AERIALS ARE IMPORTANT

Radio Record, Volume IV, Issue 19, 21 November 1930, Page 3

AERIALS ARE IMPORTANT

Points people usually forget Broadcast from 2YA by

Colin W.

Smith

lena) HERE is probably no portion y 2 of the outfit that enables one to hear broadcast reception that is subject to more comment than the aerial and the earth system. They are vitally important if good reception is to be enjoyed. Nevertheless, they frequently receive very, Scant attention and it is not unco fe listeners to forget the aerial so as signals can be received. In fact’ they will blame everything else possible before ever thinking about it, and it is marvellous how far some folk will go before suspecting that essential part of the outfit. I was rather amused by a story told me by a country acquaintance who had not long installed his set. It went splendidly for a while, then became very weak indeed. They looked over the aerial as they had been told, but could discover nothing amiss so had to call in a dealer. A couple of hours of baffling search ensued without result. The set was even substituted, but no improvement. ; "Lhen, and not till then, was the aerial ' Poperly inspected. Yes, the trouble was there all right-a partial connec--tion between the aerial and the earth. where the wires passed under the window. And this is only typical of a dozen other examples I could: mention, , so the aerial and earth are really important,

I think, before we discuss the. practical points in connection with aerials a brief explanation of the function of an aerial should be of interest. In dealing with the transmitting station last week I explained how the aerial and the earth are really two plates of a huge condenser, This applies equally .well to the receiving aerial and earth and if those, plates are not as big as they should be, are faultily insulated, or are otherwise not efficient, then a good deal is going to be missed. Between the. aerial and the earth ‘is the tuning coil in the aerial circuit of. your set and the waves collected by the aerial have to find their.way through this before they can get. to earth. By offering a high resistance to one set of waves the others cannot find their way to earth and conée-

quently only the one station can be heard. OW, we will consider some of the practical points in erecting an aerjal. The first is the site. In this most people, particularly the- townsfolk who have only a small section and have to erect their aerial wherever there is room, have little choice. Where possible, get it clear of objects that are connected to earth. If the wire must pass over a roof, very close to a down pipe or a tree, the efficiency will be lowered. Hills ‘affect an aerial, for they, and trees, too, for that matter, cast a shadow as it were, over the aeral. Keep the installation away from ‘power and telephone lines and, if possible, have the aerial at right angles to high. voltage electric lines. Many

people are _ seriously handicapped through being too close to lines and pick up a great deal of noise, but they will find if the aerial is made fairly short and kept at right angles’ the noise will be minimised. For the present do not worry about the directional effect. We shall have more to say about that later on. Having now a rough idea where your aerial should be erected, let us consider next the length. This varies according to the type of set used, For receivers using six or more valves 40 to 50 feet is ample. Jor five valves 60 to 70 feet, four valves 80 to 100 feet, three valves 100 to 120 feet, one valve and crystal 120 to 150 feet. Measurements have shown that after 100 feet the amount of signal pickup in proportion to the increase in length is very small and, except in cases such as crystal sets, where every available impulse must be collected, there is nothing to be gained in having a long aerial. In fact, it is a decided disadvantage with big sets, particularly as regards selectivity. By selectivity I mean the ability of sets to separate one station from another. "THE height is also very important. An aerial should be as high as possible, not necessarily from the ground but from the nearest earthed object, (Concluded on page 27.)

Types of Aerial

(Continued from page 3.) If you have an aerial 50 feet high and it passes over the roof of a building 30 feet high and this roof is connected to earth, then your aerial is not as good as that of your neighbour whose masts are only 30 feet high. Furthermore, a high aerial brings in a considerable amount more noise than a low one, It is interesting to note that whereas static decreases rapidly as height decreases, the signal strength does not decrease as rapidly. This is why many people have two aerials, one only 10 or 15 feet above the ground. They use this on nearby stations when interference is bad. If your aerial is too high try the effect of a series condenser of .00025 mfds. So much for the height, the length and the situation. Let us consider the other practical points as regards the lead-in, the masts and the insulators, for they are all important. Select good large insulators, and do not be afraid of using plenty of them. The egg insulator is the most common one and you will recognise it by its shape. There is a hole at each end and a groove which runs from this hole down the longer part of the insulator. When connecting the aerial to the halyard arrange the insulator so that one passes through the other like two interlocked rings. This means that if the insulator breaks the wires hold one another. Insulators need cleaning fairly regularly. To facilitate this arrange

Yyour aerial on pulleys so that it may be lowered, but do not do as I did when ‘ erecting my masts-have the halyard wires thinner than the space between the pulley wheel and its frame. The wire will most certainly jam before the masts are halfway up, and it is not the easiest of tasks letting the mast down and putting it up again, especially if they are steel and liable to break. Specially tarred rope is better than a wire for a halyard. For the actual aerial wire select fairly heavy stranded wire, with an enamelled coat, as it is much better than the plain wire, which corrodes. Now I know some of you will object to enamelled wire, saying that enamel is an insulator, and therefore must retard the radio waves reaching your set. It certainly is an insulator, but not against the high frequency waves sent out by the transmitter. You will remember I explained to you what frequency méant last week. Directly the waves strike the aerial wire they pass right through the enamel and _ become changed in form to travel along the outside of each separate wire to your set. If the wire becomes corroded the electricity passing along the skin is retarded. Rubber insulated wire is good, but it is heavy and usually ex- . pensive. SEE that the lead-in drops straight to the set and prevent it turning corners. If possible, arrange your aerial to run back under the overhead wire, The lead can be taken from "the dead centre, in which case the of your aerial is equal to the lend-in plus the distance from the junction to one end. On the other hand, it may be taken from one end, which is the best. The lead should be insulated, so that it has no chance of touching any metal objects. Keep it off the side of the

house by a stay and an insulator, Particularly avoid twisting it round anything, as this, as those who are technically versed can see, decreases the capacity to earth, and allows some of the power to escape. Bringing it through the walls to the set is a problem. On one hand we must consider the efficiency of the installation, and on the other the house itself. The best. way is certainly to bore a hole through the wall and take a piece of heavily insulated wire through. A less destructive plan is to have a flat strip of copper and solder the aerial lead-in to one end and the wire that is going to the set to the other. Do not use spring clips, as they become corroded and the efficiency is very greatly decreased thereby. if there has been any necessity for joints in your aerial system see that they are soldered and covered well with insulated tape. Try if possible not to have joints, as no matter how carefully they are made they are less efficient than an unjoined aerial. Now that we have brought the aerial inside we must take it to the set, and here again we must remember the principle that the lead must go as direct as possible. If you have to cross a room it will be better to run underneath the carpet or linoleum than around the picture rail, but if you want good reception make an effort to have this inside lead as short as possible. Above all, do not twist it round nails. If the wire needs support use a stand-off insulator made by having special insnlators in dresser hooks.

Now that the aerial installation is complete let us look for a moment at the ground. ‘With the increase in electrically-operated sets this is losing a great deal of its importance for many of them earth automatically through the mains. Some, I have found, work better without an ordinary earth, but usually a good earth is a slight improvement. With battery sets a good earth is absolutely essential if good results are to be expected. Take your wire from the set as short a distance as possible to a good contact. with the ground. This can be a cold-water pipe, a buried copper plate, an old washing copper buried, sheets of iron buried, or a pipe driven 6 feet in the ground. Here are some facts which will no doubt interest you:-Moist warm earth is the best conductor; cold and dry the worst. Nothing is to be gained by surrounding or filling the earth with charcoal, but salt and water poured over the earth connection ensures a good contact. The diameter of the earth pipe is of no importance, and when several pipes are used they must be separated by 6 feet or more to be effective; 6 feet is the optimum depth in which they should be driven. Not infrequently electrical interference is introduced through the water pipe. Gas and hot water pipes must never be used as earth contacts, neither must the earth be shared with that of the telephone or electric light. Shifting your earth is frequently a means of lessening power interference.

WE have now completed an installa» tion. Let us return to the aerial and consider one or two of the side- : lines. Multiple wire aerials attract a great deal of attention, and not a few have asked if two wires are not better than one. If separated by about six feet they are, but they will not give anything like double the. stregnth of one aerial. With an aerial of standard length there is nothing to be gained with multiple wires. Where aerials have to be very short extra wire will be of use. Spreaders of hardwood, well painted, should be used to keep the wires apart. The electric mains can be used as an aerial if a special adaptor is used. It is not, however, as good as an outside © aerial. Inside aerials attract a good deal of attention, principally because they are easily erected and are not unsightly. They can be either between the ceiling and the roof, across the room, or round the picture railing. Although very useful where an outside aerial can not be erected, it will not bring in very strong signals. The loop aerial is not very much used, It needs a special aerial circuit, and its action is different from the ordinary aerial, Do not think you can attach the loop to your ground and aerial and binding posts and expect to get results, It must repace the first coil in your set. Loops were compulsory with superheterodyne receivers until an improved model was brought out, which can be worked from an ordinary outside aerial,

Another type or aerial which is very popular in country districts is the Beverage aerial. This is about 850 feet long, but only 6 to 9 feet from the ground, and of ordinary galvanised wire. The distant end is connected to earth through a resistance of about 600 ohms, It is strongly directional and must point in the direction from which the signals are to be received. It brings in very little static, but the stations come in very well. It is well worth trying if one has the room. The directional effect of the ordinary aerial need not be considered-only when the aerial and lead-in are in a definite proportion, which is impracticable in the ordinary installations is the tirectional effect of any importance.