GB859119A - Radio frequency networks - Google Patents

Abstract

859,119. High-frequency bridge networks. ALFORD, A. May 17, 1957, No. 15694/57. Class 40(8). [Also in Groups XXXVI and XL(c)] A high-frequency hybrid circuit comprises a U-shaped tubular conductor 48 approximately one half-wavelength long carrying terminals I, V, II, A and B respectively at the ends of the limbs, at the bight of the U and half way along the limbs, the hollow space within the conductor accommodating a second conductor 4 which extends from terminal I to an intermediate point along the corresponding limb where it is brought through a hole in this limb and either directly connected or coupled to the second limb. As shown, the conductor 4 is continued for a short distance as an inner conductor within the second limb. In the region of the cross-over the conductor 4 may be surrounded by tubular screens 9, 10 which are connected to the U-shaped outer conductor. The outer conductors of the terminals II, A and B are (as shown) provided by a tubular casing 1 which surrounds the whole of the U-shaped conductor 48. The limbs of the U may be separated by a longitudinal metal rib 17 connected to the casing. In a modification (Figs. 6, 6a not shown), the circuit is realized using striplines. Two superimposed layers of insulation are used, the conductor 4 and the U-shaped conductor 48 being arranged as conductive strips respectively on either side of one of the layers. The outermost side of the other layer carries a ground-plane corresponding to the casing 1. If an input is connected to terminal I, a minimum output is derived at terminal II when equal loads are connected to terminals A and B. The output at II increases when there is a difference in either the resistances or reactances of the two loads. The circuit may therefore be used to adjust the impedances of a series of load units to a given standard. When used as an impedance measuring bridge, a standard resistive load is connected to terminal A and the unknown impedance to terminal B, the amplitude and phase of the reflection coeffiicent (referred to the standard load) of the impedance being determined by measuring the signal at terminal II. In order to provide a reference signal, the standard load 22, Fig. 2, is in the form of a voltage divider which diverts a small amount of the power at terminal A of hybrid network 21 into an auxiliary load 24. Amplitude comparisons may then be made by connecting a receiver in turn to terminal II and across the load 24. For phase comparison, a slotted line 23 is connected between the standard load 22 and the termination 24, a small proportion of the power in the slotted line being picked up by a sliding probe 25 and coupled to terminal I of a hybrid network 27 similar to the network of Fig. 4. Terminal A<SP>1</SP> of network 27 is connected to a matching resistor R27 and terminals II of networks 21, 27 are connected together. Terminal B<SP>1</SP> is connected through a resistance pad 28 to a receiver 30A and meter 40. The latter indicates the sum of the voltages from the slider 25 and terminal II of hybrid network 21. For a given position of the slider the voltages are out of phase and a minimum signal is observed. The relative phase of the voltages may then be found by taking in account the phase delays in the various connections. In a further method, a reactance having a known reflection coefficient is connected in place of the unknown impedance. The positions of the slider 25 giving minimum output for the known reactance and the unknown impedance give a direct measure of the unknown phase reflection coefficient. The standard reactance may take the form of a short length of coaxial transmission line 31, 32, Fig. 3, terminated by a matched load 33. An iris 34 which may be directly connected to either the outer conductor (as shown) or the inner conductor provides a reactance whose reflection coefficient may be accurately computed. The reactance may be provided by a notch in the inner conductor instead of an iris. The reference signal may be derived from terminal V instead of from terminal A through the voltage divider 22. The inner conductor 41 of terminal V extends within the limb of the U-shaped conductor 48 to a position opposite terminal A where it is brought through a hole in this limb and connected to the opposite limb. When the modified bridge is used in the circuit of Fig. 2, the slotted line 23 is connected to terminal V. If the hybrid network is designed for lower frequencies, e.g. about 50 mc/s., the length of the structure may be reduced by bending it back along itself.