Measuring intensity of electric field produced by electromagnetic pulse with aid of active probes

The performance of wideband active probes for measuring the electric field of a weak electromagnetic pulse within the Fraunhofer region of a strong radiation source is evaluated in terms of the proportionality factor between the electric field intensity and the voltage across the probe load, this factor also being the equivalent gauge length of such a probe. Such a probe is usually made of a thin wire segment, placed above a metal surface perpendicular to it, and connected at the lower end through a cable to an impedance load. The current distribution along such a probe is calculated by the method of superposition of traveling waves, whereupon the equivalent gauge length is determined in terms of actual geometrical dimensions and electrical circuit parameters. For a capacitive load with a very large time constant this gauge length does not depend on the frequency of incoming signals. Moreover, in the usual cases of a load capacitance much larger than the stray probe to earth capacitance, the load voltage is directly proportional to the quantity (h + 1/2 l)l (h- length of connecting cable in the field of the electromagnetic pulse, l- actual length of probe) and the probe behaves like a miniature antenna with top feed.