Atmospheric Noise

Lightning discharges consist of one or more transients having current peaks of perhaps 10,000 to 100,000 amperes, and each lasting a few microseconds. Individual lightning discharges produce impulse noise, defined⁴⁸ as "noise characterized by transient disturbances separated in time by quiescent intervals." The electromagnetic energy radiated by lightning discharges follows the laws of radio propagation discussed in Chapter 12. Thus, all other factors being the same, static is weak in daytime and strong at night; it also follows seasonal variations. Static is usually not particularly bothersome in the frequency bands above 30 megacycles, unless it comes from a local discharge.

Because of the impulse nature of static, it may cause shock excitation⁴⁵ in a receiver, causing damped oscillations, blocking, and other operating difficulties. Methods of reducing interference from static include the use of directional transmitting and receiving antennas, with the latter located in regions where static is least bothersome (page 512). To some extent at least, it is possible to obtain a high signal-to-noise ratio by using a strong transmitted signal. Furthermore, certain receiving circuits (page 507) have been developed that reduce the effects of static. However, when static is continuous, most of the methods that have been used are ineffective. In such instances, systems using frequency modulation are advantageous.