Short Wave Reception

Radio Record, Volume III, Issue 23, 20 December 1929, Page 31

Short Wave Reception

Its Present Simplicity

[ oomseeeeriaiaiene Not many wireless enthusiasts rea- * lise how exceedingly simple long--distance 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 -not particularly suitable for theework, 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 handles 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 to-day, due largely to the introduction of the Rein--artz circuit, which in one form or another is used in neatly all short-wave sets. In this cirenit 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 a(tirely eliminated, at any rate on ve-lengths down to 20 metres. Below this. limit the frequencies involyed are of such appalling magnitude that body capacity effects are still apt to manifest themselves to some small extent. These can, however, be got rid of completely simply by setting the tuning condensers some 6in. 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 enor‘mously to the present-day simplicity of short-wave reception as the introduc‘tion, of the slow-motion dihl, which enables exceedingly fine adjustments of the~veriable condensers to be made, even by those who have not what may be termed as "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 désired 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. The receiving set required for shortwave reception is exceedingly simple. It consists of nothing more than a. detector valve, with the previouslymentioned 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 audiofrequency: stage. _ In the modern short-wave set there are only two controls that require simultaneous adjustments; the grid tuning condenser of the detector valve, and the reaction. condenser. The former haying 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 re-tuned. 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 reception 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. -_