A Simple and Accurate Analysis of Conductivity Loss in Millimeter-Wave Helical Slow-Wave Structures
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A Simple and Accurate Analysis of Conductivity Loss in Millimeter-Wave Helical Slow-Wave Structures S. K. Datta & Lalit Kumar & B. N. Basu
Received: 8 September 2008 / Accepted: 4 December 2008 / Published online: 19 December 2008 # Springer Science + Business Media, LLC 2008
Abstract Electromagnetic field analysis of a helix slow-wave structure was carried out and a closed form expression was derived for the inductance per unit length of the transmission-line equivalent circuit of the structure, taking into account the actual helix tape dimensions and surface current on the helix over the actual metallic area of the tape. The expression of the inductance per unit length, thus obtained, was used for estimating the increment in the inductance per unit length caused due to penetration of the magnetic flux into the conducting surfaces following Wheeler’s incremental inductance rule, which was subsequently interpreted for the attenuation constant of the propagating structure. The analysis was computationally simple and accurate, and accrues the accuracy of 3D electromagnetic analysis by allowing the use of dispersion characteristics obtainable from any standard electromagnetic modeling. The approach was benchmarked against measurement for two practical structures, and excellent agreement was observed. The analysis was subsequently applied to demonstrate the effects of conductivity on the attenuation constant of a typical broadband millimeter-wave helical slow-wave structure with respect to helix materials and copper plating on the helix, surface finish of the helix, dielectric loading effect and effect of high temperature operation – a comparative study of various such aspects are covered. Keywords Conductivity loss . Circuit loss . Circuit attenuation . Helix slow-wave structure (SWS) . Helix traveling-wave tube (TWT) . Wheeler’s incremental inductance rule. Abbreviations mm-wave Millimeter-wave SWS Slow-wave structure TWT Traveling-wave tube S. K. Datta (*) : L. Kumar Microwave Tube Research and Development Centre, Defence Research and Development Organization, Bharat Electronics Complex, Jalahalli Post, Bangalore 560013, India e-mail: [email protected] B. N. Basu College of Engineering and Technology, Lodhipur, Rajput Road, Moradabad 244001, India e-mail: [email protected]
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J Infrared Milli Terahz Waves (2009) 30:381–392
1 Introduction The helix continues to be considered as the best choice as the slow-wave structure (SWS) for wideband electronic warfare as well as high efficiency space-qualified traveling-wave tubes (TWTs). A typical helical slow-wave structure consists of a tungsten tape-helix supported inside a metal envelope by three dielectric support-rods placed at azimuthal symmetry (Fig. 1). The construction of the structure allows control of dispersion characteristics by anisotropic and/or inhomogeneous loading of the helix, thereby making it possible to maintain wideband interaction between the electron beam and RF waves thus making the structure suitable for wideband TWTs. On the other hand, in
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