SHORT-WAVE TROUBLES

Radio Record, Volume I, Issue 48, 15 June 1928, Page 16

SHORT-WAVE TROUBLES

| THE "DEAD SPOT" DIFFICULTY A QUESTION OF CHOKE. THOSE who are investigating the interesting possibilities of short wares 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 aerial characteristics is probably the n..:t usual trouble. These occur most commonly when the receiver is tuned to a frequency at or near the fundamental frequency of the aerial 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 frequeney range used in short-wave receivers, In a broadeust receiver the frequeney ratio between the maximum and the minimum is roughly 3 to 1. 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 probleim to make a choke satisfactory from 200 to 10 metres, or over a 20 to 1 fre quency range. CHOKE COIL OPERATION. . Tf a choke i~ to have general utility se it may be used in all classes 0: circuits it must be operated above its

‘natural period, For example, if a choke is to be used at 1000 kilocycles (360 metres) if must be designed so its natural period or frequency is lower than this. For all frequencies aboye 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 equiyalent 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 imveomnee 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 aeross the output of the tube it virtually short-cireuits it, RESISTOR IN SERIA, For this season ir is advisable to use about a 10,000 olim resistor in series with the choke for very high freqenecies. This resistance is Jow enough so the voltage drop through it is vot excessive, and yet it very materially increases the total impedance at the high frequencies. At the lower frequencies the impedance of the choke inereases and this 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 men-

tioned apvove. in many ecaseS an attempt to overcome this is made by inereasing the capacity of the renegeration 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 shortwave set is one of the "converters" designed to plug into the detector socket the yoltage may be inereased 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 eliminate: this is usually unnecessary, as the regulation of the éliminator is high enough on this tap to automatically increase the voltage quite a bit and compensate for most of the drop.