Patent GB-581167-A (GB)
Title: Improvements in or relating to high frequency electric circuit arrangements
IPC: H01P1/24
Filed: 1941-05-05
Abstract: 581,167. Combined transmitting and receiving apparatus; coil-less resonators. CORK, E. C., and BLUMLEIN, A. D. May 5, 1941, No. 5807. [Classes 40 (iii) and 40 (v)] [Also in Group XXXVI] A circuit suitable for the measurement or comparison of impedances at high-frequencies is arranged so as to avoid undesirable radiation or coupling effects. A coaxial feeder has an internal conductor 1, Fig. 1 (Prov.), and a surrounding screen 2 which branches at a point 3 into portions 4, 5, the internal conductor 1 continuing within the portion 4 and making contact with the upper end of the portion 5 at the point 6. Alternatively, the conductor 1 may emerge from the portion 4 at an intermediate point and make contact with a corresponding point on the portion 5. A cylindrical sheath 7 surrounds the coaxial feeder and is closed at its lower end by a conducting ring 8 which is joined to the screen 2. The upper end of the sheath 7 divides into two side tubes 9, 10 and forms with conductors 11, 12 connected to the upper ends of the portions 4, 5 of the screen 2 coaxial feeders, to the outer ends of which impedances 13, 14 to be compared or balanced are connected. A source of highfrequency oscillations 15 is connected between the conductor 1 and the screen 2 outside the sheath 7, and a high-frequency detector 16, such as a diode, is connected between the point 3 and an adjacent point on the sheath 7. The detector may, alternatively, be outside the sheath 7 and connected to the point 3 and sheath 7 over a coaxial feeder. Each of the portions 4, 5 may be a quarter wave-length long, high-frequency voltages of large amplitude and opposite phase then being developed at their upper ends so that these portions operate as high impedance bridge ratio arms. The distance between the point 3 and the ring 8 may be a quarter wave-length so that a high impedance is presented at the point 3. An unknown impedance 13 and an adjustable impedance 14 form the other two arms of the bridge. The source 15 and detector 16 may be interchanged. A suitable adjustable impedance for use as the impedance 14 is shown in Fig. 1 (Comp.), and comprises a variable resistance element in the form of a terminated coaxial line and a variable reactance element also in the form of a terminated coaxial line. The variable resistance consists of the coaxial line comprising the cylindrical conductor 21 having an internal conductor 22 supported centrally within it. The ends of the conductor 22 are secured to, but insulated from, conducting pistons 23, 24 slidable within the conductor 21. The ends of the conductor 22 are connected to the pistons 23, 24 by resistances 25, 26, which may either be of the grid leak type or lengths of coaxial cable pf sufficiently high attenuation to avoid reflections and therefore acting as non - inductive resistances. The variable reactance comprises a further cylindrical conductor 27 branching from the conductor 21 and having a central conductor 28 supported within it. A conducting piston 29 slides on the conductor 28 so as to form a low impedance connection between the conductors 27, 28. The conductor 28 is slidably connected to the conductor 22. A cylindrical conductor 30 and central conductor 31 are connected to the conductors 10, 12, Fig. 1 (Prov.), for effecting connection to the common point of the conductors 22, 28. The resistive impedance at this point is varied by sliding the conductor 22 and the pistons 23, 24 inside the conductor 21. The reactive impedance is varied by sliding the piston 29 within the tube 27. Alternatively, the pistons 23, 24 may be fixed within the conductor 21 and the conductors 27, 30, 28, 31 may be slidable along the conductor 22. The conductor 27 may be cranked so as to have its axis parallel to the axis of the conductor 22. An aerial may be adjusted to match a feeder by coupling one end of the feeder to the aerial and the other end in place of the impedance 13. The impedance 14 required to balance the bridge is noted and the aerial adjustment modified until the bridge balances with a purely resistive impedance 14, showing an absence of reflection from the aerial. For neutralizing couplings between high-frequency circuits, for example between -transmitter and receiver in pulse reflection systems for detecting aircraft, the transmitter may be connected in place of the generator 15, the receiver in place of the detector 16, and the aerial in place of the impedance, 13, the impedance 14 being equal to that of the aerial.