Note: this is a copy of the pages from the X1W-1 manual that deal with the subject. It is presented here as a convenience.
Interpreting The Data
The EMP sensor is essentially a radio receiver that is most sensitive in the VLF portion of the spectrum. The energy released from a lightning discharge is greatest in this portion of the spectrum. A simple low pass filter followed by a darlington-pair of transistors makes up the receiver. Nominally, it takes a pulse that will produce a Vin >= 1.4V to turn on the output transistor. This is due to the two forward Vbe drops of 0.7V on each of the transistors used in the darlington-pair. Not only does this improve the gain of the circuit, but also serves to offer a large degree of immunity to man-made noise and interference (i.e., under normal circumstances only a lightning discharge is capable firing the output).
Since the receiver is very similar to the old “crystal” radios, an efficient ground plane is required for operation. In fact, the effectiveness of the ground plane will be the greatest factor in determining the sensitivity of the receiver. The second most important factor is height. The higher the sensor (i.e., the antenna), the greater the sensitivity.
From the above discussion, it can be concluded that sensitivity is dependent on height above ground and the presence of an adequate ground plane. A height of 20 to 30 feet seems to work best. An effective counterpoise, or ground, can be made using ground rods. However experience has proven that the best performance of the sensor is when the steel frame of a large building is used as the counterpoise.
Interpreting the Data
Sensor response will vary greatly from one location to another. It is not unusual, especially during the summer months, for the sensor to hear several strikes every hour. The higher the sensor is and the more efficient the ground is, the more strikes the sensor will detect.
So how do you interpret the data? There are no hard and fast numbers. At the EVVWXN location, if the sensor trips, there is an active storm within about 30 miles of the QTH. However, at OWBWXN, there is almost constant activity. At NEWWXN, the threshold for when a storm is in the immediate area is around 200 hits in a five minute period. If you graph the strike data, it will help put things into perspective. Looking at numbers on a page can be misleading. If it helps, imagine listening to a ball game on AM during a summer evening. You may hear static crashes from time-to-time faintly in the background. As the storm draws closer, the amplitude and frequency of the static crashes increases. It's the same here. Over a period of time, you will begin to get a feel for how your sensor performs.