Radio Broadcast (May 1928-Apr 1929)

266 RADIO BROADCAST SEPTEMBER, 1928 R.F.Choke FIG. 3. A 5-METER RECEIVER CIRCUIT This receiver, which is of the double-detection variety, is primarily a c.w. set, as explained in the text. It may be altered to a phone set by dropping off the heterodyne and adding a ymeter heterodyne to beat with the incoming signal as in the more usual broadcast receivers. Ci , C2 and Cj are trimmed vernier condensers with a maximum capacity of 1 5 mmfd. Li consists of from 1 to 4 turns on a 1" diameter, depending on the rest of the circuit. C5 should not be over o.oooi mfd., if used. Some primary turns may have to be removed fromthe first i.f .transformer to compensate for the effect of C^. The second and third if. transformers, I.F., will not need this. C6 is a 25-mmfd. variable condenser tuning oscillator coils Ly and L8, which are an i.f. transformer similar to the others. IV ork first without the oscillator and with a.c. plate supply at the transmitter, and then change the transmitter to d.c. and use heterodyne. The sensitivity may be improved by returning the i.f. grids to a potentiometer across the A-battery. A metal panel and baseplate are very desirable. FARTHER AFIELD \TOW it is time to put the receiver into the *• ' family car and to go around the neighborhood exploring. Here each man may be his own guide and I will go no further than to show some of the local effects we have found around 2CSM, 2eb and ioa. In Figure 4 are shown some curves taken near ioa to show that the 5-meter wave signally fails to act as had been predicted. It does not die off 100 yards from the station as we had been told by all authorities, but compares very nicely with normal waves. It does not, in fact, die off as fast near the station as does the 20meter wave which we all know to be so effective at great distances. Similar curves were run at 2EB at least 18 months ago with very similar results, but the data for these is not at the moment available to me. They are here mentioned because they brought out another very interesting point, which is to say they showed us that normal points would fall on the curve even though measured behind houses where bad screening had been expected. On the other hand points behind hills were weak, as one might expect. II J q I 1 * eat ditfana DISTANCE,MILES FIG. 4 When the effect of a hill in the line of transmission is investigated one finds effects such as shown in Fig. 5, which shows the effect of a certain round knob of a hill near Hartford. The shadow shape indicates the area in which the screening effect of the hill made reception weak or impossible, while the light lines are drawn in to suggest the general manner in which the wave fronts are deformed. Note the similarity to that of water ripples passing a rock. A variety of curious effects can be observed. With a 160-meter signal one loses a great deal of intensity when the receiver is carried through a street crossed by an elevated railway structure. The 5-meter signal, on the other hand, cheerfully ignores the structure, probably because the openings in the structure are very large as compared to the wavelength. Again, when passing under wire lines coming from the general direction of the transmitter one finds a very strong "bump" in the 160-meter signal and little or none in the 5-meter signal — which is rather difficult to explain. Many such explorations were made in the radio flivver mentioned in the first article, likewise with various other cars. The intensities were measured and curves plotted from them. Reflector systems were tried and the general result obtained shows that the parabola does not seem to be the best reflector shape, also that the reflector wires are of best effect when not of the antenna length but a trifle longer. DISTANCE EFFECTS ALL of the foregoing is not spoken of to discourage attempts at long and medium distance work. On the contrary it seems that what has ailed the 5-meter game is that there has been entirely too much short-distance work and too little attempt at long distance work. Not only must it be remembered that in turn the 200, 100, 40, 20, and 10-meter waves have defied theory and been of practical use, but in addition there has already been not a little reasonably good long distance 5-meter work. Of course we know that 20-meter work has limitations as to time and distance, and we suspect that this is even more the case with the newer 10-meter wave. Hence it is quite possible that the 5-meter wave will show its usefulness at a distance under such restricted combinations of time, weather, and distance that we may not utilize these possibilities at all and confine it to local " beam " work only. However, in view of the limited work that has been done, the few failures do not warrant dropping the wavelength. For those that care to try it there is another possibility. Somewhat over a year ago the Jahrbuch described some Telefunken experiments in which various short waves were projected upward by a huge reflector that could be rolled about on the German field which acts as an equivalent for the wgy "radio-acres" at Schenectady. Some very high angles were found successful for transmission to Buenos Aires. Norvell Douglas thereupon suggested a test with the same device at 5 meters and the possibility that a slightly diffused beam shot vertically or nearly vertically upward might have a rather good chance of covering a considerable area, no matter what the exact height of the reflecting layer, if there is one that works under those circumstances at 5 meters. A reflector for the purpose is still waiting at 9EHT untii we can try the thing fully, which must of necessity be delayed until early September, since Douglas is at present engaged in some J-meter work at General Electric's research laboratory. WHAT IS LEFT? \ A / E HAVE, then, a number of interesting * possibilities to back up our interesting actualities. One who has been at this 5-meter affair for quite a considerable while assures you that the working out of these possibilities is an enchanting game, made doubly so by the fact that everyone else has not done the same thing 10,000 times. Suppose then that a few of us try this Telefunken-Douglas near-vertical or wholly vertical beam while a few of the rest of us build decent double-detection receivers and listen for them. There is room also for some "straight" receivers and transmitters such as have been suggested. Certainly there are too many oneway 5-meter contacts in existence to let the twoway possibilities go on in their present state. I am willing to help as 1 may, which is to say I'm perfectly willing to answer letters if their writers will but recall that stamped envelopes with a return address greatly encourage a reply! FIG. 5