The Importance of a Good Earth

Radio Record, Volume II, Issue 31, 15 February 1929, Page 26

The Importance of a Good Earth

Ground Connections for all Conditions

(By

R. W.

H.

CONSIDERABLE proportion of the wireless receiving sets used to-day have their performances impaired by the fact that they are not properly earthed. There is a oD3 «prevailing impression that any kind of water-pipe-or even & gas-pipe-makes a_ satisfactory earth, whilst many of those who use a -buried earth plate of one kind or another are content to put a piece of metal with a- wire attached to it into a hole dug in some odd corner, believing that they have done all that is necessary. As we shall see in a moment, waterpipe earths may be unsatisfactory, gaspipe earths usually are, (besides being illegal), and buried earths depend for their goodness or otherwise upon @ number of important considerations. What is the Earth? First of all, perhaps, we.had better see what is meant in ‘electrical parlance by the term earth. If you take a piece of string a yard or’ so in length, put your foot on. one end of it, and move the other rapidly about, wave forms will pass down the string. One end, that upon which your foot rests, is fixed, and the string shows its greatest variations from the straight line at the end held by the hand which is moving it to and fro. Oscillating potentials behave in very much the same way in a collector system consisting of an aerial and earth with a coil between them. Dtectrically the term "earth" means zero potential and corresponds to the fixed end of the string. If the earth connection is as nearly perfect as may be one end of the aerial coil is at ‘earth potential and _ practically the whole of the oscillations corresponding to the movements of the string then take place across the aerial coil. When this is so the maximum signal strength is obtained in the wireless re-

ceiving set. The grid of the first valve is receiving the greatest possible voltage changes and the valve is, therefore, able to do its work effectively. But suppose that there is a high resistance between the low potential end of the aerial coil and the earth connection what will be the result? Only a portion of the voltage changes will occur across the aerial coil, the rest taking place across the resistance. It follows that the voltage changes ap-

SEES! plied to the grid of the first valve will be smaller than they ought to be and that signal strength will suffer. We can at once see why it is important to keep the earth lead as short as possible; any wire, whether straight or wound into a coil, possesses both resistance and inductance, hence the longer the earth lead the greater will be the oscillating potentials along it, and therefore the greater the loss to the receiving set itself. Clearly, too, the lead must be of heavy wire in order to keep down the resistance. Remember that though a resistance may have a small direct current value its oscillating current value may be very much higher.

Water-pipes as Earths. Not by any means do all water-pipes make good earths. I have seen earth leads connected to pipes which, on examination, turned out to run simply from cisterns to taps. These are usually hopelessly bad. The only kind of water-pipe that can make a good earth is what is known as the "rising main," that is, a pipe in direct connection with the underground system. In this case we have a metal

pipe containing always a column of water. If the joints in the pipe are bad from an electrical point of view the water is there to bridge them, But there is difficulty: in making a really satisfactory connection for a wire ‘to such a pipe. Water mains are usually of iron, a metal to which it is exceedingly difficult to solder. The only really satisfactory method is to clean a portion of the pipe until it is quite bright and then to fix the earth lead to it by means of a firmly-clamped clip. No water-pipe, however, should be accepted as the perfect earth until experiments have been made in other directions. Gas-pipes are thoroughly bad since they contain no conducting column and their joints are often sealed with a compound which has a high electrical resistance. In addition there must always be a certain element of danger in making use of them for the purpose and most gas companies rightly prohibit the practice. . The Buried Earth. BH next come to the buried earth, which is excellent so long as care is taken to see that it is put in a suitable place. Don’t imagine that every part of the soil is always a good earth contact. It isn’t! In many places. the upper layers of the soil

consist largely of gravel, which allows moisture to percolate very easily, and in fine weather soon becomes quite dry. Dry gravelly soil may have a very high electrical resistance. An earth connection buried in it may, therefore, lead to surprisingly bad reception results. HME A case in point came the way of the writer a couple of years ago. A friend who was the owner of a yery good receiving set complained that ‘he could hear nothing but the local station and not very much of that. Since the aerial and the set itself were mane reproach, we decided to investigat earth. On inquiring, I was told that this consisted of a copper plate buried some three feet deep close to the house. We dug down to it, the soil proving to be, as I expected, of a gravelly nature. We then deepened the hole and about a foot further down came upon clay. Immediately the earth plate was placed in contact with the clay signal strength from the local station increased by at least 50 per cent. and no difficulty was found in receiving a. number of other transmissions. Clay is a mois-ture-laden substance which is ideal for earth connection. If you want good results, let your motto be get down to clay. When a Good Earth is Impossible © T may, however, happen ‘that the gravel stratum is so deep that it is impracticable to sink a hole right through it for the earth connection. There are two possible alternatives. The first is to discard the earth connection altogether, and to make use in its stead of a counterpoise. The counterpoise is really a second edition of the aerial, suspended six feet or so above the surface of the ground and just as carefully insulated. In some instances (particularly where interference’ from trams, electric railways, and so on is a nuisance) the counterpoise may give better results than an earth, and generally speaking’ it makes for rather greater selectivity. It has, however, one great drawback ; it means more wires in one’s garden. A simple form of counterpoise which is sometimes effective may be contrived by stapling down fifteen or twenty yards of insulated wire under the carpet in the room in which the receiving set is situated. The wire should be arranged more or less_; spiral form. This device, will not answer very well in rooms of an upper story. The second tip is one that will be found useful in many localities. Fix in your mind’s eye an imaginary line

eee THE summer months have brought more forcibly before every set owner the need of a good ground connection. Correspondents almost weekly convment on different types they have tried, quite often with remarkable success. Tt pays to ex. ‘periment, and to those interested in radio reception the following article is very suggestive.

on the. ground immediately under the nerial, Five or six feet on either side: bs: this make narrow, shallow trenches running the whole length of the aerial and parallel with it. They can be made quite easily with the point of a pickaxe. In these: bury lengths of the bare cabled wire generally used for aerials, connecting: the ends of the wires together close to‘ the. house. 'Lo the junction of the two wires. fix the end of the earth lead. The triple joint should be soundly. soldered by "sweating" the. wires together. Any plumber will undertake the job if it is beyond your own powers. This form of earth has many great advantages. Once the wires’ are buried they are entirely out of the way and will require no further attention for years. In wet weather they act as fa true earth, since moisture-laden wil is a good conductor. In dry weather yery little loss of efficiency is observed, for even if the soil becomes desiccated: the wires have a counterpoise effect. In fact many who have put this hint into practice have’ found that they no longer have such a huge difference, as once was the case, between dry weather and wet weather signal strength. In Clay Soils, GIVEN clay soil at a reasonable depth one of the best forms of earth connection is an ordinary seven-pound biscuit tin: whose labels: have been

scrubbed off. An earth lead of sevenstranded wire can. be soldered to it at as many points, thus ensuring a good contact. The tin is buried at a depth sufficient to ensure contact with clay and an iron pipe long enough to reach a foot’ or so above the surface of the soil is placed within it. A few small holes’ may be drilled in the tin. When the earth is in position the mouth of ‘tthe pipe protrudes from the ground. In dry weather water can be poured into it. The water percolates through the holes drilledin the tin, moistening the surrounding clay and ensuring a good contact. at. all times. . It is, however, of no use making 2 first-rate earth: plate unless you assure yourself that the contact between it and the low potential end of the aerial coil is as good as it can be. There may be @ high resistance between this point of the coil and the aerial terminal of the set if thin wire is used for the connection or if proper joints are not made. Other possible points at which resistance may occur are in the earth wire itself, if this is of too small gauge or between it and the receiving set should there be a dirty or loose connection at either end of the lead-in tube. ‘It often pays to experiment with earths of different kinds if one is anxious to obtain the best results from a receiving set designed for long-dis-tance work. Don’t be content witb any kind of earth connection: put in haphazard. Try the water-pipe, the buried earth, and the counterpoise. It is most interesting to compare the results obtained. by these different systems, and often it is found that one ef them gives a surprising improvement both in the quality of reception and in the number of stations added to the log, A De Luxe Earth. N earth-a de luxe type-that experiment has shown to be remarkably efficient may be installed in the following way. To a copper plate measuring some four feet by two a strap of copper tape long enough to reach to the terminal of the lead-in tube is riveted. It must be riveted. and not soldered or brazed, in order to preclude all possibility of electrolytic action. The plate is buried vertically beneath the aerial. The connecting strap must be. kept well away from walls and so on, a distance piece provided with an insulator being used if need be. The strap should also be taut, so that it cannot sway in a wind. To ensure a low resistance contact the earth plate is packed round with coke or some similar moistureholding. material. Yor all kinds of earth plate the riveting tip is a good one for soldered connections are not to be relied upon underground, and there is no doubt that they do give rise to electrolytic action owing to the presence of several dissimilar metals at the joint. In some very acid soils biscuit "tins" or other iron earth plates are destroyed very rapidly, and rivetted copper is much to be preferred.