SHORT WAVE SET TROUBLES

Sun (Auckland), Volume II, Issue 487, 17 October 1928, Page 18

SHORT WAVE SET TROUBLES

Dead Spots In Regeneration Control Reduced With Choke

Those who are investigating the interesting possibilities of short waves for the first time will find a number of things to contend with that are of only slight importance in the broadcast band. The question of “dead spots*’ due to antenna characteristics is probably the most common trouble. These occur most commonly when the receiver is tuned to a frequency at or near the fundamental frequency of the antenna or one of its harmonics. Another reason for the dead spots arises from the difficulty in securing a radio-frequency choke which will have a uniformly high impedance to the enormous frequency range used in short-wave receivers. In a broadcast receiver the frequency ratio between the maximum and the minimum is roughly three to one. This is the same ratio that exists between say 30 and 10 metres, although the whole “band” in this case is only twenty metres wide. From this it may be seen that it is a difficult problem to make a choke satisfactory from 200 to 10 metres, or over a 20 to 1 frequency range. Chpke Coil Operation. If a choke is to have general utility so. it may be all classes of circuits, it must be operated above its natural period. For example, if a choke is to be .used at 1000 kilocycles (300 metres), it must be designed so its natural period of frequency is lower than this. For all frequencies above the natural period the choke acts as a condenser, that is, it has capacitative reactance. This is necessary, since if it were not inductive it would be equivalent to introducing a coil in the plate circuit of a tube. If a shunt or parallel plate feed circuit is used this would result in regeneration and an unstable circuit. If, then, the choke is made so it acts as a condenser at 1500 kilocycles, which is the lowest frequency to which a short-wave receiver ordinarily tunes it will necessarily have a very low impedance at 30,000 kilocycles (10 metres).

The best chokes that are available have an equivalent capacity of about three micromicrofarads at this frequency, and although this is an extremely small capacity the frequency is so high that the reactance is only approximately 1755 ohms. This capacity is so low that if it is introduced across the output of the tube it virtually short circuits it. Resistor in Series. v - For this reason it. is advisable to use. about a 10,000 ohm resistor >a series with the choke for very high frequencies. This resistance is low enough so the voltage drop through it is not excessive and yet it very materially increases the total impedance at the high frequencies. At the lower frequencies the impedance of the choke increases and thh makes it possible to use a small regeneration condenser. In the commercial short-wave receivers on the market it is often difficult to secure good regeneration due to the difficulty mentioned above. In many cases an attempt to overcome this is made by increasing the capacity of the regeneration condenser to 250 or more micromicrofarads which is double that of the tuning condenser. By using the suggested combination it is possible to secure good regeneration with less than 100 micromicrofarads, so a midget condenser of this capacity may be used with a saving in initial expense. Compensating Plate Voltage. To compensate for the drop in the 10,000 ohm resistor it may be advisable to increase the detector plate voltage to 67.5 volts. Where the short-wave set is one of the “converters,” designed to plug into the detector socket the voltage may be increased by connecting a 22.5 volt battery of the “C” type in series with the plate circuit. If the converter is used with a set having a B eliminator that is usually unnecessary, as the regulation of the eliminator is high enough on this tap to automatically increase the voltage quite a bit and compensate for most of the drop

THE THOROUGHNESS OF JAPAN It is said that the Japanese, in many ways, are one of the most conservative nations on irth. Yet they are always prepared to accept the best. in Western culture, and discard all tht worst features of European civilisation. Their army and navy are living examples of the national desire of modern Japan to keep abreast of the times, while their builings. tern of local governmen'. commerce, newspapers and so forth are all evidence that the little brown people of the Orient are fully awake to the advantages of modern scientific thought and rrogress. It was only to be expected, therefore, that wireless would immediate!’ appeal to this intellectually restless race. Typical of the of Nip’-‘~ i " is a letter received lately by 2BL from a radio fan in which written in English, reads: “Gentlemen, I receive your station every night by loud speaker. T will send you some radio books .nd photographs by other mail ‘on this day And I will send you mr* ‘ - on radio by phonogranh record that I will make in a short time.” A uniqv"' experience, indeed, for 2BL to receive a record made in Japan of the station’s own broadcast music.

A CURIOUS S.O.S. “Hello, is that the 2FC announcer? Well, I want your advice.” Mr. Halbert, who was on duty that night, did not commit himself: he is used to the question. He certainly was not prepared for the extraordinary difficulty the distressed listener presented. “What am I to do? There is a tremendous swarm of bees in the garden. I don’t want them, and I cannot get rid of them. Can you help me?” The request was unique, and ip a minute a broadcast appeal was made for practical help. Almost instantly the telephone bell rang sharply. “I say, tell that chap at Kogarah to get a kerosene tin and beat it as hard as he can. He’ll soon have 'the bees inside!” The suggestion—as old as the hills, but none the less effective—produced a speedy and very appreciative note of thanks from the unwilling owner of the bees.

15s 6d—Mechanics' Overalls Suits— At Bert Marshall’s.