ON SHORT WAVES

Sun (Auckland), Volume III, Issue 855, 26 December 1929, Page 7

ON SHORT WAVES

SIMPLICITY OF SETS DISTANT RECEPTION Not many wireless enthusiasts realise how exceedingly simple long-dis-tance reception upon the higher frequencies is at the present time. Five or six years ago, when short-wave circuits were still in their infancy, and when the components available were uot particularly suitable for the work, it was difficult to tune in to any station with a wave-length much below 100 metres. Body capacity effects were so pronounced that those who were then attracted by short-wave reception were commonly believed to do their coarse tuning by means of extension bandies, several feet in length, and to make their fine adjustments by the simple, if not very restful, process of moving from point to point in the room in which the set was installed. ■ Matters are very different today, due largely to the introduction of the Reinartz circuit, which in one form or another is used in nearly all shortwave sets. In this circuit one set of vanes in both the grid tuning condenser and that which controls reaction is at earth potential. By so arranging the circuit that the low potential vanes are those connected to the spindles of the condensers body capacity effects may be entirely eliminated, at any rate on wave-lengths down to 20 metres. Below this limit the frequencies involved are of such appalling magnitude that body capacity effects are still apt to manifest themselves to some small extent. EBONITE CONNECTORS These can, however, be got rid of completely simply by setting the tuning condensers some Gin back from the panel, and by using ebonite connectors between their spindles and the tuning dials. There are actually so many fine transmissions above 20 metres that a set without this particular refinement will enable reception over amazing distances to be obtained without any difficulty. One factor that has contributed enormously to the present-day simplicity of short-wave reception is the introduction of the slow-motion dial, which enables exceedingly fine adjustments of the veriable condensers -o be made, even by those who have

not what may be termed “wireless hands.” For most ordinary purposes dials with a gear ratio of eight to one or 10 to one will be found excellent, but if finer adjustments are desired either geared condensers or geared dials can be obtained with ratios up to 200 to 1. For reception down to 12 metres it is not necessary to use anything more delicate than an eight to one slow-motion dial. SIMPLE DESIGN The receiving set required for short-wave reception is exceedingly simple. It consists of nothing more than a detector valve, with the previously mentioned circuit, and two stages of audio-frequency amplification, which should be so arranged that one or both can be used at will. This can be done by means of either switches or jacks. It is best to search for stations with the headphones, using only one audio-fre-quency stage.

In the modern short-wave set there are only two controls that require simultaneous adjustments; the gridtuning condenser of the detector valve, and the reaction condenser. The former having been set at zero, the reading of the latter is gradually increased until a slight rustling noise indicates that the set is in gentle oscillation. The reading of the grid tuning condenser is then slowly raised, the set being kept gently oscillating by moving when necessary the knob of the reaction condenser. As soon as a carrier wave is heard the reaction coupling is loosened and the grid of the detector valve is retuned. The most sensitive condition for the set is just below the point of oscillation. On no account should an attempt be made to tune to the silent point between squeals. Once speech or music has been heard signal strength is brought up by small adjustments of both condensers, care being taken to see that the set is not actually oscillating. If the signal is now of good strength, the second audio-frequency valve may be thrown into circuit and a change made from headphones to loudspeaker. On the short waves one is seldom bothered by heterodyne or Morse interference. Enormous distances can be spanned and -eception from most distant countries obtained. At the present time reception on the broadcast band is to some extent discounted by the prevalence of seasonal interference, the short waves offer a fascinating opportunity of receiving broadcast programmes from almost every country in the world.