Patent GB-989566-A (GB)
Title: Improvements in or relating to radio-frequency guards
IPC: H04B1/10
Filed: 1962-12-28
Abstract: 989,566. Radio - frequency attenuators. MINISTER OF AVIATION. Dec. 28, 1962 [Dec. 29, 1961], No. 48783/62. Heading H1W. A guard for preventing damage to electronic circuits, particularly electro-explosive devices, from spurious high-frequency signals, comprises a supply line, e.g. a copper wire, surrounded by a highly conductive shield and a further inner shield in the form of an elongated metal member connected electrically between the line and the outer shield and offering sufficiently high D.C. shunt resistance to enable transmission of supply currents. As shown in Fig. 4, the supply wire 62 is surrounded by a shield 61 in the form of a conductive tube. The wire is connected at 65 to the closed end of an inner shield 64 whose open end is connected by means of a highly conductive disc 67 to the outer shield 61. The inner shield is made of pure iron and its dimensions are such that the shunt conductance offered to the line is insufficient to interfere with normal power at frequencies up to e.g. 400 c/s. passed along the line. The attenuation offered to high frequencies is approximately equal to the real part of the propagation function of the inner shield which is given (approximately) by (1+j)##Á#/2, where Á=initial permeability, #=conductivity and #=2#f.Iron has the highest Á#product up to 10<SP>10</SP> c/s. and up to this frequency the low-pass characteristic of the inner shield is a simple function of frequency. Attenuation is associated with a loss wave, the penetration of which into the metal is determined by the skin depth # =#2/#Á#, the energy of the wave being dissipated in the form of heat. It is necessary therefore to interpose a heat shield between the guard and the load or to place the guard in the supply line at some distance from the load. In both cases, the outer shield extends continuously around both the guard and the load. Where a balanced two-wire system is used, separate guards sharing the same outer shield may be associated with each wire. Alternatively, in cases where the guard is connected directly to the load, one wire may be attached to the casing of the guard. The construction shown in Fig. 4 may be modified by corrugating the tubing, by replacing it with a corrugated disc 104, Fig. 11 (the corrugations increasing the path length through the inner shield) or by using a number of concentric tubes 114, Fig. 12, connected to each other alternately at either end. In a further modification the tube is closed and connected to the supply wire at both ends, the centre of the tube being connected to the outer shield by a conductive annulus extending between the tube and the shield. In an embodiment of this type shown in Fig. 2, the conductor 32 and iron shield 34 form a coiled concentric line 36. The inner and outer conductors are insulated from each other except at points 75, 76 where outer conductor is sealed off and connected to the inner conductor by solder. Equal parts of the coil lie on either side of a central conductive disc 26, the concentric line being passed through apertures in this disc and in two conductive end discs 33. The outer conductor is insulated from the end discs but electrically connected to the central disc. The construction is completed by two shielding caps 28 (upper one only shown) of silver-plated brass which conductively engage the three discs. A low-loss capacitor 69, Fig. 4, may be connected across the input to the guard, the capacitor having such a value that an antiresonance frequency occurs within the passband.