Patent GB-773699-A (GB)
Title: Improvements relating to high frequency impedance measuring devices
Inventor: LAEMMEL ARTHUR E
IPC: G01R27/04
Filed: 1955-02-22
Abstract: 773,699. Voltage impedance measurements. POLYTECHNIC INSTITUTE OF BROOKLYN. Feb. 22, 1955, No. 5336/55. Class 37. [Also in Group XL (b)] An impedance measuring device comprises a transmission line junction having an input arm supplying H.F. electromagnetic wave energy, a load arm connected to an unknown impedance, and a third arm comprising a circular waveguide section coupled at right angles at one end to the input and load arms, and dimensioned below cut-off at the operating frequency, there being provided a directional pick-up element within the circular guide at a location spaced from the junction which is rotatable about the guide axis to explore the field therein at different angular positions, and mode-filtering means being introduced between the pick-up element and the junction for eliminating all wave modes except those producing an elliptically polarized wave within the waveguide section .containing the pick-up element, In Figs. 2, 3 a coaxial input arm A1 is joined at right angles to a coaxial output arm A2 terminated in a load impedance of value Z and is collinear with a coaxial arm A3 terminated in a variable capacitance wherein a sleeve 9 connected to the outer conductor is axially adjustable over an enlarged end portion. of the inner conductor B3 by screw 10 with reference to scale 11. A measuring arm A4 connected to the junction at right angles to the other arms comprises a circular waveguide section dimensioned below cut-off for all operating frequencies, within and about the axis of which there is rotatable a conductive cylinder 13 carrying a dial 14 referred to a pointer 15; the cylinder containing a directional loop pick-up 12 and a detector 16 coupled to an external measuring instrument over lead 16a. The pick-up is positioned sufficiently far from the junction to predominately respond to H 11  modes while attenuating other modes, which are further-reduced by surrounding the loop by a conducting sleeve 18 whose open end is bridged by plural conductors 19 normally to the plane of the loop. The resulting field in the arm A4 is resolvable into two mutually perpendicular H 11  modes, and it is shown that, if the capacitance is adjusted to unit reactance normalized with reference to line A3, I is the load current and # denotes the position of the pick-up angularly referred to a reference plane, the rotational voltage variation pattern is given by Vr = I (jZr cos # - sin 0) plotted in Fig. 3 as a curve about the axis of arm A4, in which 2# radians = a wavelength. The load impedance Z is shown to be given by Z = (1-j p tan # min.), where. p is the voltage standing wave ratio Vmax./V min. and # min. is the angle of the probe for minimum voltage pick-up referred to the reference plane bisecting the angle between arms A1 and A2; the requisite parameters being determined by adjustment of the pick-up for minimum voltage with capacitance 9 set for unit reactance at the working frequency. The reference plane is determinable by shorting the load impedance and adjusting the pick-up for zero voltage. It is shown that by substituting a resistance of unit value normalized with reference to line A3 for the corresponding capacitance, the real and imaginary components of the load impedance are interchanged in the measured value thereof. Figs. 5, 6 show a modification wherein the input and load arms A1 and A2 are adjacent aligned sections of a rectangular waveguide feeding load 7; the circular measuring arm A4 being connected thereto at right angles through an aperture of a broad wall of the waveguide offset from the axis thereof by a distance dependent on the working frequency. A pick-up 12 associated with a rectifier is rotatable about the axis of the side-arm by dial 14 referred to pointer 15 as before, while the H 11  mode is propagated and all other modes attenuated by an insulating plug 20 of appropriate dimensions and dielectric constant inserted in arm A4 between the junction and the pick-up loop. Alternatively a series of plugs separated by air sections may be used to widen the bandwidth. It is shown that the voltage at the pick-up varies with the angle between the transverse plane of the waveguide through the side arm axis and the normal to the loop in the manner set forth above. The side arms may be mounted on a sliding section of the waveguide for adjustment of the offset, or alternatively a timing screw 21 may be threaded through the wall of the side arm in the reference plane for frequency adjustment. The measuring side arm is also applicable to a coaxial line (Figs. 7 and 8, not shown); one H 11  mode being excited by the inner conductor and the other by an adjustable probe extending through the outer conductor into the dielectric in the transverse plane of the line through the arm axia and at right angles thereto; the arm being offset as before. The input arm may be a coaxial line, and the output arm a waveguide, or vice versa (Fig. 9, not shown) while the measuring waveguide section A4 may be terminated in a rotary section 23 at its upper end, which is coupled symmetrically at right angles to one end of a rectangular waveguide 24; the pick-up element 25 intersecting the waveguide at its other end and parallel to the measuring section (Fig. 10). The pick-up element may be rotated (Fig. 11) by a spindle 26 carrying a transparent arm 28 which moves over a stationary circular Smith chart 27 (described in the literature referred to) bearing a series of curves representing the resistance and reactance components of the impedance to be determined. The arm 28 is longitudinally graduated in values of the voltage standing wave ratio, and carries a sliding cursor 30 set to the value determined as above, after which the pick-up is rotated to a voltage minimum position at which the impedance component values may be read off from the cursor. The pick-up may be arranged for axial as well as rotating movement in the measuring waveguide section A4 (Fig. 12), the operating spindle carrying an axial cylinder 31 inscribed with peripheral circles representing constant values of the reflection coefficient K of the impedance, and vertical lines representing constant values of the phase angles of K, together with intersecting curves representing values of resistance and reactance components of the impedance; all co-operating with a fixed crosswire 33. After rotating the pick-up to a minimum voltage position, it is moved axially of the measuring section until the line K = 0 registers with the crosswire, and the pick-up is again rotated until the former minimum voltage value is obtained. The value of the reflection coefficient and impedance component may then be read from the curves against the crosswire. In all of the above devices, the pick-up may be mechanically rotated at a constant speed in synchronism with the horizontal sweep of a cathode ray oscillograph vertically deflected by the pick-up voltage to continuously display the standing wave pattern.