RADIO AND ITS RECEIVERS

Sun (Auckland), Volume III, Issue 765, 11 September 1929, Page 16

RADIO AND ITS RECEIVERS

Conducted for THE SUN by

R. F. HAYCOCK.

AMATEUR BROADCASTERS Troubles of the Listener THREE MAIN DIFFICULTIES DIFFICULTY is often experienced by listeners in receiving transmissions from amateur stations. As these stations provide programmes of high quality, available at times when the main stations are not operating, they prove a useful addition to the broadcasting services. The difficulties of reception usually fall into one of three groups. The first is the complete inability to hear the stations working, which is due to the receiver in use being incapable of being tuned to a sufficiently short wave-length. The second difficulty is separating the transmissions of one station from another, while the third is an unstable receiver which tends to break into howling when tuned to the short amateur wave-lengths.

In most eases it will be found that the first difficulty—that of restricted tuning range—is encountered only in home-made receivers. The majority of commercially made sets are designed to tune down to a wave length of 200 metres. This range is insufficient to obtain all the amateur transmissions. The most effective method of extending the range of a receiver is the removal of a few turns of wire from the tuning coils. If the set be home-made, this is not difficult. The amount by which the tuner should be cut down will depend on individual sets. It can be determined by “the trial and error method.” It should be borne in mind that no Australian broadcasting station uses a longer wave length than 3AR. If 3AR can be heard after the tuning condenser has been adjusted to within a degree or two of its maximum setting, all other main stations will be within the tuning range of the set. In removing turns from the tuners, the process should be continued a few turns at a time until 3AR tunes in right at the maximum position on the dials. About the minimum settings, it will then be found that the set will tune to a very much lower wave length than formerly. If this remedy be not entirely effective, the tuning range of the set can be increased still further by using a tuning condenser of slightly greater capacity than the one in the set, and still further reducing the number of turns on the tuning coils. In many cases, it is possible to extend the tuning range of a receiver downward by either reducing the dimensions of the jaerial or by placing a variable condenser in the aerial or earth wire to the set. SEPARATION DIFFICULTIES The difficulties of separating one amateur station from another may be of two distinct kinds. The amateur stations work very much “closer together” in terms of wave length than the larger stations. They will be found crowded together on the lower end of the condenser scale In such a way that there is a separation of not more than two or three degrees between them. The first tuning difficulty arises simply from the lack of a fine adjustment on the tuner. If the set be not equipped with some form of vernier adjustment, it will be found difficult to obtain the very precise dial setting necessary to tune in the desired station and to eliminate interference.

The remedy, of course, is to fit a vernier dial, which can be bought for a few shillings. A useful aid to fine tuning when no vernier dial is used is tile fitting to the ordinary dial of a long light wooden handle arranged at right angles to the condenser shaft This handle can consist of a thin slip of wood cut from a cigar-box or from some other light material, and a way of clipping it to the dial by means of an elastic band can easily be devised. It can be removed when not required. If the handle be long enough, it will give a very fine vernier effect, because quite a large movement on its end turns the condenser through only a small angle. LACK OF SELECTIVITY Tile second difficulty of tuning into one station and eliminating others arises frofn inherent lack of selectivity in the tuning coil in the receiver. The

trouble can be reduced usually by using either a smaller aerial or by inserting a fixed condenser in the aerial wire. The equipment of the receiver with a wave-trap will also help. The most effective method of correction, however, is the improvement of the tuner itself. The trouble will usually bo experienced only in sets where the so-called direct coupled aerial system is used. In such a set, the aerial and earth wires are connected to the main tuning coil in the set. It will be found that greatly improved selectivity will be achieved by providing an auxiliary aerial coil. For the amateur stations, this coil can consist of about 15 turns of cotton-covered wire wound on to a cardboard former of such a diameter that it can be slipped inside the main tuning coil. The aerial and earth wires are disconnected from their normal terminals on the set and connected to the ends of the auxiliary coil. By sliding the auxiliary coil in and out of the main tuner it will be found that an adjustment can be reached when the amateur signals are very strong, but at the same time quite free from interference. Instability, due to the oscillation of the receiver, is a common fault on the shorter wave lengths, but there are many ways of eliminating it. Probably the most simple is to reduce the voltage which is supplied to the plate of the detector valve from the high tension battery or eliminator. When this is done, it will be found that readjustment of the reaction control becomes necessary. Another but less efficient method of checking oscillation is to reduce the filament temperature on the detector valve.

If the receiver incorporates a radio frequency amplifier, considerable stability can be obtained efficiently by reducing the filament current on the radio frequency amplifier valves. - Howling, which sometimes occurs on the shorter waves, can be checked by using a grid leak resistance of reduced value. For instance, if the set is fitted with a two-megohm leak, this should be slipped out and replaced by a leak having a resistance of half a megohm or one megohm. 3LO’S ORCHESTRA The personnel of the new National Broadcasting Orchestra for 3LO. Melbourne, has been announced by Mr. Stuart F. Doyle on behalf of the directors of the Australian Broadcasting Company, as follows:—Directorgeneral of music, Professor Bernard Heinze; director, William G. James: conductor, Percy Code; leader violin, Edouard Lambert: first violin, Bertha Jorgensen; pianist, Cecil Fraser; second violins, Guilford Bishop and Alfred Ackland; viola, Dorothy Roxburgh; *eello; Tasrna Tiernan; bass, G. C. Fraillon; flute, J. Barille; oboe Mignon Weston; clarinet, A. Anderson; trumpets, J. Geary, Walter Smith; trombone, J. Shackles; horn, L. J. Davis.

The use of a counterpoise earth instead of a direct earth connection is often of assistance in eliminating interference from outside sources, such as electric railways, tramway systems, power mains, electric motors, etc.