Reception on Ultra-Short Waves

Radio Record, Volume III, Issue 50, 27 June 1930, Page 28

Reception on Ultra-Short Waves

Many Problems Overcome

By

CATHODE

1H have now grown familiar with reception on the oncedreaded wavelengths round about 20 or 30 metres. In fact, the enthusiast who delights in facing and conquering difficulties finds little to interest him in reception on wavelengths which offer fewer troubles than did the broadcast band not very long ago. To such as these we commend as fresh fields for endeavour the wavelengths between 5 and 15 metres. One ean feel tolerably certain of inducing a receiver to work on these wave: lengths but to obtain really successful" results will provide the _ enthusidst with many hours of interesting experiment. It is almost unnecessary to stress the benefit to the community generally likely to result from’ increased knowledge regarding these ultra-short waves; every experimenter helps to increase the sum total of this knowledge. The one great feature which seems to render certain the ultimate commercial use of wavelengths of 10 metres or less is their consistency. Every short-wave listener has had ‘experience of the variation in signal strength experienced on the 20 and 40-mette bands. A station may be heard one night and not the next; it may be heard during day time and not at night; it may even fade right out of | audibility in ten minutes or so. This sort of thing is interesting but annoying. To maintain day and night communication entails changing the transmission wavelength from time to time to suit varying conditions. The reason for these variations in audibility on short waves is now fairly generally understood. The tremendous range of a small short-wave transmitter under favourable conditions is due to the existence of a stratum in the upper atmosphere in a condition of ionisation. This stratum, known as the Heaviside layer, is more or less impervious to wavelengths of the urder of 15-100 metres, and reflects them earthward. Thus the audibility of signals depends on the relation between transmitter, Heaviside layer, and receiver.

Were the height and density of the Heaviside layer constant, no difficulties would be encountered in regard to fading. -In practice, however, these things are in a state of constant change, varying from hour to hour, and particularly as between daylight and dark. Thus any degree of reliability in communication on these wavelengths is rather difficult to obtain.

Wavelengths of the order of 10 metres and less possess very high powers of penetration. That is to say, instead of being reflected earthward by the Heaviside layer, they show a tendency to pierce it and loses themselyes in the vastness of space. At first sight this would seem almost to rule them out as a means of longdistance communication. Yet, in fact, very great distances have been covered’ with these short-wavelengths, even in their present stage of development. It has been suggested that wavelengths of this order "hug" the earth’s surface, either as a result of refraction

by a condition of ionisation existing in the lower atmosphere, or as a result of certain magnetic effects exerted by the earth itself. It is highly necessary that as much data as possible regarding these wavelengths should be brought to assist the solution of these ind other problems, and it is in this direction that the amateur experimenter can prove himself invaluable.

No Radical Changes. N° radical changes from the ordinary designs of short-wave receivers are required for operation on wavelengths between 5 and 10 metres. With careful adjustment, almost any good shortwave receiver of the detector and audio type can be induced to operate successfully, smaller coils being required, of course, to bring the tuning range down. Stray capacity effects are apt to be serious, however, and to obtain reasonable ease of tuning a very small tuning condenser is an advantage. The more elaborate type of shortwave receiver embodying high-fre-quency amplification by means of screen-grid valves or neutralised threeelectrode valves is not a success for work of this kind, 15 metres being about the lowest wavelength on which any amplification can be obtained from the high-frequency valve. More promising results have been secured with circuits of the Armstrong and other super-regenerative types, while there seems to be no reason why a superheterodyne receiver should not be extremely successful. For a commencement, however, there is no doubt that it is wisest to experiment with a simple receiver of the regenerative detector type, to which a stage of audio can be added if louder Signals are desired. The greatest care must be taken in regard to layout to avoid the introduction of stray capacities, but provided this precaution is taken it is usually possible to induce the detector valve to slide into oscillation just as smoothly and gently as on the longer waves.

A circuit diagram of a detector and one audio receiver specially adapted to operation at extremely high frequencies is given in Fig. 1. It will be seen that the tuning coil is centre-tapped, as at this point there is practically no alternating potential, and by connecting the centre of the coil through .a suitable battery to A, any desired negative or positive bias may be applied to the grid of the detector valve without interfering with the alternating E.M.F, in the tuned circuit. Generally ppeaking, the detector operates best Mvith a positive bias of 3 volts ox therekbouts. — An open aerial can be coupled to one | end of the coil through a midget variable condenser shaving a maximum capacity of perhaps 50 micromicro- * farads. If a counterpoise system is employed as earth, this should be similarly coupled to the other end of the eoil. If a definite earth connection is used, however, this mus’ be made to the centre of the coil for the same reasons as are outlined above in regard to the centre-tapping. Th fact that in this circuit both ends of the tuning inductance are at high alternating potential renders it necessary to so adjust the aerial and counterpoise that they have potential variations impressed by the incoming signals at the ends coupled to the inductance Thus the length of either should noty an-odd multiple of a quarter of fhe wavelength, since this would result} in a potential node at the receiver. ‘To secure the best results with this type of receiver, one must.be prepared to experiment with several adjustments, of which the most important are the length of the derial, the aerial coupling. the reaction, and, of course, the tuning. The correct adjustment of the aerial (and counterpoise) coupling is particularly important, and if it is too great, the radiation resistance of the © aerial (which at these short wave- | lengths is of the order of 50 to 80 ohms), will prevent the detector from oscillating, while if it is too small, the signal strength will be substantially reduced. . ’ The type of constructor who will be tempted to experiment on this waveband is not likely to make anything but an excellent job of his layout and wiring. For this reason no Bsn i d layout and wiring diagram A few general hints may not He out of place, however. . Hand Capacity. DIFFICULTY may be experienced with hand-capacity effects if ordinary methods of layout are adopted. To forestall any trouble of this kind, it is an excellent plan to mount the tuning and reaction condensers on a false | panel set three or four inches back from the panel proper. The condensers are then controlled from the dial on the front panel by means of a rod of ebonite or other insulating materigly~ a dry skewer is first rate. If onegor both or the panels can be of metal, this also assists in dispelling the hand capacity bogey. | The tuning condenser should have maximum capacity of about 50 microfarads. A condenser of the midget type having small plates will be best, (Concluded on page 29,)

if Ultra Short-wave Reception (Concluded from page 28,) as at these short wave-lengths ‘the inductance of the plates themselves becomes important; moreover, the field surrounding the condenser. will be smaller and less liable to create losses in the solid dielectric supporting the plates. Similar condensers may alsy be used for the reaction control, and for the aerial and counterpoise coupling condensers, The grid condenser should be fairly large. A fixed condenser of 001 microfarads capacity: will be about right, and¢its reliability and high quality must/oe above question. The most suit able resistance for the grid leak depends to a great extent on the yalve employed, but it will almost certainly be between 0.5 and 2 megohms. For reception on wave-lengths less than about metres, a detector valve of the "Short--path" variety is desirable, as otherwise the time taken by an electron leaving the filament to reach the plate will be comparable with the period of the wave being received. WIRING must be carried:out with great care. Every connection must be stiff enough to resist the tendency to vibrate. It is thus necessary to use wire of about 16’s w.g., or thicker, and: this has the additional advantage of avoiding reactance in the leads as.the result of the appreciable inductance ssessed by thinner wire. The tuning coils are best constructed of 1-8inch or dineh copper tubing, For the 10-metres band a 4-turn coil of 13 inch diameter and 2 inch winding lJength will be required... For the 5metre band a similar coil, but with the diameter reduced to 1 inch, will be most suitable. If the coils are wound with }-inch tubing, they should be very stiff in themselves, If they are found to vibrate, they must be wound on an ebonite former. In any case, they mast be mounted with extreme rigidity, pre ferably on a block of ebonite. The circuit shown ayoids the somewhat difficult problem of finding suitable high-frequency chokes. For the benefit of those who intend using a different circuit, and one necessitating a choke, it may be mentioned that the most effective type of choke at these frequencies seems to be a simple single layer winding of 46 or 48 s.w.g., insulated wire, on an ebonite former of inch ‘diameter. About 50 or 60 turns may be used for a 5-metre choke, the numbér being increased to perhaps 150 oer 160 for reception of 10 metres, There is one thing which may militate against the popularity of the 5-10. metre receiver, and that is the dearth of ’phone stations on this band. The experimenter who can read code will be quite at home, however, and, after all, it is only a matter of a little time before modulated transmission, perhaps even broadcast stations, become quite common on the ultra-short waves.