THE SHORT-WAVE SET

Radio Record, Volume I, Issue 32, 24 February 1928, Page 3

THE SHORT-WAVE SET

POINTS ON OPERATION DEAD SPOTS. QN the broadcast wavelengths, a set can be made to oscillate easily, but on the short wayes, below 100 metres, care should be taken in the. construction of coils and the disposition of components, or trouble will arise. Dead spots (says the Melbourne ‘Lyistener-In’’) are another cause of faulty reaction control, and these may be prevented by careful adjustment of the size of the aerial coil, If the total wavelength of the aerial, earth and the aerial coil is the same as that situated at some point over the range of the grid condenser, then the set will refuse to oscillate J at this point, as the both circuits are

in resonance, and the energy absorbed by the aerial is great, and causes the! set to become "dead" Another cause | of this trouble is the- coming into resonance of the reaction coil, which produces the same effect. So that the efficiency of a shortwave receiver depends mainly on the adjustment of the coils and the distance between them. ‘he distance between the aerial and the grid coils wili determine . the amount of selectivity which the set ‘will possess, and the further the distance between these two coils the greater the selectivity, but the volume will be less. | Very Fine Tuning, A distance of half an inch should be ample, as tuning is exceptionally fine on, these wavelengths. The grid leak shonid have a high resistance of from 5 to 10 megohms, not, as is the case with the broadcast receiver, 3 megohims. ‘The grid condenser has a proportionately smali value, and should have a capacity ix the vicinity of 0.0001 microfarads. A large variable condenser can be used to tune the reaction, as, usually, this adjustment is not critical, especially with continnous wave Morse transmissions. A special valve should be selected, as some will not oscillate on such low wavelengths. -A valve of the 201A type proves excelient for these wayeleugths. The special 200A detector is liable to be very noisy and unstable, althongh it is supersensitive on the broadcast bands. In some improperly, built shortwave receivers a howl occurs just on the point of oscillation. This is not noticeable in the detector stage, and goes to prove that the trouble exists in the andio stages. The trouble j

4 can be fixed by burning the filamenta of the audio valves berond normat brilliancy, but this is bad prace tice, and reduces the effective life of the valves. The hest method of eliminating the trouble is to corte pletely shield -the detector stage from the audio stages. This should effectively eliminate the howl. Sepats ate rheostat controls should also he fitted as such a how! would be caused by using the one rheostat to control all the filaments. Radio Frequency, Old valves in the audio stages alsé howl, so that it wil] be seen that there are many sources of this trouble, Radio frequency amplification {apart from the new double-grid valve) offers no advantage under 100 metres, and when stations from all orer the world are received with a single valve it will be seen that such amplifiers are not A ee eR eee. cee terrae AF necessary. In any case tuning is diffix . cult enough with the addition of radio stages which complicate tuning to a . very great extent. Most of the low wave transmissions — are in code, so tliat a so-called distortion trausformer with an extremely high ratio cata be used to amplify the signals before reaching the amplifying valve. Such an arrangement produces frightful results with speech and music, but is worth another stage of audio amplification as regards the it crease in volume. A high MU valve of the new. type gives a further amplification unobtainable in any other tube. Bearing all these hints in mind, the best can be obtained from a short Wave receiver which is giving trouble due to unstable regeneration,