Radio Broadcast (May 1928-Apr 1929)

Record Details:

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98 RADIO BROADCAST DECEMBER, 1928 More Data on Underground Aerials SO FAR as we are concerned the following quotation from a letter from our good friend Dr. G. W. Pickard closes the subject of trick and underground antennas: — "On page 259 of the September issue of Radio Broadcast I notice an appeal for definite quantitative data on the underground antenna. Probably by this time you have found the various references necessary, but in case you have not, I'll give some of the desired facts. "As you know, there are underground antennas and antennas. Some of these consist of plates or coils of wire, variously insulated, but aside from the sucker type of radio fan, no one has taken them seriously, and so far as I am aware, no quantitative measurements have been made. But the real, more-or-less-useful type of underground antenna, consisting of a long, straight insulated wire buried at a slight depth in the ground, is the subject of a considerable technical literature, and its reception characteristics are quite well known. "First, consider 'ShortWave Reception and Transmission on Ground Wires (Subterranean and Submarine) ' by A. Hoyt Taylor, Proceedings I. R. E. Vol. 7, No. 4, August, 19 19. Taylor points out that the buried wire antenna is strongly directive, receiving signals best in the direction of its length, and but feebly from directions normal to the wire. He also explains that reception is possible because of a tilt in the wave-front, which gives a component of the electric vector parallel to the wire. "Next, take my paper, ' Static Elimination by Directional Reception,' Proceedings I. R. E.. Vol. 8, No. 5, October, 1920, wherein it is explained that if static and signal come from different directions, properly oriented directional aerials will eliminate more static than signal. "Finally, consult 'The Wave-Front Angle in Radiotelegraphy' by L. W. Austin, Washington Academy of Science Journal, pages 101-106, March 4, 1921, wherein it is shown that waves are slightly tilted forward in the direction of propagation, this tilt being small and of the order of o°-3°. "It is obvious without going further into the literature of the subject that buried wires often give better signal-static ratios than does the conventional open antenna, simply because they receive directionally. "Now for a numerical answer to Radio Broadcast's question; what is the relative signal strength of a fifty-foot wire in the open, and the same length of wire buried? If the wire in the open is truly vertical, and the earthed wire is hori zontal and but lightly covered with earth, reception on the buried wire will be somewhere between o and 3 per cent, of that on the vertical wire, depending upon soil resistance and the consequent tilt of the wave. If the wire is buried quitedeeply, both theory and Taylor agree that the wave tilt will increase, and hence a somewhat better signal will be obtained on the more deeply buried wire. If you really wish to pursue this matter to the bitter end, that is, literally run it into the Telegraphy by Zenneck, O.00025 = Total number of turns in P aftdS FIG. '■). THE IMPROVED "LAB" CIRCUIT RECEIVER 1 — vwwwv Xc 0.001 FIG. 3 round, see Wireless McGraw-Hill, 1915, particularly pages 260-262 and Figs. 310-317 showing examples of tilted waves for different soil constants. "You see, there is no mystery, no magic about this matter. There is no division of the wave from the transmitter into two distinct parts, one traveling under and the other over, the earth's surface. There is no inexplicable filtering action in a layer of dirt which will strain the static out and let the signal through. A long buried wire is merely an inefficient but directional antenna, and, if it can be aimed at the signal and not at the static, it will give a favorable signal-static ratio. "But more power to you in your attack upon the thousand and one fake contraptions which grow like weeds upon our unfortunate roofs, burrow foolishly in the ground and litter the tables of the uninformed fan. I am afraid an appeal to reason will not reach effectively the class you would protect, therefore, the best way would be to make fun of these fakes. With wire cones, triangles and other Euclidian-looking objects on the roof, weird tangles of wire in pits on the front lawn, Geppert and other dingusses guarding the radio receiver, a poor, puzzled radio wave must scratch its head and wonder how it would ever get in." Is a 112 Tube Needed in the First stage? FIG. 4 FROM time to time we see the statement in an article about high-quality audio amplifiers that it is a good plan to use a 1 12-type tube in the detector socket because of its low output impedance. The object is that under these conditions the first audio transformer works out of a tube whose impedance is, ostensibly, 5000 ohms, instead of 12,000 ohms for the 201-A. Therefore, the articles argue that the low-frequency response will be better. We have often suspected this to be a piece of nonsense, and recent tests made in the Laboratory by Howard Rhodes on a two-stage Sangamo amplifier have proved our contention. Little or nothing has been said about what happens to the high-frequency response of an amplifier when the impedance out of which it works is changed. It seems to be assumed tacitly that nothing happens, or if it does, the difference does not matter. This may be part of the general negligence on the part of amplifier designers to consider the high frequencies as unimportant, occasioned without a doubt by the unreasonable demand on the part of listeners for low-frequency tones all out of proportion to their natural values. Mr. Rhodes's figures (Table II) show that working a good amplifier out of an impedance for which it was not designed may ruin its characteristic. Rp ohms TABLE II Frequency cycles 60 1000 2000 4000 6000 5000 -1.7 0 +1.3 +2.8 +2.6 10.000 -1.7 0 +0.6 0 -1..3 20,000 -2.0 -0.5 -1.1 -3.5 -5.2 30,000 -2.3 -1.0 -3.2 -8.0 -8.6 TU The amplifier consisted of two Sangamo type-A transformers, a cx-327 first-stage and cx-350 second-stage tube. The output device consisted of a General Radio Type 587-D which is made up of a 15-henry choke and two 2-mfd. condensers. The current into 4000 ohms was read while the input voltage was kept constant. The table gives the necessary data on the measurements. The current at 1000 cycles when the amplifier worked out of 10,000 ohms was taken as a zero level; all other adjustments of frequency and input impedance are compared to this value. Thus, at 60 cycles and worked out of 5000 ohms, the amplifier is down 1.7 TU compared to 1000 cycles when worked out of 10,000 ohms. The amplifier has the best characteristic when worked out of 10,000 ohms, and it is assumed it was designed with this impedance in mind. Working it out of 5000 ohms causes a rise at the high frequencies where the secondary capacity begins to resonate with the leakage reactance of the transformer. Perhaps this is because lower input resistance is equal to lower reflected resistance in this resonant circuit so that the resonance becomes more pronounced. This theory is borne out by the fact that many amplifiers, which are perfectly stable when operated out of 10,000 ohms, begin to oscillate badly at 5000 or 6000 cycles when worked out of low impedances. The Sangamo amplifier begins to fall off when worked out of 20,000 ohms but is still much better than many when worked out of the impedances for which they were designed. The truth of the matter is that a well-designed transformer is engineered with some particular input resistance in mind. When worked out of this resistance the characteristic will be the best, if this resistance is something else the characteristic goes bad. If the resistance is 5000 ohms a 1 12-type detector tube will give better over-all frequency response; if it is 10,000 ohms a 201-A detector will be better. The question remains, what is the output impedance of an average detector circuit? Who knows? — Keith Henney.