RADIO AND ITS RECEIVERS

Sun (Auckland), Volume 1, Issue 24, 20 April 1927, Page 7

RADIO AND ITS RECEIVERS

AERIALS PART 2.

By

"RAYTHEON.”

Continuing from the general considerations of last week, we now come to the different main types of aerials used in conjunction with broadcast receivers. The commonest forms in use are the inverted “L,” the “T” and the single straight-line antenna. The inverted “L” receives its name from its shape, see Fig. 1. The wire or wires are suspended between two masts or other supports and the leadin is taken from the end nearest to where the set is to be located. If the distance between the supports is short, say 50ft., use two wires in the horizontal portion spaced not less than 3ft. apart. This makes one of the most satisfactory aerials in a short space. However, this type of aerial is slightly directional, but it is an advantage when receiving distant stations in a given direction, as it receives the signals with better intensity when the lead-in points in the direction of the transmitting station. Where a single wire is used there will be no need to make a joint at the insulator at the end where the lead-in is taken down (if the wire used in the horizontal is equivalent to that required by the underwriters’ regulations). Instead, it may be looped round the insulator, twisted twice round the horizontal wiie and led straight to the arrester, care being taken to keep it away from the building as mentioned later. Under such cirsumstances not much more than 30ft of insulated lead-in cable should be required from the arrester to the set. The horizontal portion should be about 35ft above the ground, although it will be better at 50 or 60ft. The “T” type, see Fig. 2, is probably next in common usage. As the title implies the lead-in is taken from the centre of the horizontal portion. If of a single wire type the total length of this part should be about one hundred feet. It is important that the lead-in is taken from the exact centre, as if not there may be trouble - in securing sharp tuning. If two or more wires are used in the horizontal these must be spaced at least two feet apart. In this case also care must be taken to see that the joints are properly soldered,, mechanically and electrically. The height of this type applies as with the inverted “L.” If insulated cable is used as lead-in from the horizontal to the arrester, and from the latter to the set, about 60ft will be required. The straight line aerial usually has a short lead-in coming almost direct to the set, see Fig. 3. As a rule this type is used where there is only the space for the erection of one mast, or one support available. Again, it usually has the advantage that the wire can be run in one piece, thus avoiding any jointing whatever. Only the necessary insulated cable for connection from the arrester to the set will be required. ERECTION With the site chosen and assured that it will be clear of all metallic surroundings as well as trees, the masts or supports are fastened rigidly and guyed tightly. Make sure that when in place between these the aerial will not be across or under power-line, lighting, or telephone wires, or across a roadway. The actual aerial or horizontal portion is now swung between the two insulators fastened to the supports or halyards. It is as well to fit halyards to the supports so that the aerial can be raised and lowered at will or when necessity requires. If the mast has to be lowered each time it will become annoying. The lead-in may be a continuation of the same wire, or else soldered to the horizontal as may be required. One is as good as the other, providing the joint is properly soldered and taped. In the “L” or straight-line type the lead-in should be taken away at the insulator, and not a foot or two away. By doing this greater strength is given to the lead-in and it is not so likely to break away by constant swaying. In bringing the lead-in down to the window, care should be taken to see that proper insulation is used. If the building is of steel or concrete it is advisable to keep the lead-in away from the walls, at least three or four feet. In fact, it does no harm with any type of building. With a wooden building porcelain bobbin or the usual stand-off insulators can be used. AT THE ARRESTER The lead-in is connected first to the lightning arrester (the latter being mounted outside for preference close to the window or lead-in point, sheltered from the rain). Good heavy terminals are usually provided on these devices so the connection can easily be made. A pipe should be driven into the ground below the arrester about six inches away from the building, and to this a ground clamp secured. A lead is now taken (No. 14 wire or equivalent) from the other terminal of the arrester to this clamp. This completes aerial and lightning protection. The receiving set having been placed in its permanent positon, the aerial terminal of arrester is connected to the lead-in terminal or strip, protruding through the wall of the building. That portion inside the building is now connected to the aerial binding-post on the set. If this cable is run along the upper part of the walls or picture moulding and kept in place with insulated staples it will look much neater. A ground wire is now taken from the set to a ground clamp fixed on to the nearest waterpipe. Failing a waterpipe, a special earth is usuallj' - put down. Avoid where possible connecting the earth wire of the set to the clamp which is connected to the arrester. INSULATORS The difference between glazed porcelain, glass, or pyrex is hardly worth considering, as long as the insulator is well designed. If an insulator is not well made it is as bad as none at all. In buying see that they are free from detrimental chips and cracks. A short one, well made, is as good for receiving purposes as a large one, although as a rule the latter are made stronger in proportion. One error that many amateurs make is to put too many insulators in their aerial system. One good insulator at each end of the aerial is all that is required for a receiving antenna. ATTACHING GROUND CLAMP When attaching the ground clamp to the pipe, the latter must be cleaned free of dirt, enamel or paint, and the clamp must be fastened as tightly as possible. A few layers of tape wrapped round the clamp after the connection has been made will protect it from corrosion. Periodically examine your aerial and see everything is O.K. Clean off any dirt which may have collected on the insulators. Sven glass insulators will collect dirt which will cause a leakage. Next week it is hoped to give a short article on “Grounds,” which really comes in natural sequence to the one already given. U.S. BROADCASTERS INCREASE POWER According to the “U.S. Radio Service Bulletin” just to hand, the following stations have recently increased their power to 5,000 watts: —KFAU, Boise, Idaho; KOB, State College, N. Mexico; WAIU, Columbus, Ohio; WAMD, Minneapolis, Minn: WCAR, San Antonio, Texas; WIBO, Chicago, Inninois; WRBW, Buffalo, N.Y.; WRHM, Minneapolis, Minn.

Station WOK. Homewood. Illinois, has increased its power to 6,000 watts and wave-length to 410 metres. As advice is received from time to time of station changes across the Pacific, readers will be advised, as with these powers doubtless several of them will be heard this winter. The writer understands also that several American stations have matters under way for more special New Zealand broacasts this winter. PREVENTING CORROSION OF ACCUMULATOR TERMINALS It will often be noted that when storage or acumulator type “A” and “B” batteries have been left in circuit for some time, a coating of verdigris usually forms on the connection terminals due to chemical action within the batteries. Such a corrosive coating causes poor connections and flickering valves, and also grating noises in the set. By giving the terminals a light coating of vaseline this trouble may be prevented. NOTES BY THE WAY If you are by chance experiencing interference from your neighbour’s receiving set try placing your aerial at right angles to his. Although this may not completely eliminate the trouble it will quite probably minimise the nuisance. In placing “B” batteries on shelves, in a cupboard, or in the set cabinet, place them in the position intended by the manufacturers. Thus a vertical type should be kept in the vertical position and the flat type should be kept in that position. Placing the batteries on their sides is likely to result in a considerably shortened life. This is due to the fact that the semiliquid contents of the cells is liable to leak out and so cause a short circuit. It is surprising that experimenters here have not made a greater use of the toroid type of inductance coil, but prefer to keep to the older style. Probably the coil is a little more difficult to make, but it has quite an advantage over the proverbial cylindrical type, in that the “field’' is more confined with therefore less likelihood of interaction between coils caused by “stray” fields. Probably the lack of reliable data is the main reason. CORRESPONDENTS’ QUERIES “Dry Cell/' Parnell.—Has a fourvalve regenerative receiver and complains that he is troubled with crackling noises every few seconds, when receiving Australian stations or even 3YA, Christchurch. The trouble, however, is not apparent when listening to IYA or when the aerial is disconnected. (Ans.). It is quite clear from our correspondent’s letter that his trouble is one with which we all have, unfortunately, to contend, and which is proverbially known as “static.” The fact that it is not heard when the aerial is disconnected confirms this. The fact that it is not noticed vrihen receiving IYA is, of course, due to the fact that the proximity and power of the station does not necessitate working of the set to its most sensitive point, and, consequently, weaker interfering signals are inaudible. On the other hand, for the reception of distant stations the regeneration control is brought into play in order to build up the signals, with the natural result that the weak interfering signals caused by “static” are also proportionately increased. If the crackling is very severe it may point to a leakage in the lighting or power lines, which may be near to the receiver, rather than the usual atmospheric discharge.