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Sun (Auckland), Volume I, Issue 303, 14 March 1928, Page 14

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UNDERGROUND WIRELESS EXPERIMENTS WITH A PORTABLE By H. F. Goodman, late Captain Royal Engineers Having, during the early days of the war. carried out experiments with spark sets on ground antennae, I was glad when the opportunity presented itself recently to test a valve set underground. For obvious reasons the use of much power during the war was undesirable and, until the advent of the thermionic valve, underground or even close to the ground antennae was found impracticable, tremendous power being necessary to transmit only a few hundred yards.

Communication by wireless to-aay is so far removed from those strenuous days of 1915 that the average listener-in may not appreciate fully the difficulties the Wireless Corps encountered in its endeavour to organise and maintain the lines of communication necessary for an army in the field. To-day one simply turns a small filament switch, rotates the condenser dial and there one has station after station to choose from, a few thousand miles being “no trouble” to the modern valve set.

One man can carry, erect and control to-day an outfit having a far greater range than the army set of 1915, which required for its transport erection and control no less than 12 men, 10 horses, two wagons and one motor-cycle.

ARMY’S UNCEASING EFFORTS These army stations used different types of aerials, but they were necessarily large and high. Apart from the question of transport the height of the masts offered such disadvantages that determined and continuous efforts were made to discover a circuit that would work over a short distance without this high antennae. Men, money and materials were thrown into the work. Experimental stations were erected in various parts of England. The army and the universities were combed for men with special qualifications. A factory was erected for the building of sets and the trying-out of new circuits and new ideas, but all without appreciable result. The so-called high-powered stations would not function on ground antennae. Certain results, however, were noted, and may prove of interest in view of the recent tests: 1. The lower the aerial the greater the capacity in the aerial circuit. So marked was this result that in sets with fixed wave lengths extra inductance had to be added to ensure sharp tuning. 2. Directional effects became more noticeable as the aerial was lowered. 3. A heavily insulated aerial lying on the ground proved useless. FORTY-SIX FEET DOWN For my underground test a fivevalve Travler portable set, having a frame aerial only 13in by 12in, was used. The construction of a number of sewage tunnels offered such an inviting chance that one fine afternoon I sought the contractor’ spermission and commenced operations. The tunnels, all 4ft 6in high, 2ft 3in wide, and varying considerably in length and depth, are situated exactly six air miles from IYA, and are screened for a distance of approrimately three miles by a range of hills, and all gave similar results. They were in course of construction, having been driven 103 ft at a depth of 46ft. Before entering the tunnel, the set was accurately adjusted for the greatest volume and all readings were noted. Volume was good with the frame aerial in any position, although drectional effects were noticeable^ After penetrating 10 feet directional effects were more apparent, and the set had a tendency to howl. At 30 feet only weak signals were heard unless the aerial was absolutely directional to IYA. Moving the aerial either way off the line reduced the volume tremendously; in fact, it was scarcely audible on the speaker. The set howled badly, and to correct this the audio filament current had to be considerably reduced. At 103 feet the tunnel face was reached. Directional effects at this point were most marked, the aerial having to be absolutely directional to the transmitting station, which was at a right angle to the direction of the tunnel. One inch out of line and down went the signal strength. Three inches out and signals were not audible. The set still had a tendency to howl, and the filament current — audio only—was again reduced without loss of volume. That volume was marvellous, although it mav have been accentuated by the confined space. While my experiment may be of any scientific value, it is of interest to find that placing the aerial close to or under the ground produced the same results as were recorded on the old spark sets, i.e., increased capacity in the aerial circuit and marked directional effects. MEMORIES Reclining in that muddy tunnel, listening to loud, perfectly reproduced signals, I was forcibly reminded of our war-time efforts and of the great advance made in wireless science since that time. What a priceless possession this portable would have been to my old corps! If there is ever another war I shall, I know, meet many old sapper friends and we shall laugh most heartily when we remind each other of the gear we were expected to “get through” on during 1915.

H.T. ELIMINATOR VOLTAGE It is a well-known fact that the voltage given by H.T. eliminators depends largely on the current which is being consumed at the moment of taking the voltage reading, and in some cases the output voltage drops very rapidly when currents even as small as 10 m.a. are being taken. Bad cases of faulty regulation are generally due to the use of inferior components, such as chokes of high resistancy, but a certain drop of voltage with increasing current consumption is unavoid - able. _ , . The plan usually recommended when measuring the output voltage is to employ a high resistance voltmeter, which consumes a very small current. This is all very well, up to a point, but again there is a probability of faulty readings owing to the fact that the voltmeter now takes less current than the receiver on which the unit is to be used, and the voltage indicated will therefore be greater than that obtained in actual practice. The best plan is to plot a current voltage output curve for the unit.

A voltmeter is connected across the output terminals and in parallel with it is a milliameter and variable resistance in series. The voltmeter should not take more than one or two m.a. for a full scale reading. The variable resistance should be variable between about 10,000 and 100,000 ohms, such as a tapped potentiometer. The current taken by the voltmeter should be added to the milliameter reading", and the voltage current curve plotted. From which, knowing the drain of the receiver in question, we can immediately state what voltage the eliminator is delivering.