WIRELESS

Wanganui Chronicle, Volume LXXXI, Issue 19034, 11 June 1924, Page 3

WIRELESS

Amplification. The above word truly covers a multitude of evils. The average wireless amateur starts off with a crystal set, very soon finds this too absurdly simple, so builds or buys a single valve set; in a short time this again proves too simple and he longs for more worlds to conquer, in other words, more valves to play with. “Facile decensus averni” is a true saying, particularly referring to wireless amateurs. An liberal translation might be “the descent into the pit of amplification is easy.” It seems so simple to buy another valve, an inter-valve transformer, another rheostat and a few terminals; hook them up and away she goes! But will she go properly or will the result of the stage of amplification simply be distortion? The answer to this question depends on the design of the amplifier and the way it is used. In the first place amplification may be either high-frequency or lowfrequency; i.e., the signals may be amplified either before detection or aftei detection. In this connection it is well to remember that the square law for valve rectification roughly applies. This means that up to a certian point the, energy applied to the grid of the detector tube will be reproduced in the plate circuit roughly squared in valve. Thus, if two units of energy are applied to the grid the resultant variation in plate current will be four units, while if four units are applied to the grid the resultant variation will be six teen units. It can thus be seen quite easily that it is advantageous to apply as much energy as possible to the grid. Where weak signals are being dealt with high-frequency amplification is therefore of great value. Another advantage of this method of amplification is that the selectivity of the set is greatly increased, since each stage of high-frequency is really a filter circuit which will narrow the wave-length band. - Unfortunately, high-frequency amplification on wave-lengths below 300 metres is, to put it mildly, very difficult to manage. For this reason most amateurs shy clear of it, even above 300 metres. Now that the neutrodyne method hjis been invented, however, the difficulties have bvez* greatly minimised, and amateurs who are desirous of receiving long distance telephony with anything like clearness can be well advised to investigate the neutrodyne. In its essentials the neutrodyne principle simply consists of a method of neutralising the inter-elec-trode capacity of the high-frequency valves, and thus preventing them from oscillating. It should be mentioned that the main reason for the difficulty experienced on short waves is the tendency of the high-frequency valves to oscillate. This tendency is sometimes overcome by placing grid bias on the amplifying valves, but this method savours of brute force, and is not nearly so effective as the neutrodyne method, which achieves the desired end by means of a very small condenser coupled back from the grid circuit of the second valve to the plate circuit of the first. This is extremely small,

and may be made with two half-pen-nies separated by a piece of mica. Its effect is to stabilise the valve by neutralising inter-electrode capacity and thus preventing feed-back from the plate circuit to the grid circuit, which is the cause of oscillation on short wave-lengths. Another important point in high-frequency working is the design of the inter-valve transformers. These should be made so that the capacity between the windings is small; a good type consists of a slot cut in a disc of ebonite, say two inches in diameter. The windings should be of thin wire, 34 or 36 gauge, and well insulated from each other. For working with the complete neutrodyne set a special type of transformer is used which has a double primary winding. This is not essential, however, as the neutrodyne condenser can be applied to any highfrequency amplifier, except the resistance type, where it is not required. For very low waves high-frequency amplification is not highly effective except that it increases the selectivity of the set. Turning now to low-fre-quency amplification a very different set of circumstances prevails. In the first place high frequencies are left behind with the detector tube, most of them at any rate. Thus attention can be confined to a band of audible frequencies. It should be mentioned here that low-frequency amplification ; s successful only where there is an audible, through perhaps very faint, signal perceptable with the ’phones in the plate circuit of the detector tube. Lowfrequency amplification will not bring a signal out of nowhere; it simply mag nifies the signal already there. Herein lies the reason for a large amount of the distortion experienced by amateurs They are endeavouring to amplify a signal which is really not strong enough to operate the detector tube; to do this they must use reaction to the utmost possible degree and also force the amplifying tubes as much as possible. The result is distortion as inevitable as night follows day. A further cause of distortion is the phenomenon known as resonance, which is at times a blessing and a curse. Musical notes vary in frequency from something like a periodicity of 34 per second to several thousands per second. To amplify perfectly the apparatus should be capable of dealing with frequencies between these limits without accentuating any particular frequency. Actually such a performance is extremely difficult because most circuits possess a resonant frequency to which they make the greatest response. In practice inter-valve transformers, telephones and loud speakers are usually so designed that the resonant point is nt about 800 to 1000 per second, which is roughly the mean of speech frequencies. Thus the majority of speech fre-

quencies fall near the resonant point and distortion is minimised, although not prevented. When music is trans* mitted, however, the low notes and the high notes are largely lost and thus distortion is apparent. This is to 6omn extent remedied in the transmitting station by suitable placing of the various musical instruments, but this is only a partial cure since the overtones of some instruments are bound to be lost. When the amateur starts to amplify the music he accentuates the resonance difficulty with every stage he adds until the final result is horrible in the extreme. A partial rem edy for this state of affairs exists, which has not yet been developed much in this country. Amateurs who ware working on long waves a few years ago

will no doubt remember .the resistance —capacity coupling method of highfrequency amplification. This method is applicable to low-frequency also r.lthough this fact is not generally recognised. Exactly the same circuit 15 used except that the amplifying valves follow the detector tube instead of preceding it. The resistances have a value of 80,000 ohms., while the intervalve condensers have a value of 1 microfarad approximately. The amplification per stage with this method is not so great as with the usual transformer method, but the results arc much better from the point of view of purity and freedom from distortion. The method is very largely used in broadcasting stations for the amplitica tion of music and speech prior to its being applied to the modulator valves. Reaction and Its Control.,

Tiie majority of amateurs seem to find some diiiiculty in controlling reaction on short waves. By controlling is meant causing reaction as well as preventing it. It should ba understood that reaction can be obtained m two ways; the first is electro magnetic reaction and the second electro-stacic reaction. The eelctroinagnetie method is that used, in the standard three-coil curcuit, in which reaction is obtained by coupling a coil in the plant circuit to a coil in the grid circuit. These coils are commonly known as the reaction and secondary coils respectively. The reaction which occurs is due to the actual magnetic coupling between these coils. Now it should be understood that the plate current variations are composed of high frequency as well as low frequency waves, since the valve is not a perfect detector. It is the high, frequency component of the plate current which causes reaction and this point is most important. No hindrance should be placed on the flow of the nigh frequency component and accordingly the plate circuit should be roughly tuned to the frequency of the incoming waves. This is where most amateurs make the mistake. When they find it difficult to obtain reaction on short waves they immediately jump to the conclusion that the reaction coil is too small. It may be too small, but on the other hand it may just as likely to large. In this connection is is well to consider the electrostatic method of obtaining reaction. If the plate circuit is tuned to the same frequency as the grid circuit, and the frequency is high (i.e., the wavelength low), energy will be fed back from the plate to the grid owing to the capacity between these elements. This will cause reaction just as effectively as the electro-mag-netic method mentioned above, and there is no necessity for magnetic coupling between the two circuits. This is the reason why sometimes it will be found that reaction is obtained with the reaction coil connected cither way round. In this case electro-static reaction is occurring because the two circuits are tuned to the same frequency. It should be realised easily that electro-static reaction is most effective on very short wavelengths since the inter-electrode capacity is small. This is most useful really since it is on the short waves that electromagnetic reaction is most difficult to obtain. The elec-tro-static method can be used in the following ways. A small variable condenser, say .0003 mfd. can be shunted across the reaction coil and used to tune the plate circuit. Very delicate control of reaction can be obtained in this way since both elec-tro-static and electro-magnetic reaction can be employed. Alternatively the reaction coil may be removed and a small variometer connected in its place. By tuning this it will be found that reaction can be obtained. This latter method is very largely used in the United States, jvhere valves with large elements are generally employed.

Telephony From England. It was announced by the Amalga mated Wireless Company a few days ago that telephony had been successfully transmitted from Poldhu in England to Australia. The wavelength was not announced and the w’hole matter was largely shrouded in mystery. The beam method was hinted at in the report although this was not actually used. Reports to hand from England show that the tests were probably made on 95 metres and the ordinary method was used. The power used is about 20 kilowatts. English amateurs have been getting the full benefit of these tests since many of them are working around 100 metres for trans-Atlantic reception from KDKA. Just imagine 20 kilowatts let loose a few miles away from you. 2YB used about 15 watts! Nutt said! ♦ » • • General Notes A writer in an English wireless paper mentioned the low power record created by Mr. C. McClurcan, when he worked N.Z. from Sydney

on .004 watt. This writer was deluged with letters, mostly frankly incredulous, asking whether a misprint had occurred. He was able to confirm the report, in fact he stated that the distance worked was 1500 miles and not 1200, as first stated. Plate voltage used 15, plate current .25 milliamp. Distance 1500 miles! The Pope is one of the latest converts to the benefits of wireless. A receiving installation has been installed in the Vatican.

The Great Western Railway (England) is conducting experiments to determine whether broadcasting can be successfully received in a train travelling at 80 miles per hour. A double wire aerial has been installed along the roof of a coach. If success is obtained it is intended to instal sets for the benefit of passengers, who would be able to listen in for a small charge. A local amateur reports reception of six different N.Z. broadcasting stations on Sunday night. 2YM (Gisborne) was the best of the bunch Confirmation has been received by a local amateur of his reception of signals from 6AVJ, Mr. Melvin S. Wood, Gardena, California on March 23 last. The card sent by Mr. Melvin is headed “.American Radio Re-, lay League Station 6AVJ.” The transmitting apparatus mentioned consists of an inductively coupled Hartley circuit, feed by a 100 watt synchronous rectifier, 2200 volts on plate, at 325 milliamps. Radiation 5.1 amps. Antenna a 9 wire cage 7 feet in diameter at each end and 2 feet in diameter at centre. 90 foot towers at each end. Antenna 50 feet long. Mr. Wood reports that on March 23 he was using one 50 watt tube so that the distance covered is Quite good. A rather humorous note at the bottom of the card states that he is a member of the

“Key Klick Klan.” Apparently the Klu Klux Klan is treated as a joke in California, since it is parodied quite freely. Mr. Wood uses a Grebe CRI3 receiving set, also a nine tube heterodyne set, so that his station is evidently well set up.