Tips and Jottings

Radio Record, Volume II, Issue 13, 23 November 1928, Page 27

Tips and Jottings

Aerial Wire and Oxidisation. ANY people are under the impression that a brand-new aerial wire is more efficient than one which has been in use for some time. This impression is the result of the popular belief that the coating of oxide which forms on copper or brass wires when exposed to the air increases the resistance of such tarnished wire to highfrequency currents.. Exhaustive tests, however, have proved that the theory is to all intents and purposes a fallacy. It is perfectly true that aerials do sometimes lose their pristine efficiency, but this falling-off in energy can generally be traced to an accumulation of dirt on the insulators, corrosion of the connections between. aerial and lead-in, dirty or loose connection at the

lead-in terminal. It is a good idea to overhaul the aerial at least once a year, but it is not at all necessary to emery paper the wire! Grid Bias from the Mains. ACCUMULATOR cells are sometimes thought to be a good solution of the grid biasing problem, but a small thermionic rectifier will ‘ be found cheaper and give less trouble. It is inadvisable to attempt to obtain grid biasing potentials by a potential divider associated with the H.T. battery eliminator, or to use the "free grid bias" system by causing the anode current to ereate a voltage drop across the grid biasing resistance. Use a small independent rectifier if a high grid biasing potential is required for the output stage. After smoothing, a shunt resistance should be floated across the leads. This need only pass a milliampere yet it will keep the voltage constant as well as protect the smoothing condensers.