A design goal of the GP-1 project is that all components should be readily available from common retail sources. No exotic items are required. The few pieces of non-electronic hardware can be obtained from local hardware stores or home centers. Sources for electronic components used by the authors, are listed in the Reference section.
Used in conjunction with the Antenna Array, the Signal Source generates a magnetic field (B) that approximates the field from distant lightning. By simply orienting the position of the Source inside the Antenna Array crossed loops, a signal from any direction can be simulated. Negative (NR) or positive (PR) return stroke simulations may be selected by panel switch, S2. Intracloud or cloud-ground return strokes are selected by S1 “Biph” (biphasic) or “Mono” (monophasic) positions, respectively. Near or far stroke signals are approximated by S3 being in the “Hi” or “Lo” position. A voltage field (E) signal, of identical shape and timing to that of the magnetic field, is also generated and is connected in place of the e-field antenna at J0 on the Preamplifiers-Integrators circuit box at the antenna base. The above switches produce the same changes in the voltage field signal. In addition, S5 allows selection of the signal source to approximate the capacitance of the e-field antenna plate (100 pF) or to appear as a very stiff low impedance by selection “100pF” or “Short”, respectively. The low impedance source selection is required if the antenna preamplifier E-field input capacitance must be quite large (see attenuation discussion in Antenna Array). Thus, virtually all GP-1 performance characteristics can be tested and appropriate adjustments made. Because the Signal Source gets rather rough use, it should be constructed in a robust manner.
A small metal box houses the circuit board and batteries and supports the PVC pipe loop. The five switches, S1-S5, E-field output binding posts, and the LED are all mounted on the front face of the box. A pair of Schedule 40, 3/4-inch tees are modified by removing half the material on one arm by two saw cuts on the center lines, as shown. These cuts provide a platform for the circuit board. The pair of tees are fastened to the box base with small angle brackets. A pair of 1-1/8-inch holes are punched in the front and the opposing faces of the box base for the short lengths of 3/4-inch PVC pipe to insert into the tees. The rest of the pipe loop is completed with four 90-degree elbows and three lengths of pipe. After carefully dry fitting the pipes and elbows, cement all joints together.
To most easily thread the wire through the pipe loop for the required six turns (L1 on the lower right of the schematic diagram), first put a string leader in place. Tie a small weight, such as a 1/4-inch nut, to the light string. With the plane of the loop vertical and a tee opening upward, drop the nut to the bottom of the first elbow. Rotate the loop 90 degrees and allow the nut to fall to the next elbow while feeding the string. Repeat three more times so that the nut and its string emerge from the other tee. Next tie the string leader to one end of a 28-foot piece of stranded, insulated wire; wire gauge is not critical. While pulling on the far end of the leader, feed the wire into the other tee and pull the wire completely around the pipe loop until it emerges and overlaps the wire entering the opposite tee. Untie the leader and temporarily tape the free end of the wire to the other wire crossing between the tees. Now, grasp the two wires and push the loop around to make a second, third, etc. turn until six full turns are in place and the taped end is between the tees. Remove the tape and then push the loops a bit further so that a few inches of the wire end emerge. Cut the other end of the wire so that there is, again, a few inches emerging.
The loop wire end emerging from the front panel-side tee is the hi terminal of L1 and the other end the grounded terminal. At this time, connect the grounded end to a solder lug bolted to the box base. By polarizing the loop in this manner, the front panel will always face the direction from which the simulated signal is coming; for example, when the Source front panel faces northeast when inside the Antenna Array, a signal from the northeast is simulated.
All the panel items are mounted to the box base and the four 4-cell AA battery holders bolted to the box top. To keep the batteries from falling out of their holders, invert the box from its normal orientation. Now the box "top" is actually the box bottom in this construction. After connecting the battery holders in series, they are wired to a small 4-pin polarized connector as shown on the lower left of the schematic diagram. The mating connector can be wired to the Off-On switch, S4, and to a ground lug bolted to the box base.
A circuit board is available for this Signal Source from FAR Circuits. See the References section for details. If you prefer to fabricate your own board, then a suggested procedure follows.
Fabricate the circuit board from the artwork by any method you choose. We have had very good success with the toner transfer method when using the manufacturer's heat press (fuser). With this method, the artwork should be sized to the exact dimensions indicated and then either Xerox-machine copied or laser-printed to the special toner transfer paper. Make sure that the final toner transfer paper image looks exactly (wrong reading) as shown on the artwork. This is then fused to the bare, clean copper board and etched according to the manufacturer's instructions. In our hands, fusing with a household flatiron gave uncertain results.
After cleaning and drilling the board, two jumpers are installed on the component side. Following this, the remaining components are installed on this same component side. Flying leads from the circuit board are connected to the various front panel components. A ground lead is connected to the box base and the remaining L1 lead to the hi board loop pad. The circuit board is mounted by wedging it in the gap between the pair of modified PVC tees.
No adjustments are required for the Source. The Source performance can be tested by noting that the LED flashes briefly about once per second when the unit is switched on. Obtaining the waveshapes as shown will confirm that it is performing properly. Leave the temporary, taped auxiliary loop in place, as it will be used for testing the Preamplifiers-Integrators circuit board.
Battery life is several tens of hours, but care should be taken to switch the unit off when not in use.