Patent GB-744479-A (GB)
Title: Electrical voltage and/or current indicator for high frequency coaxial lines
Inventor: BIRD JAMES RAYMOND
IPC: G01R19/145
Filed: 1952-12-19
Abstract: 744,479. Bridge measurement of current voltage and energy in coaxial transmission lines. BIRD ELECTRONIC CORPORATION. Dec. 19, 1952, No. 32323/52. Class 37. [Also in Group XL (b)] Apparatus for measuring the current or voltage in either direction of a coaxial transmission line comprises a coaxial insertion section connected in the line comprising an outer and an inner tube, the latter containing two measuring circuits comprising back-to-back characteristic impedance terminations each with concentrically extending rectifier and load resistance and decoupling capacitance elements all within the bore of the inner tube and coupled through capacitance decoupled resistances extending radially of the bore of the outer tube to externally mounted galvanometers, each measuring circuit being equivalent to a bridge network sensitive to energy travelling in one direction only comprising three fixed resistances and the line characteristic impedance diagonally connected between the sending and receiving ends ; the remaining diagonal containing a rectifier and the measuring galvanometer. Such a section for measuring the line current and/or voltage, or the forward and reverse energy, comprises (Figs. 1, 4) an outer metal tube 1 jointed at 3 to metal end sections 4 terminated in the threaded tubular connectors 7 enclosing cylindrical insulators 23 and central conducting rods 22, and sealed by locating ring 24 and sealing ring 25 against the oil or gas dielectric filling the section. The inner conductor comprises a metal tube section 10 centrally supported by a perforated dielectric ring 11 carried by tube 1, symmetrically engaging hollow metal sections 18 which have conical ends socketed to receive plug 21 of conducting rods 22. Within the slotted resilient ends of the central section 10 two minor coaxial elements 31 are opposedly inserted each comprising a metal tube 32, characteristically terminated in an impedance substantially the same as that of the main coaxial by a tapered end portion 33 curved in accordance with Specification 641,607, co-operating with a central resistor for reflectionless termination, which is spacedly supported within the section 10 by annular shoulder 35 in opposition to the outer diameter thereof. Within the open end of each shoulder 35 is fitted a shouldered tubular insulator 17 whose other end is engaged by a flanged metal sleeve 38 embraced by the slotted resilient end of the corresponding section 18. The composite inner conductor has the same outer diameter throughout, and the peripheral surfaces of the insulators 17 are coated with resistive films constituting series resistance SRf and SRr which are connected to the adjacent metal flanges. The split resilient tapered ends of the coaxial elements 31 grip and are connected to central carbon film resistors TRf and TRr (which may be of the kind described in Specification 702,357) whose free ends are connected to metal rods 44. The latter are connected to carbon film resistors BRf and BRr which respectively extend through apertures in elements 31 and section 10 radially of the main coaxial section so that their free ends may be adjustably engaged by resilient fingers 76 of tubular metal screws 77 in the threaded bores of bosses 78 extending radially of the tube 1 and closed by caps 97. Each rod 44 is also connected to one pole of a germanium rectifier cartridge 48 inserted in a dielectric sleeve within insulator 17, whose other pole is soldered to the centre electrode of a button load capacitor 56 whose outer ring 57 is soldered within sleeve 38. A load resistor 54 terminating in a collar 60 is held within a dielectric sleeve 65 by a spring 61 to connect the centre of each load capacitor through a filter circuit, comprising button capacitors 62, 63 soldered within sections 18 and resistor 55 connected therebetween to a metal rod 67 seated within a dielectric bush 66 inserted within the conical part of each section. A carbon film resistor 80 having a metal end engaging a hole in each of the rods 67 extends at right-angles thereto through apertures in the bush 66 and the element 18 and radially of the main coaxial to engage its resistive surface with the annular slidable resilient contact fingers of a cup 83 carried by a guide-rod 84 extending through a dielectric cup 88 within a clearance bored bush 85 threaded within a boss 86 of the end section 4. The guide rod 84 passes through the centre eyelet of a button decoupling capacitor 90 whose outer ring is clamped between a metal washer 91 over a sealing ring 93, and a closure sleeve threaded into the boss 86, which contains an insulating sleeve 94 supporting a tubular contact element 95 soldered to the guide rod, which is itself soldered to the centre of capacitor 90 when the value of resistor 83 has been correctly adjusted. All non-variable contacts which are not soldered are consolidated by bands of silver paint and oil or gas dielectric is charged into the section through plugs 97, while galvanometers Gf and Gr are connected between contacts 95 and the outer tube 1. In operation (Fig. 6) the insertion section is connected between a signal generator and a termination network 100 of the kind described in U.S.A. Specification 2,556,642, through coaxial changeover switches enabling high-frequency energy to be transmitted through the symmetrical section either from left to right or from right to left, and each galvanometer is connected in series with a calibration resistor 106 and a compensating resistor 105 which may be switched out when the termination network 100 affords no D.C. path, to compensate for the increased D.C. resistance of the galvanometer circuits due to the inclusion of resistors TR and BR therein. When H.F. energy travels from left to right from signal generator to characteristic termination network, the portion of the section to the right of the axis of symmetry 29 effectively presents to the incoming energy the equivalent bridge circuit shown in Fig. 7, in which arm b<SP>1</SP>, c<SP>1</SP> is the characteristic impedance Zo, b<SP>1</SP>, e<SP>1</SP> is the resistor BRr, e<SP>1</SP> f<SP>1</SP> is the resistor TRr and c<SP>1</SP> f<SP>1</SP> is the resistor SRr, and by design TRr=Zo so that Zo/SRr = BRr/TRr for balance, and BRr is adjustable by screw 77 for zero reading of the galvanometer Gr. Interchange of the signal generator and characteristic termination network presents a similar bridge circuit comprising Zo, BRf, TRf and SRf to energy travelling along the line from right to left across the axis of symmetry, and BRf may be balanced for zero reading of galvanometer Gf by screw 77 as above. Following this preliminary adjustment the section may be inserted in a cable within which the H.F. energy is to be measured, and a wave travelling from left to right and entering the section encounters the effective circuit shown in Fig. 8 in which a H.F. voltage appears across series resistor SRf proportional to the line current and is vectorially combined with the fraction across resistor TRf of the voltage appearing across the main coaxial conductors and the potential divider comprising resistors BRf and TRf. The combined voltage is rectified by rectifier 48 and condenser 56 to a D.C. which deflects the galvanometer Gf to give an accurate measurement of the energy in the line from left to right provided that resistor BRf has been previously adjusted as described to give a zero indication for power flowing from right to left in the line. If the line is imperfectly terminated so that reflected energy flows from right to left, it encounters a circuit similar to Fig. 8 but comprising resistors SRr and BRr and TRr which deflects galvanometer Gr to indicate the reflected energy, and a bridge circuit similar to Fig. 7 but comprising resistor BRf, SRf and TRf such that galvanometer Gf gives zero indication. The net energy passing over the line may then be determined. The deflection of galvanometer Gf (or Gr) may be caused to measure line current by omitting BRf (or BRr) and to measure current by replacing the series resistor SRf (or SRr) by a low resistance conductive member. The apparatus may be inserted in the antenna lead of a transmitter, and its output may be applied to shut down the latter in the event of antenna damage.