As an example, the helix-pitch proﬁle of the considered tube was modiﬁed for improving the yield (Table 1), maintaining sim- ilar small-signal gain (62.6-dB average gain and 5% ﬂatness). In particularly, according to the previously exposed theory, the helix pitch in the lower helix-pitch section (section 4) was incremented to 0.064 cm. The yield for the original and improved tubes was compared for both the cases of random (independent random variation of the helix pitch of each section) error and systematic (same helix-pitch variation for all the sections) error [5]. A max- imum helix-pitch tolerance of 3% was considered. Three hundred different tubes were simulated for each value of helix-pitch error. For the purpose of this study, a tube was considered acceptable if the average gain was included in the 1.5% range with respect to the nominal value and the ﬂatness was lower than 6%. A remark- able yield improvement is obtained for both random (Fig. 2) and systematic (Fig. 3) errors. The small histograms included in the ﬁgures show that the percentage increment of the yield obtained for the different helix-pitch error values is, in some cases, higher than 25%. CONCLUSION TWT yield degradation in case of small-signal gain, caused by the tube sections with lower helix pitch, has been studied. The analysis of the small-signal gain per unit length of a helix slow-wave structure provides useful information for predicting the yield be- haviour. It was demonstrated that a suitable adjustment of the helix-pitch proﬁle, hence maintaining similar small-signal gain performance, signiﬁcantly improves the yield. REFERENCES 1. J.X. Qiu, D.K. Abe, T.M. Antonsen Jr., B.G. Danly, and B. Levush, Traveling-wave tube ampliﬁer performance evaluation and design op- timization for applications in digital communications with multilevel modulations, IEEE Trans Microwave Theory Tech 51 (2003), 1911– 1919. 2. D.K. Abe, B. Levush, T.M. Antonsen Jr., D.R. Whaley, and B.G. Danly, Design of a linear C-band helix TWT for digital communications experiments using the Christine suite of large-signal codes, IEEE Trans Plasma Sci 30 (2002), 1053–1062. 3. D.M. Goebel, R.R. Liou, W.L. Menninger, X. Zhai, and E.A. Adler, Development of linear traveling wave tubes for telecommunications applications, IEEE Trans Electron Devices 48 (2001), 74 – 81. 4. V. Srivastava, R. Carter, B. Ravinder, A.K. Sinha, and S.N. Joshi, Design of helix slow-wave structures for high efﬁciency TWTs, IEEE Trans Electron Devices 47 (2000), 2438 –2442. 5. D.M. Goebel, A.C. Schneider, W.L. Menninger, and J.M. Weekly, Gain increase through the of life in traveling wave tubes, IEEE Trans Elec- tron Devices 50 (2003), 1117–1124. 6. S. D’Agostino and C. Paoloni, A study on helix pitch tolerance impact on TWT small-signal gain, IEEE Electron Device Lett 23 (2002), 746 –748. 7. S. D’Agostino, F. Emma, and C. Paoloni, Accurate analysis of helix slow wave structures, IEEE Trans Electron Devices 45 (1998), 1605– 1613. 8. S. D’Agostino, F. Emma, and C. Paoloni, Sensitivity analysis of TWT’s small signal gain based on the effect of rod shape and dimensions, IEEE Trans Electron Devices 47 (2000), 1457–1462. 9. J.R. Pierce, Traveling wave tubes, Van Nostrand, New York, 1950. © 2005 Wiley Periodicals, Inc. EXPLORATION OF THE INTEGRATION OF A PASSIVE COPLANAR ISOLATOR BASED ON THIN MAGNETIC FILMS S. Capraro, 1,2 T. Rouiller, 1 M. Le Berre, 3 J. P. Chatelon, 1 B. Bayard, 1 D. Barbier, 3 and J. J. Rousseau 1 1 DIOM University of Saint-Etienne 23 rue Michelon 42023 Saint-Etienne cedex, France 2 LAHC University of Savoie Bat. Le Chablais 73376 Le Bourget du Lac, France 3 LPM UMR 5511, INSA Lyon 7 av. Jean Capelle 69621 Villeurbanne cedex, France Received 14 March 2005 ABSTRACT: In order to miniaturize and self-bias a coplanar isolator, new method is to use thin magnetic ﬁlms. The results presented in this paper show that this component behaves like an isolator in the 50-GHz band. The inﬂuence of the magnetic and conductor ﬁlms for increasing the isolator properties is studied. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 435– 437, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop. 21009 Key words: coplanar isolator; barium ferrite; thin ﬁlms; passive com- ponent 1. INTRODUCTION Microwave ferrite devices allow the control of microwave propa- gation using a static or switchable DC magnetic ﬁeld. The devices can be reciprocal or nonreciprocal, linear or nonlinear, and their development requires an understanding of magnetic materials, electromagnetic theory, and microwave-circuit theory. It appears that these devices will continue to play an important role in microwave technology in future years. Nowadays, these components use bulk materials which are not compatible with the technology of semiconductors. Thus, the achievement of thin ﬁlms can be a major step in the miniaturiza- tion of such electronic devices [1]. Figure 3 Comparison between the yield (percentage of acceptable tubes) of the TWT computed for the original and the improved helix-pitch proﬁle for the case of systematic error (the inset shows the yield-improvement histograms) MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 46, No. 5, September 5 2005 435