Radio Broadcast (May 1928-Apr 1929)

A Modulator for the 1929 ShortWave Transmitter A WRITER on the Hartford Times tells me that all radio men write stories badly; that is, they always leave the thrill until the last, instead of putting it into the headline. In this three-part tale of improved short-wave transmission, I seem to be guilty of this offense, for I am waiting until the third installment to speak of the uses of such transmitters, and, of course, that is where the thrill comes in. For the present we shall give additional data on the constant-frequency transmitter described last month, and explain those devices which make it useful, which is to say the key or microphone. A transmitter without modulation is useless, since it can send out only a "carrier wave." To place variations on that carrier which a radio receiving set can "unscramble" into code or voice is the business of the apparatus described in this article. It was explained in the October issue that voice, or radio-phone transmission, has the advantage of speed while, with the same power, radio-telegraph transmission has the advantage of greater range. Very evidently these two systems may be used with the same transmitter to excellent advantage, and it is my suggestion that provision be made for both. CONCERNING INTERFERENCE 1*0 MANY amateurs a recommendation of radiophone is as a rag to a bull; it causes them to paw up dirt and bellow about the awful interference caused by the microphone-operated sets. This opinion is a bit out of date. Two years ago amateur oscillators were so unstable that the addition of modulating equipment did have a distressing way of causing the signal to smear over a wider band than one cares to think of. Also, the transmitters did exactly the same thing when keyed, but, in that case, the interference consisted of mumbles, thumps, and "yips" which were harder to identify. Since that time our preaching for better tuned circuits .has had a good effect, and the coming of crystal-control has set examples. As a result the number of good amateur radiophones has increased so that we must withdraw the accusation against the phone. To sum up — with an oscillator-controlled battery set we may modulate with key or voice as we please with no fear of " wobbulation " being added to "modulation." The circuit of the oscillator-amplifier set-up is shown in Fig. 3. THE CIRCUIT ARRANGEMENT THE circuit is the almost standard Heising constant-current arrangement. In Fig. 2A we review the general principle. Current from the battery flows through the high-inductance choke, L, to the point B, then divides and returns in two parts through the two resistances Ri and R2. If we suddenly change the setting of the variable By ROBERT S. KRUSE resistence R2 we find (by watching the meter 1) that the current through Ri changes for only a moment when this is done. The explanation is this; the choke L is (after the fashion of inductances) an electrical "stand-patter," i.e., always opposing a sudden change. Thus, if we suddenly FIG. 1 . TOP VIEW OF MODULATOR UHIT reduce R2 we do not change the total current through the choke immediately but simply change its division between the two paths, therefore, the current through A drops sharply until L* finally agrees to allow more current to pass. On the other hand, if we suddenly raise the resistance of R2 we force additional current A TRANSMITTER, according to Mr. Kruse, without a means of modulation is as bad as a ship without a sail. Inasmuch as the transmitter itself was described in October, a transmitter that is designed to stay on its assigned frequency, the modulation equipment necessarily had to follow. Here it is. — The Editor through Ri until L permits the total current to die down. Thus, the effect of any sudden changes in R2 causes corresponding sudden changes in the current passing through Ri. In Fig. 2B we show how this principle is used in our set. The oscillator tube feeds radio-frequency power to the grid of the r.f. amplifier tube where it is amplified and passed to the circuit L2 C3. The current is then transferred to the antenna, but this does not concern us just now. The plate power for the amplifier tube is supplied through a choke coil L, of at least 6 henries, just as the current in Fig. 2A was supplied. The radiofrequency choke (r.f.c.) is to prevent the r.f. 38 wandering into L and the tube "Mod" where it would be wasted. The vacuum tube labeled "Mod," which is used as a voice-operated resistance, is substituted in the circuit in place of the resistor R2. A microphone and amplifier feed voice-currents to the grid of this tube, and, as they cause the voltage of this grid to change, the current to the amplifier tube varies, causing corresponding changes in the r.f. current in the L; C2 circuit from which the antenna power is taken. It is only fair to say that this method of operation does not permit the highest amplification to take place in the amplifier and that a "straight" r.f. amplifier should be added if the full power-rating of that size of tube is desired. However, this added complexity does not seem warranted in a small set since the range increase is not large. The circuit arrangement in complete form is shown in Fig. 3. The picture in Fig. 1 shows how simply the whole works goes together. In listing the particular makes of apparatus which are given below, the writer has no particular desire to favor any manufacturer but rather his motive is to remove uncertainty by listing those parts which have been used with satisfaction in short-wave transmitters. In several cases it was found that other equipment, which seemingly had every right to be as good, was thoroughly unfitted for this somewhat special purpose. If changes in the assembly are considered necessary they should, therefore, be made one at a time and the effect noted. The parts used in my set-up are as follows: LIST OF APPARATUS Mike — Western Electric type 348BVV or Federal type 260W. Ri — Used to reduce microphone current to proper value. Not necessary in most cases. R2 — Gain control. Frost 100,000or 200, ooo-ohm type. R3 — Shunt resistance, exchangeable in clip to suit tube and transformer used. 1 megohm is usually satisfactory. Rs — Filament rheostat, 6-ohm. L — General Radio choke, type 485-S. TRi — General Radio type 485-M (for single button mike) TR — General Radio type 485-D SW — Switch to cut off d.c. filaments and microphone. No a.c. circuits should go through or near this switch. CONCERNING TUBE EQUIPMENT COR the sake of simplicity, and to minimize 1 A.C. hum, it is recommended strongly that for voice operation 2oiA-type tubes be used in the oscillator (see p. 344 of October Radio Broadcast) and in the first socket of the modulator, and that their filaments be operated from