Patent GB-566843-A (GB)
Title: Improvements in and relating to ultra high frequency impedance measuring apparatus
IPC: G01R27/02
Filed: 1943-09-09
Abstract: 566,843. Impedance measurements. BRITISH THOMSON-HOUSTON CO., Ltd. Sept. 9, 1943, No. 14792. Convention date, Sept. 15, 1942. [Class 37] Apparatus for measuring an ultra-high frequency impedance comprises a rod and two tubes concentrically arranged, the tubes comprising a first transmission line and the inner tube and the rod comprising a second transmission line, an ultra-high frequency source of voltage arranged to supply the first transmission line at one end, means for measuring the voltages at the remote end of the line, and at a point one-quarter wavelength from the remote end, means for connecting the second line in series with a known impedance across the remote end of the first transmission line and means for varying the length of the second transmission line. As shown in Fig. 1, a transmission line 11 is supplied at one end from an ultra-high frequency source 10, and has a voltmeter V2 at its other end. A voltmeter V1 is placed one-quarter of a wavelength from V2 by adjusting the position of V1 until the V2 ratio - is a maximum. In this condition, V1 V2 the ratio -- is proportional to any impedance V1 connected across V2. Accordingly, there is now connected across V2 the impedance to be measured, ZX in series with a transmission line 12, the length of which can be adjusted by means of a movable short-circuiting member 13. The member 13 is adjusted to give a minimum V2 value of -- corresponding to a minimum value V1 of the effective impedance across V2, which occurs when the reactive component of ZX has been neutralized by the reactance of the line 12. This reactance is determined from the dimensions of the line 12 and hence the reactance of ZX is determined. The slide 13 is then moved to a non-resonant condition and V2 the new ratio -- is determined. The ratio V1 V2 between the two values of -- is the same as V1 the ratio between the corresponding impedances and, since the reactance of ZX has been determined and the two values of reactance of line 12 can be calculated, the resistive impedance of ZX can be determined. Fig. 2 shows a practical construction of the apparatus. Concentric tubes 15, 16 correspond to line 11, and tube 15 and an inner rod 14 correspond to line 12. A valve voltmeter V1 is connected to concentric brushes 21, 22 movable by a member 23 having a scale 25 so as to be one-quarter wavelength from the lower end across which the voltmeter V2 is connected. Energy from the high frequency source 10 is introduced through coupling members 17, 18. The impedance ZX is connected across the lower ends of 14 and 16 and the movable short-circuiting member 13 for conductors 14, 15 is adjusted by member 19 co-operating with a scale 20. The voltmeters V1, V2 are of the valve rectifier type, having screened leads 27, 28 brought out to sensitive meters having adjacent scales 29, 30. Conductor 16 is slotted at 31 to allow the passage of lead 27. In practice, the value of V2/V1 is read for a number of positions of bridge 13 and a resonance curve is plotted from which the values at resonance are read to give an exact determination of the reactance of ZX. Then, using this value, a number of the other readings are used to give a mean value of the resistive component.