A suitable antenna is needed -- see Atmospherics Monitor [1]. This is constructed of seven lengths of approx. 1mm (1/32") diameter enamelled copper wire, soldered together as one at the base. The tips are sharpened with a file, to improve sensitivity. These seven copper "javelins" are each 70 mm (3") long from the point at which they separate from their base, and form a 70mm (3") diameter circle at their tips. At their base, the "javelins" are soldered to the core of a 15 m (15 yd) length of screened microphone cable. This must have a quality screen, which cheap substitutes sometimes do not have. The cable's core, at the antenna end of the cable, together with its insulation, protrudes from the screen by 100mm (4") as shown in the diagram. That is, 100 mm (4") of the outer screen and insulation is cut away. The copper "javelins" are fixed into the top of a length of PVC piping. The microphone cable is then secured with a cable tie, to ensure that it will not pull loose. The whole assembly is mounted about 2½ metres (2½yards) off the ground at its tips, and preferably half a metre (half a yard) or more away from any exterior wall. Directly beneath the antenna, at the ground, a section of the microphone cable's screen needs to be exposed, so that it "shows through" the insulation, and this is securely connected to a metal stake in the ground. For safety purposes, the remaining length of microphone cable (several metres/yards) is buried in the ground, and its far end is taken to the circuit (pin 3 of IC1). The screen of the microphone cable is wired to the circuit’s 0V (pin 4 of IC1). You may click on the diagram to enlarge.
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