The Reduction of Interference from Static

Radio Record, Volume I, Issue 30, 10 February 1928, Page 3

The Reduction of Interference from Static

Some Further Points of Interest to Constructors

"By

M.I.R.

E.

‘ OLLOWING on last week’s discussion regarding the reducton of static interference by meals of the directional effect of aerials and loops by taking advautage of a difference in ditection of arrival of the static and the desired signals, it is proposed to outline some more points of interest. It will be remembered that it was shown that a method of sorting out signal from static, or at least increasing the static to signal ratio in favour of the signal, could be put into operation by pointing a directional aerial.towards the signal in such a way that its receptivity would be at a maximum in that direction, but its receptivity would be relatively tess, and preferably a minimum towards the direction of static. FIELD FOR RESEARCH, This has opened up a tremeudonsly wide field for research, and eminent engineers and physicists have devoted many years of research to improving methods of putting this principle into operation, and thus arming the receiver . with a much-needed defence against-the attacks of its most bitter enemy. The simple directional aerial is of little use because its directivity is more a theoretical than a practical consideration: Unless the design is carried to an extreme by having a wire about 150 feet or more long, run along the ground on insulators, and only a few inches distaut from the ground, then there is little advantage to be gained from it. The effective height of such an aetial would naturally be far short of one, say, twenty-five or thirty feet high, but nevertheless a surprising number of stations can be tutied in with a multivalve set, owing to the flexibility of the radio-frequency valves and their liability to give enormous amplifications on extremely weak signals. This type of aerial may also be formed by laying a well insulated cable along the top of the ground, the one end being attached to the receiver and the free. end pointing towards the station it is

desired to receive. The free end should be well insulated of course. Another form is often referred to as an ‘‘underground aerial’? and consists of the before described well-insulated cable buried.in the ground up to two or three feet and still pointing its free end towards the distant station. RADIO TRANSMITTER DiS: TURBANCES, | As has been described previously in -this column a radio transmitter sends out two disturbances, one of: which is in the "aether’ and consists of electro-magnetic waves which travel through space while the other represents electric currents which, travel through the earth. ‘lo just what extent these disturbances affect the receiver depends on two factors, the first of which is the wavelength or frequency of the currents used at the transmitter, and the second is the nature of construction or electrical conductivity of the intervening ground, With a given wavelength and salt-water -between two stations, providing certain constants of the stations are known, the actual percentage ratio of aerial to earth components of the wave transmitted and received can be dedetermine with satisfactory accuracy. Immediately, however, the intervening earth consists of ground of unknow1 conductivity, the computation becomes very wnsatisfactory. Tt has also been explained in these columns that the amount of energy picked up by a receiver (and therefore the signal strength) is dependent on the height of an aerial (from an electrical and not necessarily a geometrical point of view). The higher the electrical height the greater the energy received from the waye arriving from the transmitter. "UNDERGROUND AERIALS." An application of these principles to the question of low highly directional aerials and buried wires resolves itself down to a consideration of the waveslength in use and the nature of the

ground at the receiver as well as the ground intervening between the tramsmitter and receiver. The -effective height of an aerial may be very great owing to the fact that the ground underneath it is composed of very poor material which might just as well be considered an insulation and it may be necessary to go down, say twenty feet, to get to good material. If such an aerial is twenty geometrical feet high above the ground, then the total geometrical height would be reckoned at forty feet with the added ‘height’ under ground. Providing the earth connection is buried twenty feet into the good material the advantages: offering will be made use of otherwise anything picked up by increased height will be used up in aerial resistance. RECEPTIVITY OF LOW AERIALS. It will mow be seen that it is a very difficult problem to determine the, receptivity of low or buried aerials because for a start, it is mot known just what the ratio of space to earth currents are, nor to just what extent the earth currents which do arrive will affect the receiving system because the wire slightly above earth should act as a true aerial, while the buried wire should respond to the earth currents only. The wire laid along the surface of the ground should receive nothing whatsoever if the ground _ it is laid down on is good wet soil because it should be non-inductive theoretically. Its geometrical height above earth would consist of the thickness of the insulation of the wire as a matter of fact! This would actually be so if the surface of the ground consisted of copper-sheet for instance. In order to*make certain that only earth currents are picked up it is customary to lay the insulated wire in brackish water as the latter has a lower resistance than even salt water. It is claimed by the exponents of the undergroutd aerial that there is very great freedom from static interference because the greater percentage of static

arises in the atmospliere and that .static waves are purely space waves. ‘his is so, but the space waves tetid to induce currents in the surface of the earth, and these are picked up in the aerial, and, furthermore, there are natural as well asman-made earth currents which tend to disturb such a pick-up system materjally, and do not haye an appreciable effect on an overhead system. DIRECTIVE AERIALS. Considerel briefly, these directive forms of aerial show considerable advantages over orthodox designs, providing the user is prepared to instal a couple of extra valves in the receiver to make up for the loss of sensitivity .due to the smaller pick-up of the derial. Naturally such aerials can only be used to receive in a given direction unless several of them are installed: in such a way that all the stations it is desired to receive are capable of being picked up on one or other of them. However, in New Zealand the average receiver is used on the Australian stations, principally when distant reception is being carried out and static is causing most interference, Hence, anywhere in New Zealaud, if au aerial is laid down in such a manner as to cause the free end to point towards Sydtiey, the aerial will satisfactorily enough include Brisbane and Melbourne in its compass. There are one or two points of practical interest and one is that thé selfcapacity to earth of the low or buried aerial is much greater than one of or- | thodox design, and in order to have a fair length as well, the tuning of the system may be.found to be difficult. It is therefore wise to instal a_ series condenser between the lead to the receiver and the aerial terminal, This condenser should preferably be a_ variable one, although a fixed one of a value of .0003 will be found to work well enough. A variable condenser with a maximum capacity of .0005 is desirable, especially with the buried wire. To get best results, this condenser should be varied with respect (to the first tuning dial on the receiver

until. maximum signals are obtained. Any variation of the series condenser will call for a slight readjustment of the first dial or aerial tuning control of the receiver. It is ye possible where there Is plenty of open ground to lay down an aerial twice or three times as long as usual and still get good signals with sharper directive effects. Lhe series condenser is now an absolute necessity, and it should be variable, ‘The Beveridge aerial has been mentioned in these columns before, and this takes the form of a wire actually longer than would be called for in a straight-out tuning system. In this case, two wires are used, one as the aerial, and the other as a transmission wire to bring the energy from the aerial to the receiver -and at the same time make the aerial system reversible in direction. A special resistance is joined between the far end of the aerial and earth to assist this effect at different receivers working at different wave-lengihs are energised by tliis system. It should be noted that directive systems are not really practicable on shortwave systems, for instance, below 100 metres. ‘It was mentioned at the commencement of this article that a factor determining the energy transferred to the receiver was the nature of the earth between the stations as regulating the amount of earth current which would arrive at the receiver. As before-mentioned, this varies with wave-length. On wave-lengths above 10,000 metres probably over 50 per cent. of the energy received comes through the earth, but below 100 metres the earth currents dissipate themselves owing to the resistance of the earth and they do not carry more than a few miles at most. Palpably, therefore buried earths are quite impracticable for the reception of the ultra-short wave stations, which are such a source of interest at present. Those of an experimental turn of mind will find great interest in trying out stunts on the lines of the few pointers treated in: this discussion.