Die vorliegende Arbeit soll einen ]3eitrag zur Theorie der Spin-Gitterrelaxafion in Molektilkristalten rnit Quadrupolkopplung liefern. Es wird versucht, die Megergebnisse yon. DEHMELT und I~Rf3GER1, 2 am Dichlor/~thylen (CHCI=CHC1) theoretisch zu deuten. Die schwache Ternperaturabh/~ngigkeit der Kernquadrupolresonanzfrequenz kann erklgrt werden, wenn man die Molektile irn Gitterverband als quantenrneehanische harrnonische Oszillatoren auffal3t, die Torsionsschwingungen ausftihren und deren Amplitude ternperaturabhgngig ist. Dabei ist zu bedenken, dab der Quadrupolkern fiber den elektrischen Feldgradiellten der hom6opolaren Bindungselektronen energetisch rnit dern iibrigen Molekfil gekoppelt ist. Dutch diese Kopptung lassen sich auch die beobachteten kurzen Spin-Gitterrelaxationszeiten t 1 erkl~ren, wenn man die Dichte des FouRInR-Spektrums der Molekiilbewegung irn Bereich der Quadrupolresonanzfrequenz beriicksichtigt. Theoretisch wird gezeigt, dab bei Zugrundelegung der MeBergebnisse sich eine Molekfiltorsionsfrequenz ergibt, die in gleicher Gr6Benordnung liegt, wie die dutch RaMAs-Untersuchungen an an~eren Molekfilkristallen gefundenen Werte. Die theoretisch errnittelten Grenzen ffir die Spin-Gitterrelaxationszeit t I schlieBen den Megwert in sich ein. Experiment und Theorie konnten damit in befriedigende {3bereinstimrnung gebracht werden.Einleitung.
After a short review on the different attenuation measuring techniques, some special points of view in the application of the power method are described. By employing exactly measured standard attenuators in cascade, the modified measuring equipment can be used over a range of more than 100dB. An error analysis is given considering all essential sources of systematical errors the limits of which are calculated.It is shown that the systematical error of the system described is less than 1 10-4 dB for very small attenuation values. It is in the order of ISAI = 1 10-4 (1 + A ) dB over the whole range, when A is the numerical value of the attenuation (in dB) being measured.
Some examples are given of mechanically and electrically stable attenuation transfer standards, developed or assembled for the international intercomparisons GT-RF 75-A 3 and 78-1 proposed by the radio frequency working group of the CCE. A 4-port directional coupler with matched loads terminating the ports of the side arm and a switchable short interrupting the power flow in the side arm between two coupling sections was constructed for 0.01 dB in waveguide R 140 at 15 GHz. At frequencies of 8 GHz < v < 18 GHz in coaxial systems, switchable transfer standards consisting of commercially-available components incorporating a waveguide directional coupler and two isolators are the most stable and have the best repeatability. For comparing 100 dB at 30 MHz, two transfer standards applying the WBCO principles (using an R 100 rectangular waveguide) were developed, one of them for insertion into and removal from the line, the other fitted with a coaxial switch of optimal repeatability for remaining in line during the measurement.The total uncertainty of these standards is < 0.01 dB at 100 dB.
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