Transmitting valves for short waves

The use of high-power valves on wave-lengths below 60m. brought in its train a new series of troubles which have had to be overcome by the valve designer. The main difficulties are: heating effects caused by h.f. losses in dielectrics, heating effects caused by the very large h.f. currents encountered, external insulation difficulties due to the ease with which air breaks down at very high frequencies, the requirement of low electrode capacity. The first difficulty, which is the most serious, has been overcome by the use of shields, or screens, which are electrically connected to the anode and so disposed as to protect the dielectric at its most vulnerable points. In a similar way the grid and filament may be fitted so as to reduce the intensity of the field where it cuts the glass. The latest form of valve is of the double-ended type, with the anode in the middle and the filament and grid sealed in from opposite ends. In this type the water jacket is made self-contained, which is found to be generally more satisfactory, especially since the cooling water for short wave transmitters is circulated on the closed-circuit system and, hence, the anode does not become appreciably coated with deposit. The article concludes with a table of the valves at present manufactured which consist of four having maximum power outputs of 1, 1.5, 10 and 15kW respectively.