Variable attenuators

796,238. Transmission line attenuators. STANDARD TELEPHONES & CABLES, Ltd. April 27, 1956 [May 2, 1955], No. 12976/56. Class 40 (8). [Also in Group XL (c)] An attenuator for a strip transmission line comprises an attenuating card lying adjacent the line and associated with means for distorting it. The line comprises a narrow planar conductor 19 and a wide planar conductor 21 separated by a solid dielectric layer 20. The card 22 is wider than the conductor 19 and is shaped as shown in Fig. 2 to cause minimum reflection in the line. The card is slightly flexible and may be made of a plate of phenol formaldehyde coated with a thin film of graphite. To increase the attenuation the edges of the card are urged towards the wide conductor 21 by means of pairs of thumbscrews 24 which act through a pressure plate 23 made of lossy dielectric material. Throughout a certain range the attenuation is proportional to the pressure. In a modification (Fig. 4), the card 22 is supported near its longitudinal edges by elongated lossy dielectric members 25. Pressure is applied through a shaft 27 and a lossy pressure plate 26. A pointer 30 driven by a rack and pinion gear indicates the degree of attenuation. In a further modification (Fig. 5, not shown), for producing amplitude modulation of the transmitted energy the pressure plate is made of magnetic material and is subject to the field of an electromagnet whose windings carry a signal current. Specification 704,050 is referred to.