The Effect of Direction

Radio Record, Volume II, Issue 16, 2 November 1928, Page 27

The Effect of Direction

Important Considerations in Erecting the Aerial ©

ANY a wireless listener is quite unable to select from a number of possible positions the most favourable one in which to suspend the aerial. For many town and suburban dwellers it is a case of erecting an aerial in a limited or restricted position or not at all. At the same time the more fortunate amateur will generally find that there exists around his residence several possible positions in which an aerial can be erected. There is no doubt that direction has a great deal to do with reception, and in considering the qualities of different outdoor types it will be found that there are merely two, the IL and the T. The L Type. (THE ever-popular L type-that is, with a lead-in at one end-attains directional properties in consequence of the fact that the "fall" ends of the transmitted radio waves tend*to travel through space more easily than they do at lower levels. — Therefore at distances of several miles from a broadcasting station the upper portions of the waves are slightly in advance of the lower parts. The waves, in fact, bend outwards, roughly in the shape of an inverted I. The ideal state of affairs would obtain when the IL shape of the waves coincided with the L shape of the aerial. This, however, is never attained, there being only a tendency to do so. From these facts its becomes evident that. the inverted L type will always receive best when it points in the direction in which the waves are coming. that is, when the lead-in of the aerial is taken from the end nearest’ the broadeasting station.

This -type of aerial,- which points at an angle away from the direction of travel waves, is less effective in directional properties, its effectiveness decreasing as the angle which the aerial subtends to the direction of the waves increases. Thus it will be seen that an inverted L type aerial which runs exactly at right angles to the direction of travel of the radio waves is the least efficient of any, so far as directional properties go. The T Type. (SOMING now to the T type aerial, which is decidedly less popular than the former type, it will be apparent upon reflection that a T type aerial really consists of two inverted L type aerials combined together. Thus an aerial of this nature cannot be as directional in properties as a small inverted. L-aerial can. A T type aerial is directional in two opposite directions at the same time. Thus if the T-aerial runs due north and south it will receive most efficiently stations which are situated at those points of the compass. If, on the other hand, the aerial runs from east to west, the northern and southern stations will come in least efficiently. The directional properties of an overhead aerial tend to increase as the proportion of the length of the downward lead to the horizontal portion of the aerial is increased (that is the higher and shorter the aerial), and in some instances this effect can be very marked, especially in the reception of distant stations. Other Considerations. NATURALLY at very close ranges to a. broadcasting station the lie of

an efficiently erected aerial system does not make much appreciable. difference to the strength of reception, always assuming, of course, that a reasonably good set is employed. It is when the aerial system is situated a distance beyond six or seven ‘miles from the station that these directional influences of an aerial come most into play. More especially is this directional effect a feature with crystal and single vyaktve sets. On establishing a receiving station with a weak receiver some (istance from the. transmitting station, every precaution to obtain maximum strength should be employed. It is then that the direction of the aerial is of most importance. Thus it is that the amateur who is erecting a new aerial, or who has cause to feel dissatisfied with his present one, should take into consideration the selection of position. The directional properties of an overhead aerial. become really more important in those cases, in which it is de-

sired to obtain constant and reliable distant reception from overseas stations. For not only can the reception strength of the distant station be augmented considerably by a judicious selection or re-selection of an overhead aerial’s position, but the nearby .station can also be more easily tuned out by such means. PARE, for instance, a case in which an aerial runs north to south, the local station being situated northwards, and the distant overseas stations being located in an easterly and westerly direction. In such a case, if the aerial were taken down and reerected so that it ran east and west. the lead-in being taken from the end nearest the stations desired, say the west, the distant stations would be received more favourably, whilst, at the same time, the local station could be eliminated with least difficulty. NATURALLY, it is not always possible to redesign an aerial system

so conveniently, as has been recommended. Nevertheless, some sort of a compromise between having an aerial directional to the local station, and directional to a required distant station, can generally be made. , At the same time, the actual collecting efficiency of an aerial system should not be sacrificed for its directional qualities. A table is appended, suggesting directions of aerials for the different stations received in New Zealand. nu, New Zealand stationsWhere these four are chiefly required, a fT aerial running from N.E. to S.W. bb. Australian stations. The best results will be obtained from an inverted IL aerial running

from N.W. to S.B. with the lead-in at the N.W. end, 7 c. American stationsAn inverted L, running N.H. to S.W., with lead-in in N.BE, end. i. Japanese stationsAn inverted L, running N. to L. Leg in N.

WHERE a powerful receiver is used and the Australian and New Zealand stations can be brought in without having to study directional effect, it would be advisable to build the aerial to bring in the Americans or the Jans. A compromise is often made by running the aerial in two directions by the use of a third mast. In this case, the lead-in should . come from the centre mast.