Radio Broadcast (May 1928-Apr 1929)

Working on 5 Meters By ROBERT S. KRUSE THE preceding 5-meter story in August Radio Broadcast told of the ancestry, birth and childhood of the 5-meter band. All biographies are required to have such a first chapter, which the author expects the reader to skip. The reader usually does skip it; everyone is satisfied and ready to begin the main story, which shall happen here and now. Since this present story is to undertake the large job of describing a complete radiophone or c.w. circuit operative at a wavelength of 5 meters (60,000 kilocycles, if you like bulky terms) we must needs proceed logically or become lost. It seems simplest therefore to start with the transmitter and proceed via the sending antenna, receiving antenna, receiver and headset to the Receiving Experimenter. It is hoped that this will leave over enough time to go back and see what curious things the wave has done en route. THE TRANSMITTER IT SEEMS like obvious nonsense to say that * the business of the sending set is to generate a decent phone or c.w. signal, yet the average operator or experimenter never seems to think of this. He is perfectly happy if the meter in the antenna circuit moves far across the scale. That the speech or code being transmitted cannot be "unscrambled" seems to him to indicate that the man at the other end is distinctly dull and incapable. This is then a plea for more than the usual attention to proper transmitter operation, whether the output carry speech or dots and dashes. The usual way of getting at the 5-meter business appears to be that of "sneaking down on the wave." This unfortunate habit has just received new impetus by the opening of the amateur band of 10-10. 7 meters, which is unfortunate for 5 meters, since the 10-meter band can be worked with normal circuits in the transmitter while 5 meters demands changes. The former therefore has little educational value toward learning to handle 5 meters. The first requirement of the 5-meter circuit is of course that it work at 5 meters, the second that it do so steadily and the third that the apparatus be neither too special nor too anxious to "blow up" in operation. All of these needs are met by the circuits shown in Fig. 1. Little explanation needs to be made other than that provided by the caption of the figure, except in the case of the modulator, D. When telephony is desired this is interposed between the B supply and RFCi of any of the circuits shown. If the oscillator is a ux-210 or a uv-202, the modulator will be a similar tube and the transformer a modulation transformer. The C-bias on the modulator must be adjusted by ear until decent speech results. If such speech cannot be obtained the microphone or the transformer are to be suspected. If once abused many microphones never recover; on the other hand, many of the modulation transformers now available cannot operate properly with the normal microphone CT'HIS is the second of Mr. Kruse's -*■ articles to appear in Radio Broadcast, supplying experimenters with data on transmission and reception on 5 meters. Now that the bread has been cast upon the waters, the editors and Mr. Kruse are anxious to know who is gathering in the loaves. We would like to hear from all of those to whom the articles have proved interesting. If you have done or expect to do any work on 5 meters — or if you are doing any work at all on short waves — drop us a note telling what you are doing and what material you would like to see published in future issues. — The Editor. current flowing through them — having been designed for the costly double-button microphones or for some weird sort of "mike "that cannot be found on the market. If the oscillator tube is of a larger sort, such as the ux-852 or the UV-204A we must use the same sort of tube as a modulator. Naturally such a tube cannot be operated directly from the modulation transformer; therefore we must put a stage or two of audio amplification in between to boost the gain to get a decent percentage modulation. Any good audio amplifier can be used. One has only to remember that the microphone can operate a 1 12A tube very nicely but that its plate current is a bit high for the average audio transformer; therefore the 201A is a good first tube when used with 90-135 volts and the appropriate bias. The second tube may be a 171 or a 210 and the third tube a 210 or an 852. In each case the last tube should be fed by the next smaller tube, but one can stand a jump from a first -stage 201 A to a second-stage 210. Biases are in general a bit higher than for ordinary amplification and things are improved all around if there is a control in the shape of a 500,000-ohm Frost rheostat across the secondary of the first audio transformer. If at any point in the circuit it seems not possible to find a transformer to fit the job one can always rig up an impedance-capacity-impedance coupling as shown at E. The iron core choke in the B lead must be designed liberally enough to carry the plate current of the tube and have an inductance of at least 6 henries, while the condenser at its base should have a capacity 1 or 2 mfds. The grid choke going to C can be almost anything, since the current is small. An audio transformer primary or secondary will serve if left on the original core. STARTING THE OSCILLATOR DEFORE the modulator is connected to the oscillator the latter should be working properly. This is easily found out by touching almost any part of the tuned circuit with an Eversharp pencil (advt.) while holding on to 263 the metal parts of the pencil but staying well clear of any other metal work whatever, especially any electrical circuits. If the tube is oscillating the characteristic fizzing will appear at the point of the "lead." A bit of metal such as a screwdriver will serve if nothing better is available. The bare finger will do but takes time to heal. If the tube refuses to oscillate it can frequently be caused to start by changing the points at which RFQ and RFC2 are connected to the tuned circuit, or by changing these chokes themselves or backing them up with other chokes. If this does not do the work one may change the grid bias or temporarily shift the adjustment of Q to a higher wavelength. Usually 5-meter oscillators using a ux-210 or a ux-852 give little difficulty. In fact the 852 was designed for just such work. Having the oscillator in operation one next adjusts for reasonably steady operation and then with the aid of a General Radio 5-meter wavemeter gets the wave right. The further adjustments must wait until the receiver is ready, though we may get the sending antenna up meanwhile. Figure 2 shows some suitable antennas of which the types shown for the Phelps station and the West station are simplest and therefore recommended as desirable for the beginner in 5-meter work. One can gauge the proper length for any antenna system shown here or elsewhere by remembering that for each half-wavelength we will need roughly 100 inches of antenna. Thus the West antenna system must be about 100 inches long minus something to permit use of a coupling coil, while the Phelps arrangement can have almost any convenient length if a little loading, consisting of a few turns of wire, is done at the base. A length of 500, 700, 900 or 1 100 inches works out well. The work at 2EB has been done to a large degree with the last of these lengths. In any case one must keep the antenna well away from things — trees, wires, hanging flower baskets, etc. — for absorption is severe at 5 meters. Depending on the combination chosen, one tunes the sending set up by setting the primary wavelength, i.e. the "tank" circuit, correctly with the General Radio wavemeter and then adjusting the antenna tuning condenser or cutting pieces off the antenna until the proper wavelength is obtained. In the Phelps arrangement in Fig. 2, one cannot well climb the mast to cut pi.ces off the antenna, so the adjustment is made by resetting the sending set condenser, Q, and by using a loading coil of the proper number of turns at the base of the antenna. When the lamp in the antenna glows steadily or the meter returns to the same reading promptly each time the plate supply is closed we are done for the time and can move on to the receiver. THE RECEIVER IT IS perfectly possible to build a 5-meter re* ceiver with a stage of r.f. amplification using the ux-222 screen-grid tube, but the result is a receiver with two tuning controls, and until