SHORT WAVE RECEIVERS

Evening Post, Volume CXIII, Issue 133, 9 June 1927, Page 14

SHORT WAVE RECEIVERS

The recent series of short-wave transmissions, part of a wide-spread .endeavour to extend the range of broadcasting to the utmost, has naturally aroused much interest in. the use of shortwave receivers, and no doubt manywireless enthusiasts are providing themselves with, suitable instruments. It is safe to, say that most of. the short-wave. receivers now in use could be greatly improved, by a. slight change in the valves-of their .tuning coils and condensers^ One is constantly meeting with the statement that short-wave sets are very difficult to tune. The usual expression is that it tunes extremely sharpjy, and one effect, of this critical action is that a very delicate control of the tuning dial is necessary if stations are to be found. The real reason for this defect—and it is a defect—is. that the tuning condensers used are far too large. Unfortunately there are few regular tuning condensers on the market of a small" enough capacity to be really convenient. If we consider short waves from , • the point of view of frequency it. will bo easy to see why a small capacity condenser is better than a large, one. The average broadcast receiving set is designed to receive wave-lengths from about 200 metres to 550 metres. Few modern sets, have a larger range than that. Such a set, with condensers of :up-to-date type, with a dial having 100 divisions, will tune a carrier wave out of audibility in about one division, and for most people: that is quite critical enough. A condenser of, .0035 mfd, and. a standard coil of say, 65 turns three inches 'in diameter in a length of about three inches, will give that Tange. Now, if we reduce the number of turns and retain the same condenser we can reduce the wavelength to which the circuit will tune to any degree we like. Suppose we bring it down to the neighbourhood of 30 metres—a wave-length of considerable interest just now. This will be

when there are from eight to a dozen turns on the coil; the exact number depends on several factors. Suppose, however, that the set now tunes from 20 metres to 40 metres. This is a range- apparently less than that of the broadcasting range. .. But .in actual f fact, so far as its capacity for holding" a number of stations is concerned, and in "criticality'? of tuning, the 20-40 metre band is over seven times as wide as the 200-550-metre band. If the short wave tuner ran. from 20 to 55 metres, and so had the same proportionate wave-length range as the broadcast tuner, it would really have ten times the tuning range. That is to say, the b.c. set could accommodate 50 different broadcasting waves without interference, but the short-wave set would accommodate 500. And as a corollary, each would be tuned clean out j with a movement of the dial of one-tenth of a degree. Obviously this is far too critical for comfort. By far the best short-wave tuner is one which has a condenser of far smaller capacity—say .00005 mfd. This can best be obtained by taking one of as small capacity as can be had. and removing some of- the plates. It is not a" job for i clumsy fingers, and it must be done properly if it is done at all. There is, however, another and simpler method which will work almost just as well in practice, and will be very much better than using too :aige a condenser. If a; fixed condenser of very small fixed capacity—say .00006 mfd) —is placed in series with a tuning condenser of .00025 mfd, the maximum capacity of the combination will be approximately .00005 mfd, and the minimum about one-third of that value. With an appropriate-tuning coil, this will give a range of say 20 metres to 33 metres. • . "■■ ■'■'■: The difficulty about using a tuning condenser of really small capacity is that it is hard to make a coil which will tune to a required band of wavelengths; but if one is afraid of a: little extra work in this respect, one must pay the price of going.on with a.tricky set. The easiest way out is to; use plug-in coils, with the plugs or pins well spaced in good insulation,' and to make several coils of different sizes, and try. them, either in actual reception or, if one is able to: do so, by means of a wave meter. Theij the.coils most convenient for the various short-wave bands may be selected and labelled. At a. sacrifice of appearance, a coil can be "tuned" considerably by altering its shape, if it is not fast on a 'solid former. The turns can be brought' closer together to increase' the wavelength, or spread out to decrease it; but the correctly-wound :coil is, of course, the most satisfactory.. ' ; It will certainly be fourid^ if tho small condenser is used, that 1 the 'difficulty of using a short wave set i's'vastly reduced, and the extra trouble involved is -.veil worth while". It'is true that it is necessary to change the coils to make a change from 20 to 30, or from 30 to 40 metres, for example; but the tune thus lost is far less irritating than the difficulty of adjusting a dial to a microscopically exact setting. The Auckland- station produced an unusually good programme on; Tuesday night. As the Cbristchurcn 'and "Wellington stations were':ailenty it was: probably keenly appreciated by all who were able to hear it. Unfortunately it was interrupted by particularly loud morse interference by a ship station transmitting on 450 metres..'This is, the writer understands, about the only part of the world where ships are allowed to use a 450 ihtetre. wave-length', and it would be interesting to.kmhywhy the practice is tolerated here. For , broadcast listeners to "get. on-their hind legs" about interference by ship traffic to the extent of^wahting it suppressed is absurd, but it: is' entirely proper to object to the-Use of 3a savagi* spark set on 450 metres close to tfye | coast during broadcasting hours. '