Wireless Broadcast

Star (Christchurch), Issue 17031, 3 May 1923, Page 11

Wireless Broadcast

Contributed by "Aerial."

HINTS ON VARIOMETER RECEIVERS. After comparing several kinds of circuits for receiving snort wave signals, a good many experiments will settle on the variometer set as being the most sensitive, and at the same time the easiest to control. It has several advantages over the usual tiolcler coil regenerator whether the said tickler sec is tuned by condensers or otherwise. ■’First, when the coupling between the secondary and tickler coils is tightened or loosened, the secondary tuning is somewhat- altered. Oniy a lit Lie perhaps, put quite enough to spoil things, when tuning in a ’phone, li the tickler coupling is increased, the secondary tuning condenser must be reduced a trifle, to remain exactly on a given wave length. Again, suppose a condenser tuning is being used, the full advantage of the amplifying powers of the detector bulb cannot be obtained, owing to the capacity across tne grid and filament. Such capacity always tends to reduce signal strength. In a variometer tuner the only capacity employed is the sell capacity of the variometer and tunei windings. It will be seen at once that a greater potential can be applied to the grid, resulting in louder signals. Altering the plate variometer does not affect the grid tuning in a well designed set. So much for the advantages ol variometers. The usual receiver employing these continuously variable inductances, how ever, has seldom sufficient tuning range to satisfy the average amateur. This is the one big disadvantage of variometers, but woth a. little ingenuity this can easily be overcome. There are numberless ways ot increasing the tuning range, but the three circuits shown in the accompanying diagram are about the best I have come across to date. Fig. 1 shows the usual regenerative receiver employing grid and plate variometers. Normally a set of this type tunes down to about 150 metres and up to 400 metres. The range is increased in this case by shunting fixed condensers of small value in parallel with the grid and plate coils. This is shown by the clotted lines. The condensers must be quite small if fixed condensers are used. About .0003 m.f.d. s is ample. The two condensers will not necessarily have to be exactly alike, however. Very often the variometers are not alike, the plate inductance being somewhat smaller than the grid one, when any difference exists. It- may be well to state here that to do this job properly a wravemeter is almost indispensable. It need not be calibrated accurately, however. If you know where one particular wavelength tunes in, it will do. A large variable condenser, shunted across fifteen to twenty-five turns wound on a four inch diameter is near enough. A buzzer may be connected in series with the coil to act as a driver. To get a clear note and sham tuning a noninductive resir.£.*nee must be connected i:i parallel with the buzzer coils. When all is reac.lv to start, set the variometers at their minimum setting and tune the aerial to resonance. This will be noted when a blind spot is found on the aerial tuning condenser. Blind spot means a setting where the aerial does not oscillate. Ihe aerial circuit should oscillate a little on either side of this spot. Now set the wavemeter up a few feet away, and having, started the bur.zer, tune in on the variable condenser until the buzzer signal is heard at maximum intensity in the ’phones. Make a note of the wavemeter setting, then adjust the variometers to their maximum setting, and tune the aerial to resonance as before. Tune the wave-meter to this setting and make another note. Now reduce the wave-meter condenser slightly, just four or five degrees or so. Next connect variable condensers across the pjlate variometer, arid also across the grid variometer, and the coupler secondary. Set the variometers again to their minimum tuning, and tune in with the variables across the grid and plate inductances. When tuned to tuned to resonance judge as near

as possible the 1 capacity across each coil. For instance, suppose the two variables are each of .0005 m.f.d., and the dial of the grid variable shows that about three-fifths of the condenser is in use. it will be seen at once taht the ' capacity in parallel with the windings is .0003 m.f.d. When the capacity in each case has been found approximately, the variable

condensers may be removed, and fixed j condensers of the capacity determined ,be connected in their place. This may sound rather laborious and roundabout, • but the task proves itself very interest* j ing indeed. A switch may be employI od to cut the condensers in or out at j v ill. thus giving two ranges to the set. ' L the low side gives 150 to 400 metres, he upper range should give from 385 l o nearly 700 metres, i i Circuit 2 ©hows how to obtain the

same result by another method. Here again we have a similar receiver, using precisely the same tuning inductances as before. This time, however, we do away with the fixed condensers entirely, tuning being accomplished by the use of the variometers. The only extra instrument required in this ca?fe is a single-pole, double-throw switch. This may be of the blade, or anti-capacity type, it does not matter much which. Key switches, however, have the advantage of small size, and can be mounted almost anywhere on the panel. To the blade of this switch the negative end of the filament is connected. One jaw is common to one side of eaeft variometer, while the other jaw is connected to the middle point of the plate variometer, that is, the point where the rotor winding joins the station winding. When the switch is thrown to position 1, the usual regenerative circuit is in use. Throwing the blade to position 2 puts the stator of the plate variometer in series with the grid or secondary circuit, thus acting as a loading coil. The rotor of the plate variometer now acts as a tickler coil pure and simple. Care should be taken to bring the negative end of the “ B ” battery to the rotor terminal of the plate inductance. This is quite a simple arrangement, but, to tell the whole truth, the circuit has one or two slight advantages. Loading the grid circuit with inductance in lieu of capacity flattens the tuning a little, so that the variometers will scarcely be expected to tune as sharply as they do on the lower range. Then, again, a tickler coil is introduced, with the disadvantage mentioned in the beginning of the article. These little troubles are quite com pensated for, however, as signal strength is not reduced, since capacity is kept out ot the circuit as far as possible. To sharpen the tuning, an-

other blade can be added to the switch, so that a small condenser, either fixed or variable, can be shunted across the grid windings. This will reduce signal strength a little, but will sharpen* uo the timing ot : the grid circuit wonder fully. Perhaps the best idea of the lot is the circuit shown on Fig. 3. This is somewhat unusual. J>ut the circuit has all the good points of the previous ?nes, without anv of their disadvantages. Special variometers are required this time, biit they are well worth the trouble. The ordinary variometer has somewhere in the region of 100 turns on it altogether, rotors of the spherical type are about four inches in diameter. and wide enough to accommodate fifty turns of wire, leaving u hall-inch space in the centre for the shaft. For this set. the stators and rotors ■should be made somewhat larger, and at the same time should hold more wire. Fire inches will do nicely tor the rotor diameter, which should hare approximately thirty-two turns of wire on each side ol it. Tile stators may have thirty turns on each side. This difference m the number of turns between stator and rotor will give each coil approximately the same inductance. Juke the other two circuits, this one has also two ranges, hut instead of using loading coils and condensers, the variometers are altered by the use of series-parallel switches. Looking at either variometer in Fig. will be seen that when the switch is moved to position S, the stator and rotor windings are in parallel with each other, instead of being in series ns usual. Moving io switch to position It. the coils are in series, as usual. Everybody is familiar with seriesshunt switches when used with a condenser and inductance. The same thing happens in this case. Paralleling condensers in a circuit increases the wavelength of that circuit, but the opposite thing applies in this case. When the variometer windings are in paral- \ lei, their natural period is increased, thus decreasing their wave length. On the short wave side the wave length range will be from ISO to 325 metres, and on the series side 320 to 700 metres. Quite a useful tuning range.

and all that the short wave artist wants. A short time ago, a Spreydon amateur, Mr H. H. Smith, showed mo quite a novel arrangement he had designed. Instead of having to manipulate the series parallel switches, they were of another type, and were fastened on the back of the variometers, where they worked automatically. The switches took the form of slip rings and brushes. The indicating dials were

equipped with 360 degree scales, one half of the scale reading range 1. When the dial was turned half a revolution, the connections altered themselves at the slip rings, and for range 2, the second half of the dial was used. This arrangement works well as long as the brushes make good contact. 1 have tried a good many ways of extending the range ol : variometer sets, including » Figs. 1 and 2. Each has its own good r points, and the final selection is more l or less a matter of taste. For the > wavemeter see the article m the “ Star,” April 11). I NOTES. t Many questions are being asked , about a broadcasting station for this r city. The matter is well in hand now, and a \veek or two hence should see a plant installed in a Christchurch busi- ( ness liou.se. The set will have a range of about 400 miles. It is intended .o broadcast at least two concerts per week. Real good up-to-date items are L to be given by various qualified artists. L The radiophone transmitter at Gisborne has now' installed a new generator, and has shown much improvement lately. I.Y.A. at Auckland continues to keep his end up well. Mr O’Neill, at Dunedin, lias just epened up with a big half-kilowatt set. His concert on Anzac afternoon was heard here very nicely on a single-valve set. He was even stronger the following evening, every word spoken being clearly understood. His music is very good, and very loud compared with hi® Messrs Edwards and Smith, of Spreydon, and Mr J. Orbell. of Merivale, are receiving American amateurs nearly every evening, loud enough to he heard several feet from the loud speaker. Mr Orbell uses three stages of radio-fre-quency amplification, followed by two stages of audio. The others use. a detector, two steps of audio, then another amplifier using two five-watt transmitting tubes. The output from this set goes to the Magna vox. CORRESPONDENCE. “ Listener.”—You have got things upside down. L equals V—N where V equals velocity and N equals fre quency. This is radio-frequency, not spark frequency, as you state. N is calculated from L C in the closed oscillatory circuit where L equals inductance and C equals capacity. A simple way to find current or resistance is .o use the dummy formula E over C R This means nothing as it stands. Cover up the one you want and the other two will give you the answer. Thus if the current in a circuit is wanted, cover the letter C, which leaves E over R F equals voltage, C equals current and R equals resistance. This is known a** Ohm’s law. Two factors must be known to find the third. “Pyrites.”—Your trouble is very likely in the tuning coil. See that the slider makes perfect contact all the way. You might try rewinding with enamelled wire. Yes, a loose couple*will be a big improvement over your present tuner. Phones should be 2000 or 3000 ohms for crystal work. Galena i- much more sensitive than carborundum. “ C.W.”—Try the Hartley circuit shown in these columns some time ago. Grid condenser is not absolutely necessary, but if used a grid leak of about 1000 ohms must he used also. With the aerial and counterpoise you describe, two U.V. 202’s should radiate uo to two amperes.