A modified compact antipodal Vivaldi antenna is proposed with good performance for different applications including microwave and millimeter wave imaging. A step-bystep procedure is applied in this design including conventional antipodal Vivaldi antenna (AVA), AVA with a periodic slit edge and AVA with a trapezoid-shaped dielectric lens to feature performances including wide bandwidth, small size, high gain, front-to-back ratio and directivity, modification on E-plane beam tilt, and small side lobe levels. By adding periodic slit edge at the outer brim of the antenna radiators, lower end limitation of the conventional AVA extended twice without changing the overall dimensions of the antenna. The optimized antenna is fabricated and tested and the results show that S 11 < -10 dB frequency band is from 3.4 GHz-40 GHz and it is in good agreement with simulation one. Gain of the antenna has been elevated by the periodic slit edge and the trapezoid dielectric lens at lower frequencies up to 8 dB and at higher frequencies up to 15 dB, respectively. The E-plane beam tilts and side lobe levels are reduced by the lens.Index Terms-Antipodal Vivaldi antenna (AVA), tapered slot antenna (TSA), compact wideband antenna, dielectric lens.
A B S T R A C T Non-destructive detection and evaluation of stress-induced fatigue cracks in metals is an important practical issue in several critical environments including surface transportation (steel bridges, railroad tracks, railroad car wheels, etc.), aerospace transportation (aircraft fuselage, landing gears, etc.) and power plants (steam generator tubings, etc.). Although there are several standard non-destructive evaluation techniques, near-field microwave and millimetre wave techniques have shown tremendous potential for significantly adding to the available non-destructive 'toolbox' for this purpose. This paper serves as a review of recent advances made in this area and the capabilities of these techniques for detecting cracks and evaluating their various dimensional properties including determining a crack tip location accurately. These techniques include using open-ended rectangular probes (in two distinct modes) and open-ended coaxial probes. a = broad dimension of the open-ended rectangular waveguide aperture b = narrow dimension of the open-ended rectangular waveguide aperture D = crack depth dc = direct current DM = dominant mode GHz = gigahertz h = distance between the crack and the coaxial centre HOM = higher order mode k = ratio of the distance between the probe and the waveguide wall and narrow dimension (height) of the open-ended rectangular waveguide aperture l = distance between a detector and a waveguide aperture NDE = non-destructive evaluation TE = transverse electrical TEM = transverse electric and magnetic TM = transverse magnetic 2D = two dimensional r i = radius of an internal conductor of a coaxial probe r o = radius of an external conductor of a coaxial probe W = crack width T = the distance between the probe and the waveguide wall δ = location of the crack within waveguide aperture I N T R O D U C T I O N Metal fatigue or failure usually begins from the surface. Aircraft fuselage, nuclear power plant steam generator tubings and steel bridges are examples of environments in which this type of metal failure occurs. Hence, fatigue and Correspondence: R. Zoughi.stress crack detection on metallic structures is of utmost importance to the on-line and in-service inspections of critical metallic components. Currently, there are several prominent non-destructive evaluation (NDE) techniques for detecting surface cracks in metals. Acoustic emission testing, dye penetrant testing, eddy current testing, ultrasonic testing, radiographic testing and magnetic particle testing are examples of the techniques. 1 However, each
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