CUTTING OUT INTERFERENCE

Radio Record, Volume I, Issue 48, 15 June 1928, Page 12

CUTTING OUT INTERFERENCE

WITH THE FRAME AERIAL | [NX a recent issue of "Wireless World" it is pointed out that the directional effect of a frame aerial can be considerably improved by centre-tapping the frame and connecting this tap to earth and to the valve filaments, as ; shown in the figure. The idea underlying this mode of connection is that the electrostatic pick-up, being

more or less evenly distributed over the frame, should be balanced out, leaving an approximation to pure elec-tro-magnetic pick-up. The degree of success attained in eliminating the static pick-up can be gauged by the extent to which the local station can be eliminated by turning the frame into the position of minimum signals, TPILOSE who live in large towns are very frequently annoyed by finding that all reception, save from the local station, is impossible, owing to the noises due to the electrical machinery which is now found everywhere, All moving electrically-driven machinery, from a vacuum cleaner up to an electrie tram, seems to generate

— od disturbances which are carried along | the electric light mains ("wired wire-_ less") for very considerable distances from their point of origin. These disturbances, which are usually classed together under the vague but inclusive title of "earth ngjses," make themselves known to the user of wireless receivers in the form of a wide yariety of scratches, cracks, bangs, buzzings, and indeterminate uproar of all kinds wheneyer he ventures to tune in a station at any distance. In bad eases even the local station may have its programme punctuated by the louder of the noises, while every other station may be completely blotted out by a continuous roar. Since these noises are completely untuned, it is not possible to eliminate them by making the receiver ultra-selective. PPARENTLY, however, the majority of the noises are introduced into the receiver through electrostatic rather than electromagnetic pick-up, for if the frame aerial connections shown in the figure are employed, the ratio of noises'to signals can be very considerably reduced, It is found that a frame, connected in the usual manner to the receiver, does not offer any very great relict from noise, but that if the connections shown in the figure are adopted, a very considerable improvement results. In bad cases it becomes necessary to extend the precautions beyond that of the simple centre tap by making the tapping on the frame, which must for this purpose be wound with bare wire, by means of a spring clip, adjusting its position carefully to give the minimum of interference. ., AVOID STATIC PICK-UP. Iti} lead from tuning condenser to grid, having no counterpart at the other end of the tuned circuit, must -be shielded, which implies, in practice, that the tuning condenser must be within the set, and the whole enclosed in an earthed metal screen. It is also essential to look very carefully after the leads connecting the two ends of the frame to the tuning condenser and to take every precaution to ensure that the static pick-up by them is identical. This could be achieved by using twin flex for the connections, but the high capacity between the wires mikes it wnsuitable. A good method is to make a kind of rope-ladder, the two wires forming the uprights, while the rungs consist of ebonite or wood spacing

td a strips. Alternatively, the two wires may be sewn into opposite edges of a piece of webbing, with .the connection for the centre tap, if desired, down the centre. "THESE precautions may seem very elaborate, but it must be emphasised that half measures give but little relief from the earth noises, while a whole-hearted attack on them, on the lines laid down in the present note, will at the least make it possible to obtain some sort of entertainment from any station which, on an open aerial, is not so entirely blotted out by noise that its transmissions are unintelligible, The accompanying circuit is one that will give comparative freedom from earth noises, but in most cases would only be suitable fur shortdistance reception, except in the case , of using headphones.

CHOOSING A VALYE. (THERE is now a good choice of re- * ceiving valves on the market, many of them particularly suited for some special function which in many cases requires more or less exclusive characleristics, Iixperimenters and constructors should accordingly not hesitate to mix the products of various makers if by so doing they may more neatly approach the ideal laid down in the various constructional articles. A BATTERY VOLTAGE. HE voltage of a discharged cell should never be less than 1,8 volts, A newly-charged cell may show 2.25 volts, but on standing it rapidly drops to 2.2 volts, aud your battery should show this value when connected to your set. In discharging the voltage drops gradually from 2.2 to 2 yolts, remains constant at that figure for a fairly long period, and then drops to 1.8 volts. If discharged after this the drop:to 0.5 volt is very rapid.