A portable, self-contained, compact instrument measures and records transient electric fields generated by nearby lightning strikes. This instrument complements, and in many respects is similar to, the one described in "Instrument Records Magnetic Fields Generated by Lightning" (KSC-11769), NASA Tech Briefs, Vol. 19, No. 4 (April 1995), page 38. Both instruments are designed to be placed near sensitive electronic equipment before thunderstorms begin. The data recorded by the instruments during thunderstorms can be analyzed afterward to determine whether the electromagnetic fields associated with the lightning were strong enough that they might have damaged and/or affected the operation of the sensitive equipment. Thus, the instruments provide data that can be used in deciding whether the sensitive equipment should be tested for damage and/or other effects caused by lightning. Typical installations in which the instruments could prove beneficial include outdoor sensing equipment, computer rooms, broadcasting stations, and powerplant-control rooms.

Three Orthogonal Antennas sense the three orthogonal components of the rate of change of electric field; that is, Ex, Ey, and Ez. These components and their time integrals (proportional to the electric field) are sampled and recorded for subsequent analysis.

The present instrument (see figure) includes three orthogonal antennas on an electrically conductive sphere. Each antenna senses one of the three orthogonal components of the transient electric field. The current i(t) induced in each antenna is proportional to the rate of change of the electric-field component E(t), and is given by

where t is time, A is the area of the antenna, ε is the permittivity of air (very close to ε0, the permittivity of the vacuum), and κ is a constant that expresses the concentration of the electric field in the vicinity of the antenna or a similar electrically conductive object. The spherical shape was chosen because κ for a sphere is easily determined and is found to equal 3.

In the instrument, the currents are measured to determine the rates of change of the components of the electric field. The current signals are also integrated to obtain signals proportional to the electric-field components.

The instrument includes a microprocessor that controls its overall operation. It also includes an analog-to-digital converter and a sampling clock. Under control by the microprocessor, the analog-to-digital converter samples the waveform of one component of the electric field at a rate of 10 MHz for a duration of 50 µs. (The reason for not sampling all three waveform components is simply that doing so would consume too much power.) Also under control by the microprocessor, the peak values of all three components of the electric field and their time derivatives are sampled and compared with specified threshold levels during intervals of 1 ms. The electric-field waveform sample values and their times are stored in a nonvolatile random-access memory (NVRAM). The peak electric-field and derivative sample values that exceed the threshold levels, and their times, are also stored in the NVRAM.

The stored values are subsequently read out by use of a portable computer. The instrument is powered by batteries and can operate unattended for as long as two weeks. The inclusion of the NVRAM prevents the loss of data in the event of a power failure. The batteries can be changed in the field, so that the instrument can remain in place and continue to measure the electric field without interruption.

With their 10-MHz sampling rate, both this instrument and the previously reported magnetic-field instrument measure electromagnetic fields generated by lightning more accurately than do portable commercial magnetic-field meters. Lightning waveforms typically include frequencies up to tens of megahertz, while the commercial meters, which are designed to measure magnetic fields of high-voltage power lines, are usually limited in frequency response to a few hundred hertz.

This work was done by Pedro J. Medelius and Howard James Simpson formerly of I-Net for Kennedy Space Center. KSC-11953