Discussed from an Unusual Angle

Radio Record, Volume II, Issue 37, 28 March 1929, Page 28

Discussed from an Unusual Angle

By

PENTODE

AKING radio all the year round, the winter months cer2 tainly show a great increase the interest of wireless by the general public, with per: py }haps a little increase at the ws} beginning and end of the wireless year. The radio year commences on April I, from which date all the receiving licenses are issued, and just about this time there will be some who have put off buying a receiver until they can pay for their license and obtain a full year’s benefit from it. These prospective listeners’ will no doubt welcome a few hints on the erection of the aerial and the selection of a suitable earthing arrangement.There will be some, too, who desire to improve their external radio equipment before the weather becomes too bad. Many articles have been written on the subject, but the average amateur pays far too little attention to his aerial and earth, and if the present chapter gives a few new enthusiasts an idea of the importance of these two essentials, then this talk will not have been wasted. The Use of an Aerial. Now. before commencing to describe the efficiency of different types of aerials, let us try to understand, by the process of analogy, why an aerial is used at all. Suppose a certain kind of fog or smoke existed, that in contact or close proximity to the earth ehanged to transparent air, and instead of waves in the invisible ether, the broadcasting station sent out waves of this fog or smoke. It would be interesting to follow this train of waves as they spread out in circles of ever-in-creasing diameter. This is a close analogy of what actually occurs. Imagine one of these waves coming over your own locality. What would you do if you wanted a sample. A pipe stuck up a few feet off the ground would be of "ss

very little use as the smoke is changed when near the earth. No, a long pipe, well above the roofs of surrounding buildings, and trees, would be the only way to obtain a good strong or concentrated sample. In the case of the smoke waves, the vehicle or medium upon which the waves are carried would be the air. But air is not the only commodity by which we are surrounded. The mystic word "ether" stands for a substance of which man knows very little. It pervades everything. Even if we produce a vacuum the ether remains, and it is upon this ether that radio waves are conveyed. Waves are radiated from the broadcasting station and travel outward in circles, the distance between adjacent waves depending upon the wavelength or frequency. As in the case of the assumed smoke waves, the radio waves become less dense nearer to the earth and to get strong samples of these the only way is to erect a collector high above the ground. Not only do these ether waves become weaker nearer to the ground, they become weaker when near conductors in contact with the ground. Such conductors as trees, buildings, overhead telephone or power lines, etc., absorb the energy, conveying it to earth, where it is lost. And so, to obtain strong samples from distant transmitting stations, the aerial has to be erected as high as possible, not only with regard to the earth, but as high as practicable above surrounding trees, etc. Dimensions of the Aerial. WE have seen that, an aerial must be high. Now comes the length-the

best length for average purposes. On first thinking of the subject, it will appear that the greater the length, the greater also will be the collected energy. This is quite true up toa certain point, which is governed by several conditions. As the length is increased so does the capacity between it and the ground, and it is undesirable to have capacity here. Copper wire has a certain hf. resistance, which increases with the length. This is another, although minor, reason for limitation in the length of an aerial. The principal drawback to the use of very long aerials is the effect on the selectivity of the receiver. The set becomes very broad in tuning, and to bring back selectivity one has to resort to methods which introduce losses that only counterbalance the extra energy delivered by the long aerial. Such adoptions as series aerial condensers, small aerial coil, etc, only lower the efficiency and make the set no better than if used on a shorter aerial without these "gadgets." In Hngland the permitted overall length of aerial is 100ft., thus accounting for the standard 100ft. aerial coils. It is found that using lengths above 100ft., including the lead-in, results in most cases, in very little improvement, and the recognised horizontal length is 60 to 70 feet. No specified measurements can be given regarding the height, beyond putting it as high as possible. An aerial forty feet high is better than one only thirty feet. Insulation. Having given thought to height and length, so as to collect all the energy possible, the question of insulation must not be neglected. After erecting a good high aerial, don’t lose anything through faulty insulation. Remember that the lead-in end requires more insulation than the free end. When choosing insulators, . the main points to watch are insulation, strength weight. Regarding insulation, the insulators should be highly polished, so that rain and moisture forms in globules and not in a thin film of liquid, as would be the case if they were of a dull surface. Strength and weight have to be considered if the aerial is subject to much wind pressure, or if a long span is used.

Cheap forms of glass insulators have been known to break in windy weather. Two or three good quality insulators at the free end, with at least three at the lead-in end, will give sufficient insulation in all weather. A number of readers have written in asking what advantage enamelled wire has over bare copper wire, when used for an aerial. Well, in districts near the sea, or in the thermal regions, salt spray in the first case and sulphurous gas in the second, have a corroding action upon the copper. This has no effect upon the electrical properties of the aerial unless it goes too far and increases the resistance of the wire. But when it has reached this stage, tie wire will have corroded to such an extent that a breakage will occur. To prevent this it is advisable to employ endmelled copper wire. Unlike direct ° current, radio frequency electricity passes on the outside of a conductor. If the outside is insulated, either by corrosion or enamel, the current penetrates further into the centre. In England, comparisons have been made of the high frequency resistance of a thin copper tube and a solid wire’ of the same external diameter. The resistance of the tube was actually less than that of the solid wire, proving that the mass of copper below the surface has no use for the passage of R.F. currents. It is for this reason that all the tuning coils of transmitters are made of copper tubes in the form of a coil. But this is wandering from the subject of aerials. r Types of Aerials. LASt week a correspondent wrote asking for specifications of a cage aerial and inquiring if this is the best where space limitation prevents the erection of a single wire. The correspondent would have been more correct if he had put things the other way round and inquired of the efficiency before asking for details of construction. Cage aerials are used by transmitting stations and there is no advantage to be gained by using them for reception. In fact, the added capacity of the aerial system has a distinct disadvantage when coupled to a receiving set. Measurements have been taken of the energy received on a single wire aerial, and it has been found that practically no advantage is gained »by using a similar parallel wire unless that wire is at least 6ft. distant. This opens up a field for a hint on twin aerials. Where space does not permit of a single wire aerial of 50 to 70 feet, then a twin or even a triple

aerial has to be resorted to, As the name implies, these types of aerials ‘ consist of two or three lengths of wire running parallel and kept apart by spreaders at each end. To get maximum efficiency these separate wires have to be at least 6ft apart, all well insulated, -with the lead-in taken from one end. A common mistake is often seen where a twin aerial is used with a single length of wire passing from one end, across and then back again. This is absolutely wrong, as less energy is picked up than if just one single wire of the same length was used. To be absolutely correct, all the separate wires should be of the same length and joined to the lead-in wire by a Vshaped piece soldered to both wires at each end. Dealing with single wire aerials, probably the inverted L shape is the most popular, also it is the most ef‘ficient. This, of course, is the aerial ‘in which the lead-in is taken from one end, being led down as vertically as possible to the receiver beneath. The next type to be commonly used is the one in which the lead-in is taken from some point along the horizontal span. Judging by the numerous aerials seen

nowadays, this point seems to vary in each individual aerial. Undoubtedly the best position is at thé centre point, midway between the two sets of insulators. In all cases a good joint between aerial and lead-in wire must be made. Merely twisting round and binding with tape is not sufficient. In contact with the air, bare copper becomes coated with a thin film of oxide, which is in itself an insulator. A good soldered joint is indicated, not only as a good electrical connection, but also as it is mechanically strong. . A lead-in wire should be tight. In windy weather a loose lead-in is often the cause of quite a lot of the fading experienced, especially in a sharply-tuned receiver. This is of special importance on an aerial used for shortwave reception. If, on pulling tight the lead-in fouls the root or trees, ete, it can be kept away by straining back to a fence or stake driven into the ground. At least three insulators must be used here, as the nearer to the receiver, the greater care must be exercised on insulation. Quite a good hint to keep the aerial constantly tight is the use of weights fastened to the hoist cord. This is of

greater benefit where one end is fastened to a tree that sways in the wind, thus making the aerial.alternately tight and slack. The lead-in tube must be strong. Thin tubes of 3-Sin. diameter will not last long, and it is false economy to buy them. When drilling the hole in the wall, choose a spot just underneath a sill or other slightly overhanging projection. This prevents the tube from becoming wet on the entire length outside the wall, and provides a few inches of positive insulation. It can be easily seen that a wet lead-in tube in a wet wall will provide a splendid path for leakage. Regarding lightning arresters. More than once these have been the cause of poor reception. Before mounting, try out with a pair of phones and a 221 V battery. Cases have come under the writer’s notice where arresters have provided a direct path to earth, not only for a highly-charged aerial, but also for the local station. In the back will usually be found a cavity filled with wax. If this wax has become loose or is missing, either add more molten wax or put in the oven for a few minutes to melt the loose wax and also drive out any moisture that may have collected. Reverting to the subject of aerial poles. A number of enthusiasts are prejudiced against the use of metal pipes as aerial supports on account of the conductivity of the metal, with consequent screening. Besides the extra mechanical strength and less surface exposed to the wind, the poles have no material effect upon signal strength, whether made of wood or iron. Frame Antenna. BEFORE leaving the subject of aerials, a few words on frame antenna will not be amiss. It is a common mistake for the man who knows nothing whatever of radio, to wish for a set using a frame antenna, instead of all the elaborate system of external wires. When he is told, too, that no earth connection is necessary, he is more than ever desirous that this shall be his method of reception when he purchases his set. Anyone reading this article will be interested to learn that it needs an exceptionally sensitive receiver to successfully work from a frame. It is in the sense of receiving long distance stations that is here meant. This method is really ideal if used on a small receiver for the local station. Special receivers are designed anf built for use with a frame antenna, such as the ultrodyne and the superheterodyne, but these employ from six to ten valves, and in operation are no more sensitive than a four or five-valve receiver on a good outdoor aerial. Actually, a frame antenna is 4 tuned coil replacing the first grid coil of the receiver, and when building a set of this nature the succeeding coils have to be thoroughly screened from the frame aerial coil to avoid interaction and oscillation. All Types of Earths. HAVIN G seen so many varied articles on earthing systems described in

recent journals, the writer is rather nervous to express his views on the subject. A whole host of criticisms will no doubt be raised against one or two of the following statements, but the facts are compiled upon recent lectures by leading authorities in England and America. First of all, what is a good earth? It is a positive electrical connection between the transmitter and the receiver. The transmitting aerial is sending out waves on the imaginative substance, ether. These waves are radiated into all directions, and come in contact with our aerial. As soon as these ether ‘disturbances come in contact with a conductor, minute radio frequency currents are induced. These fiow down the aerial lead into the set. Put in a very crude fashion, we take out all we can of these minute currents to operate our receivers, and the remainder flows away into the earth, presumably back to the broadcasting station, this completing the circuit. Now, to get maximum efficiency this whole path has to have the least possible resistance, the most resistance being where we want it, between the aerial and earth terminals of the set. Any external resistance, in the lead or earthing arrangements, all takes away from the efficiency of the receiver. Dealing with this resistance, it is not direct but high frequency alternating current with which we are concerned, any any experiments made have to be with electricity of this nature. It is positively known that high frequency currents travel on the outside surface of a conductor alone, hence the use of standard wire for an aerial, as it presents a much greater surface than one made of solid wire. It is not realised that the earth wire, that is the wire leading from the set to the connection with the ground, must be chosen and treated with as much care as the aerial lead in. It should preferably be insulated, and if not it must be fastened firmly to the wall or surface along which it runs. The reason for this is obvious when it is understood that the aerial and earth can be considered as one, the finishing point being the connection to the ground. If during a programme half the aerial was short-circuted, the programme would become weak, if not go altogether, due to the detuning pro- duced. Now, if a slack earth wire is blown with the wind against a wet wall it will temporarily short-circuit a portion of the aerial-earth system, and cause fading and scratching noises in the *phones or speaker. If the wire is bare, tighten it, and if trees or shrubs are liable to blow against it, thoroughly insulate it. Another point to watch is to keep the earth lead as short as possible by running in a direct line, if necessary under the floor of the house, instead of along a maze of skirting boards, round door frames, etc. The actual connection between the wire and ground pipe should also be watched, and a few sketches show various methods of tackling the job. These methods of attachment are useful when it is

desired to make a good connection to, 2 pipe through which water is flowing, making it impossible to solder. As far as actual earths go, no hard and fast rules can be laid down saying which method is absolutely the best, as individual circumstances vary. In general it will be found that a good water main makes as good an earth as anything. A pipe should be chosen that runs directly into the ground, but even this suggestion does not always hold good. In other countries, as a preventative against electrical apparatus being indiscriminately earthed to the nearest water pipe, the water-power authorities decided to adopt the use of insulated collars between the housepipes and the water-meter.. An earth attempted under these conditions would be decidedly bad, as the only conductor upon which to rely would be the water, and the purer the water the poorer would be its conductivity. Also, some water pipes run quite a distance just below the surface before they finally are connected to the main, and in dry weather a poor earth would result. So it is always just as well to try an independent earth, especially in an endeavour to cut out any local interference. It has been stated before that high frequency currents travel on the outside of a conductor. This is even correct with the earth as a conductor. Actual measurements have shown that this form of current does not penetrate far into the ground, and this depth depends upon the nature of the upper surface of the earth. In poor -surface earth, the current will penetrate deeper. Taking the amount of current flowing through the earth at different depths it is found the earth at 50ft. practically no cwrrent is flow‘ing. This condition lasts until about 15-20ft. of the earth’s surface is reached, and it can be said that practically

all of the many thousands of minute high frequency currents are all flowing within 10ft. of the ground. It 1s said that 95 per cent. of its current density will be found within the first 15 feet of the earth’s surface. This distance, of course, varies greatly.. In the case of the salt water of the sea, this distance is reduced to 4 to 5 feet. The question arises at what depth a pipe should be driven to obtain a good earth. For experimental purposes, a pipe was driven into the ground a few inches, and the resistance of this earth made by an independent electrical generator, working on a wavelength of 350 metres. The pipe was driven down a few inches at a time, and measurements taken. This was done in various parts of the country in different types of soil, and the result tabulated. With the pipe a few inches down, a resistance of 1000 to 10,000 ohms was measured. With every measurement the resistance rapidly decreased until a distance of 8ft. was driven into the ground. About this distance the resistance was practically constant. | By these experiments it is concluded that an earth pipe should be driven at least 6 to 8ft. into the ground, and in the table of results compiled from numerous positions of trial, the resistance can be reduced to about 10-15 olms. Another point of interest also enters, when the question arises as to what diameter and material to use for the earth pipe. A single pipe was driven into the ground, and its resistance as an earth calculated. A similar pipe was driven in a few inches away, and the two joined by a length of wire. The resistance of the two was found to be only a small fraction of an ohm different from that of the single pipe. This

shows that diameter of pipe counts for very little. All the above experiments were carried out using half-inch diameter galvanised iron pipes, and in practice the pipe should be of sufficient diameter to give mechanical strength while driving. The resistance does not exist between the pipe and earth so much as in the earth surrounding the pipe, and so it was decided to see how far away a second pipe had to be driven in order to reduce still further the resistance of the earth. Quoting a leading American authority: "If several pipes are driven in homogeneous soil, at different distances apart, it is found that the resistance between pipes increases as the distance between pipes up only to about six feet distance. For greater distances the resistance is practically constant and independent of the distances between the pipes." It will be now seen that to decrease the resistance of an earth, the only way to do this is to drive in another pipe at least six sfeet distance. The resistance of the two pipes connected together will then be halved. To further reduce the resistance, a further pipe can be driven at least six feet from the second pipe. Resistance does not occur between the actual pipe and the earth, but in the surrounding circle of earth up to a distance of six feet. Since the resistance depends upon the soil in this vicinity it can be considerably reduced by frequent applications of water. Comparing the table of the conductivity of fresh water with that of brine, it is evident that a little salt added to the water will prove very beneficial, besides being a naturally hydroscopic salt, it will absorb and retain the moisture from the air. Copper sulphate is no better than common salt, besides possessing the disadvantage of killing any plant or animal life with which it comes in contact. A #-inch iron pipe was driven five feet into the ground and the resistance measured. Salt water was added every day for four days and the resistance dropped steadily each day as follows: Resistance of earth before salt, 50 ohms; four pounds salt added first day, resistance 25 ohms; another four pounds added second day, resistance 20 ohms. On the third day four gallons of water was added and a final resistance of only 13 ohms was measured on the fourth day, which remained more or less constant. Before leaving the subject of earths, never under any circumstances use gas pipes for an earth. Besides being illegal and forbidden by any insurance company, it forms a very indifferent method of earthing as red lead and tow are decidedly poor.conductors and most of the pipe joints have these two ingredients in their "innards."

Also, it is inadvisable to use hot water pipes for this purpose, as they. usually go rambling round the house before burying themselves in a bit of decent ground. : Summary. 1. Aerials to be as high as possible and thoroughly insulated down to the point where it enters the receiver. 2. If twin aerials are used, the span has to be at least 6 feet and where the length is available, single wire aerial of 60 to 70 feet length is advised. 3. Lead-in and earth wires to be drawn tight, especially where the receiver employs reaction. 4. It is always advisable to try an independent earth, especially where local interference is experienced. 5. A satisfactory earth can be made by driving a Hin. or fin. galvanised iron pipe 6 to 8ft. into the ground. -To still | further lower the resistance, .one or two/ more can be driven with a distance of at least 6 feet separating any two pipes. Connect all the pipes together by a length of stranded wire. 6. If the ground surrounding the earth is dry, it can be.made more conductive by pouring round it a solution of four pounds of salt in a bucket of water. 7. Finally, do not earth to your gaspipe, as if there is a fire it will surely be put down to this by the insurance company. Give the independent earth a try-out.