RADIO NOTES

Dominion, Volume 23, Issue 284, 27 August 1930, Page 7

RADIO NOTES

By

“ETHER”

Query Corner. “Fusing Point” wants to know if it is possible to have new filamentsi inserted in valves that have burnt out. Yes, this is quite possible. In the days when valves cost thirty shillings each, or more, it was a quite popular form of rejuvenation. Many firms s?t up factories in those days to do this job, and made a success of the operation. The valve was placed in a special machine, and the glass bulb was cut neatly in two by means of a revolving diamond. The new filament was carefullyinserted, the two halves of the glass bulb were fused together, the valve evacuated, bombarded, and returned to the owner. The cost worked out at about ten shillings, in some cases less. ’ . “Careful” says: “I have got a piece of old water pipe about six feet long, and I am thinking of hammering it into the ground for an earth. Would it be suitable?” Yes, provided it is hammered into damp soil. The joint made by the wire should be soldered if possible. “Home Made” is undecided what method of coupling to use for his loudspeaker. “Which is preferable, a transformer or a choke and condenser?” There is not much to choose between the two. However, a choke coupling shunts all the audio frequencies away from the batteries, or eliminator, through the loudspeaker to earth. It thus avoids all back couplings and reduces the chances of howling. Provided a condenser of at least 2 mfds., or if possible 4 mfds., is used and the speaker is connected from earth to the condenser every satisfaction should be given by the arrangement. * “Mimic” possesses a moving coil loudspeaker. He asks if anything better has been invented yet. This is a debatable point. Some say that the condenser speaker is better, as it deals with sudden sounds such as clapping or the firing of a gun more realistically. This type of ’loudspeaker requires a direct current voltage of something like 400 to work it ’properly, in addition to the usual amplifier voltages. Theoretically it gives better response and possibly in the end it will supersede the dynamic of moving coil speaker. “Eliminator” is anxious to measure the voltage output of his eliminator. “I understand a special voltmeter is required, Why is this?” Eliminators generally have a fairly high internal resistance. If a voltmeter taking as much as or more current than the valves is used the eliminator output will drop. The best way to measure the output is with a voltmeter that takes only a fraction of a milliamp for full deflection. These meters may be but they are expensive. An indirect method of getting at the correct voltage is to measure the current passed by the plate of the valve in question. If the resistances in the eliminator are known the voltage delivered by the eliminator may be calculated by multiplying the current in amperes by the eliminator resistance in ohms. Another way is to connect up a dry battery” until the valve passes the same anode current. The voltage of the dry battery then will be the same as that supplied by the eliminator.

ECHOES FOR AIRMEN Radio Altimeter Radio has rendered still another signal .service to aviation. “It is signal” in two senses of the word. For the last few years one of the most persistent of the problems which have faced airmen has been that of knowing exactly their height above the land (states “Popular Wireless.”) During heavy or fogs, as well as during night flying, the peril due to this uncertainty has been very grave. Various instruments have at one time or another been devised to overcome the difficulty, but in no one case did it appear that complete satisfaction was given. At last, working on the theory that the interval of time required for a radio impulse to travel from an aeroplane to the earth and back again to a receiving set on the plane might afford a solution, Dr. E. F. W. Alexanderson, one of the ablest of America’s research engineers in the service of the General Electric Company of, New York, set about making experiments with a series of wireless-echo altimeters. ' After many months of patient investigation and ceaseless experimenting, Dr. Alexanderson has at length devised an instrument which appears likely to meet the 'necessities of the case. It is a smlill recording instrument which can conveniently be mounted on the cockpit panel of an aeroplane in full view of the pilot. It consists of a meter on which earth distances up to 3000 feet may be recorded. It is in the lower regions, however, that the greatest danger lies, and it is in the recording of levels down to 50 feet that the new device has proved most serviceable. Three coloured lights—green, yellow and red—indicate not "go,” “caution,” and “stop” as heretofore, but elevations of 250, 100, and 50 fe”et respectively, and the flashing of one of these lights gives the pilot immediate warning of his position with, relation to the ground. Accurate Measurements. Because the . time limit between the outgoing and the reflected radio impulse is so short, radio waves travelling with the speed of light, an indirect method of making such measurements was adopted by Dr. Alexanderson. In his experiments he used an oscillating receiver, one of the type which sends out a wave which may be picked up on other receivers as a squealing note or beat. The echo or reflected signal was picked up on the same receiver which sent out the wave. Dr. Alexanderson then discovered that , every time the aeroplane changed altitude by half a wavelength, a whistling note went through a complete •tone cycle, from low pitch to a high pitch and back again to a low pitch. By counting the cycles of the tone it was possible to measure the altitude, the measuring stick being one-half the wavelength of the aerial oscillator. Thus it comes about that by means of the meter, graduated from 3000 to 200 feet, the pilot may read his altitude within those limits at any time. The echoes indicating height are periodic, becoming stronger as the plane approaches earth. The lieriodic character of the echo, and the chance that the pilot would not see the instrument at the instant an echo was recorded, presented a problem which Dr. Alexanderson met by developing a memory meter. In this instrument the echo is recorded as altitude when it occurs, and the meter continues to hold that reading until a stronger echo indicating a lower altitude occurs. In approaching the earth the memory meter gives a continuous indication of altitude.

If depth sounding is desired when climbing, in which process the echo is becoming weaker, a push-button may be used to eliminate the memory features of the meter, and each succeeding rending is an indication of the next echo. Thus a depth-sounding may be taken at any time during the flight, whether the pjane is ascending or descending.

LIFE OF A SET ; Things That Matter The life of a radio receiving set has never been determined definitely, though many unfounded conjectures have been made by persons not thoroughly familiar with its construction (says a writer in “Radio Engineering”). Such predictions as were made varied anywhere from two to five years, none of them correct when considering a well-designed and efficiently constructed receiving set. The life of a radio set, say engineers after a series of tests to determine this factor, should, under ordinary conditions, at least equal the human span. This statement is based on the results of careful investigations conducted at a factory in RochesFer extending'over a period of several years. The modern receiver, the report reveals, is subject to mechanical wear at only three points, which are: the bearings id the variable condensers; the volume control unit, and the. “on” and “off” switch. While the parts that are subject to mechanical wear are Usually designed to be easily replaceable in regular servicing operations, each unit is selected by the aid of “breakdown” tests to give a' lifetime of service. These tests operate the moving parts of the apparatus hundreds of thousands of times, which reduced to years of operation represent a lifetime of uninterrupted service. The problem of deterioration by moisture entering the radio receiver is' Of major importance in calculating the life of the apparatus. Most well-made sets, for instance, are completely protected from the detrimental action of moisture by all coils and capacitators being sealed in metal containers, and through the application of corrosion resisting finishes to the metal work. Furthermore, there afe no liquids or electrolytic elements in the construction that'might change with time. ’ . I( . Of major importance also is the tactor of safety” provided in the electrical insulation and dielectrics employed in the chassis and the loudspeaker. The highest grade of receivers use a factor of safety that insures” against breakdown for all extremes of operation. ... /Another item of importance in the life of a receiver is the ability of the various resistors to withstand adverse climatic conditions. It has been found by field tests that wire wound resistors, for example, must have a conductor size adequate to withstand the mechanical strains of continued heating and cooling, as well as a vitreous enamelled covering over'the wire to seal against corrosion and' other damaging effects of moisture. The .wiring of the receiver merits particular consideration with regard to soldering, as long efficient life, of the receiver depends on the materials used in this operation. Soldering' Troubles. While quick and secure soldering can be done by an inexperienced worker, when so-called “acids” or soldering pastes are used, it was early found in telephone apparatus construction that these kinds of soldering fluxes introduced troubles at a later time. Some of these fluxes are conductors for electricity and by creeping along the apparatus terminals or wires cause “cross-talk” or defects in operation. In other cases the paste collects conducting dust, which in time causes electrical trouble. More than thirty-five years ago one well-known company learned that rosin flux could be used with safety in wiring the most intricate telephone, circuits without danger of defective insulation at a later date. Some of these early switchboards are still in use after, 30 years of uninterrupted service. This lifetime experience with satisfactory materials has been extended into the radio receiver field by this manufacturer. Of course it is unreasonable to expect a radio receiver to operate efficiently throughout a lifetime without receiving a reasonable amount of attention, any more than one would expect an automobile to run indefinitely without adequate service. Vacuum tubes are among the most important considerations in this respect. Their occasional replacement should be practised if the maximum efficiency is desired at all times. Even one slightly defective tube will have a detrimental effect upon the operation of the set as a whole.

VALVES OF FUTURE Two Important Factors TUNING COIL RATIOS Present-day valves are about five times more efficient than their ancestors of only four , years ago. Amplification has gone up and internal impedance has gone down. These two things always fight one another for first place in the make-up of a valve. It has been possible to obtain valve amplifications as high as modern valves for a number of years. But hitherto any increase in amplification when put to practical use have been swamped by this increase in impedance. The reason why a modern valve is so superior to. its predecessors is not its amplification but its' lower impedance. A valve with an impedance of 200,000 ohms and an amplification of ,say, 50 would be a far worse valve than one with the same amplification,and an impedance of 50,000 ohms. The latter valve when used for high frequency amplification would give results vastly superior to the former provided correct coils ‘were used. It is just as well to remember this impedance business. There is a tendency for many pople to worship a valve ampli; fication characteristic and forget everything else. A case in point may serve as an example. A screen-grid valve has been produced with an impedance of 400.000 ohms and an amplification of 1500. This sounds delightful, but do not imagine fqr one moment that you will obtain anything like an amplification of 1500 in practice. The actual amplification obtainable will (depend on the coil used. The best coils made do not have an impedance, when tuned by a condenser to a given wavelength, of more than 400,000 ohms. Most coils fall far below this and vary between 100,000 and 300,000. Using the best coil possible wound with litz wire, this valve in practice, using a 1 to 1 radio transformer (the best for this impedance), will not give an amplification of more than about 750. With more ordinary coils the amplification falls to 300. If only the impedance of this particular valve could be lowered to say 100,000 we should find radio designers dancing with joy. Practical amplifications, using a suitable step-up radio transformer of 30r4t0 1, of 5000 or from one valve could then be obtained fairly easily and two stages of radio frequency would give us results good enough for anybody. Slowly year by ♦year impedances are coining down and amplification is going up. There can be little doubt that the time is not far distant when the valves of the future will open'up possibilities not even dreamt about four years ago. Tn the meantime when new valves make their appearance glance at their amplification factors, but study their impedances. Every valve has an optimum ratio for the radio transformer coupled to it. Low impedance and a high ratio may work out Better in practice than a high impedance valve and a one to one railio transformer.

DAY AND NIGHT Why Ranges Vary USES OF SHORT WAVES Messages from the Tahiti faded out by dav, and New Zealand stations were unable to pick them up again until night came. Most of us who own radio sets are aware that the range of the set is decidedly curtailed In the daytime. In many cases quite sensitive sets have to confine themselves to a selection of New Zealand stations. Australia as a rule cannot be received in daylight. It is the same with ships’ sets working on the normal wavelengths. The hours of night are the best for long-distance work. Judging by the number of people who have asked why this should be, it may be interesting to explain it. About twenty year ago, when radio was in its youth, this phenomenon was observed for the first time. All manner of conjectures were brought forward to explain it. All manner of conjectures had been brought forward to explain how Marconi’s first messages across the Atlantic had so kindly followed the curvature of the earth, instead of shooting off into empty space as predicted by experts. A mathematician, by name Heaviside, gave the most plausible solution to the problem, and subsequent investigations, with the delicate instruments now used, have tended to confirm his theory. Above the world, he said, would be found a partially conducting layer which only formed when the sun had set. This layer, being conducting, bent or reflected the radio rays and prevented them from passing away into interstellar space. A radio wave from the Tahiti during the night would be reflected from this layer and bent back to New Zealand with very little loss of strength. The radio waves that travelled along the surface of the ocean on the other hand lose their strength far quicker and therefore do not carry so far. When the sun rises this Heaviside layer, as it is now called.- is dissipated, and the only link is by the ground wave. In the cae of broadcasting sometimes we receive both these waves, which do not arrive simultaneously, having travelled different distances, and night distortion is the result.

This Heaviside layer sounds almost too good to be true. It would indeed appear to have been specially provided so man, in the course of a few million years, might have at his command a method of signalling round the world in which he lives. Despite the almost fairy story aspect of the gift, experiment after experiment makes scientists think that there must be a definite layer as predicted. Among a whole host of measurements carried out since the war, it has been discovered that the layer is about 60 miles or so above the earth. Moreover, recent investigations go to show that there may be more than one layer. The layer, far from being passive, is in a state of restless motion all the time, jumping from 60 to 70 miles, or even more, above the earth, and causing all manner of queer fading effects. The shorter the wave the better it is reflected by the layer. For this reason, others, it is possible to signal right across the world on absurdly low power on 20 metres or so. Many ships are now fitted with short-wave sets. Had the Tahiti been so fitted for emergencies it is more than likely that she could have kept in touch with New Zealand fairly easily. The trouble, though, with the short waves, is their habit of skipping the first four or five hundred miles. For emergency work on ships some method would be required whereby the normal ship’s wavelength for SOS purposes could be keyed into a shot-wave set to work simultaneously. If only one transmitter is carried, it is more important to get in touch with nearby shipping, and for that purpose a short-wave outfit would be useless. TONE AND VOLTBIE Making Most of a Pentode Broadly speaking, the triode supplies to the speaker a more or less constant voltage at all frequencies, even though the impedance of the speaker varies widely from one frequency to another. The pentode, on the other hand, tends to provide a constant current at all frequencies (writes A. L. M. Sowerby in the “Wireless World”). . Owing to this troublesome tendency, outrageously high voltages can be developed across a loudspeaker at those frequencies for which its impedance is at its highest. >On the other hand, the working impedance of the pentode is considerably higher than that of a triode of equivalent output power, so that an artificial raising of the speaker impedance by means of a step-down transformed is often recommended for the sake of good reproduction of the lower notes. This, while shifting the frequency of maximum power-transference ifrom the extreme treble down to a much more reasonable position on the frequency scale, accentuates the development of these excessive voltages.

Although these high voltages are not dangerous to the listener, they can be dangerous to the pentode, and, in addition, they. tend very seriously to limit the output volume attainable by causing the pentode to overload on certain frequencies, and so to produce distortion when the average volume is still quite small. The best solution of the difficulty is to look after the bass with a step-doWn ratio provided by a tapped choke acting a? auto-transformer, while the upper frequencies are prevented from becoming unduly pronounced by a condenser and variable resistance connected in series and shunted across the “primary’’ of the choke. If a step-down transformer is used in place of the tapped choke these components must, of course, be placed across the primary if they are to retain the values; across the secondary a condenser of larger capacity and a resistance of lower value would be needed to produce the same results. The variable resistance acts as. an effective tone control, and by limiting the impedance of the plate circuit as a whole to its own value prevents the nigh voltages from making their appearance, and so not only safeguards the valve, but permits it to accept, without overloading, a grid swing that is so far increased that the total output from the valve is made noticeably greater. This effect, taken in conjunction with the improvement in quality that they bring about, amply repays one for the small additional expense of the added components.

STENODE RADIOSTAT Razor-Sharp Tuning QUARTZ CRYSTAL USED It is in two respects that the Stenode Radiostat differs in principle from any orthodox type of receiver, namely, the provision of a means of effecting razoredge tuning together with a compensating device for maintaining the proportionality of the various low-frequency components of the modulated wave without reducing the sharpness of tuning. _ The necessary sharpness of tuning is effected by “the use of a quartz crystal, the piezo-electric properties of which are already tyell known (writes S. O. Pearson in the “Wireless World”). Quartz crystal is perhaps among solids the one which has the most perfect mechanical elasticity, and if a suitably shaped piece of such crystal is set into longitudinal and transverse vibration by striking it, the vibrations persist for a considerable time. If the vibrating crystal is mounted between two metal plates an alternating potential difference is set up between the plates at a frequency equal to that of the mechanical vibrations within the crystal. On the other hand,

if an alternating voltage whose frequency is equal to the natural frequency of ribratfon of the crystal la applied between the plates violent mechanical vibrations are set up in the crystal. In the Stenode a quarts resonator embodying these principles comprises the main tuning device in the last intermediate frequency stage of an otherwise normal superheterodyne receiver. The use of the superheterodyne principle is essential because a quartz resonator has a fixed natural frequency and so cannot itself be tuned to stations having different wavelengths—the frequency of each station received has to be transposed to the resonant frequency of the crystal. The intermediate ..frequency used is 100 kilocycles per second, corresponding to a wavelength of 3000 metres. There are thus only two tuning controls in the receiver, one to tune the frame aerial to the wavelength of the station to be received, and the other to tune the local oscillator to produce a supersonic beat-frequency of 100 kilocycles per second. An idea of the extreme sharpness of tuning will, be obtained when it is pointed out that the 0.0003 mfd. condenser for tuning the local oscillator has in parallel with it a variable condenser of not more than 10 micro-microfarads, which is itself controlled by a slow-motion dial, and that a quarter of a turn on this dial is sufficient to tone right through a station. One complete turn represents a change of one micro-microfarad only.