break down in a short distance. The proposed antenna configuration is more compact and flexible to change attenuation level according to measurement ranges in comparison with high power measurement using high power attenuator. We designed the attenuated horn antenna with attenuation value about -96dB at 3GHz and -83dB at 10GHz. The proposed attenuated horn antenna is composed of a horn antenna, ridged waveguide and attenuated rectangular waveguide. We designed three sections individually and after designing, we combined three sections. We simulated the final antenna configuration for the performance verification. The proposed antenna configuration is applicable to receive high power microwave pulse effectively.

Design of T/R module for the wideband active array antenna

In this paper, a wideband sub-array module which has 4-channel T/R modules was built and evaluated. The T/R module has 5-bit digital MMIC phase shifter, attenuator, amplifiers, Wilkinson power dividers, etc. Two absorptive matching equalizers to get uniform gain over multi-octave bandwidth are inserted in the common part of the module. Chip-on-substrate technology was introduced to get the repeatability for the packaging of MMIC chips. To eliminate the cavity resonance of the module housing, the absorbing material is attached on the housing. Two types of the sub-array modules were built for the interface with the planar triangular array lattice. These modules are attached to a cold plate to remove the heat of the power amplifiers in the module

Emission test of electron emitter using easy-replaceable-emitter test bench

The emission test is conducted using an easy-replaceable-emitter emitter test bench. A simple cylindrical button type emitter with the diameter of 6 mm is installed vertically into an emitter cup holder. The emitter is heated by a tungsten wire heater located around the cup holder. The emitter temperature is measured by an optical pyrometer at a given heater power. A high voltage pulse power supply gives the anode-cathode gap voltage up to 20 kV with the pulse width and repetition rate of 15 us and 50 Hz, respectively. The emitted current from the emitter surface is captured at a faraday cup and is measured using current transformer. The test bench is installed in the vacuum chamber with easy access door thus the emitter can be easily replaceable. The emitters with various combinations of tungsten grain structure and impregnations will be tested under the various emitter activation processes. The detailed test result for the emitter will be presented.

W-band microstrip reflectarray with double-cross element for bandwidth improvement

A broadband microstrip reflectarray is designed with the proposed element at W-band. The element consists of two cross-shaped resonators. Based on the analysis on the resonance mechanism of the double-cross element, the reflection phase of the element is controlled to achieve broadband characteristics by adjusting the dimensions of the element. Using the proposed element, a reflectarray with diameter of 50 mm is designed at f0 = 95 GHz. At f = f0, the peak gain of 31.73 dBi is achieved with aperture efficiency of 66.1%. Also, 1-dB gain bandwidth of 15.9% is achieved at W-band.

Cold testing of quasi-optical mode converters using a generator for non-rotating high-order gyrotron modes

In this paper, we test the performance of a quasi-optical, internal-gyrotron mode converter. When cold testing mode converters, a rotating higher-order mode is commonly used. However, this requires a nontrivial design and precise alignment. We thus propose a new technique for testing gyrotron mode converters by using a simple, non-rotating, higher-order mode generator. We demonstrate the feasibility of this technique for a W-band gyrotron quasi-optical mode converter by examining the excitation of a TE6,2 mode from a non-rotating mode generator. Our results demonstrate that this new cold-test scheme is an easy and efficient method for verifying the performance of quasi-optical mode converters.

Power estimation of electromagnetic coupling effectiveness by a X-band backward wave oscillator with mode conversion

Effective electromagnetic coupling with electronic equipment at radiation is used by a high power microwave (HPM) weapons source which means a relativistic backward wave oscillator in here. In the process of researching, developing and using HPM source, we have been studying a real “effective coupling” source which is able to attack the overall target, suitable in all situation ns. The experimental results show the broken electronic equipment or circuit damages after the radiation by an X-band relativistic backward wave oscillator (RBWO) of a HPM source with mode conversion. We used 500kA-5kA RBWO. The RF power is 0.5GW-30ns at 10GHz with mode conversion radiation from TM01 to TE11.

Pulse shortening by RF breakdown in relativistic backward wave oscillator

Relativistic backward wave oscillator (RBWO) is the suitable source for generating high power electromagnetic wave (HPEM). The RBWO has a high efficiency by means of the interaction region between a backward wave and an electron beam. However, the pulse shortening problem occurs frequently in experiments of RBWO because of the RF breakdown on rippled slow wave structure. Experimental study is done 500 MW RBWO in progress.