Precipitation Static

Although many details are involved, precipitation static could be controlled if receiving systems not sensitive to the static were developed, or, if the accumulating charges on the airplane were drained away slowly and uniformly, so that no sparks and similar discharges occurred.

One of the first developments⁵⁶ was the use of the shielded loop (Fig. 27) for reception when precipitation static is bad. Such a loop is shielded so that it does not receive energy from an electric field but does receive energy from a magnetic field. Furthermore, a loop is directive and can be "aimed" in the direction of the desired station. The loop is close to the source of disturbance on the airplane and is in the induction field (page 441). Because of the high-voltage nature of the precipitation-static discharges, the electric-field component of the induction field is strong, and the magnetic component is weak. Thus, the electrically shielded loop does not pick up the same amount of noise that would be picked up by an open-wire antenna.⁵⁷ The radiation field from a distant station arrives at the airplane with equal electric and magnetic components, and the shielded loop receives the desired signal by magnetic induction (page 504). "Open-wire¹¹ antennas, insulated with polyethylene, are also used,⁵³ This insulation reduces the possibility of corona or other discharges from occurring on the receiving antenna where such discharges would be in a strategic location to cause interference. Methods of draining away the charges were early used. These have taken the form of trailing wires,⁵⁶ wicks,^52,53 and other devices. Tufts of fine stainless steel wire have been advocated.⁵³ Radio Noise Meters.⁵⁹,60,61 Various instruments and methods have been used for measuring radio noise. Cooperative work has resulted in the development of a standard instrument and methods of measuring radio noise.⁵⁸ This instrument covers the ranges of 150 to 350 kilocycles, and 540 to 18,000 kilocycles. The radio noise meter is essentially a superheterodyne radio receiver with suitable weighting arrangements in the circuit of the second detector so that the response of the measuring instrument in this circuit will have the desired characteristics.

Figure 27. An antistatic shielded loop antenna located for experimental purposes beneath an airplane. In practice the loop is usually located beneath the front portion of the main body of the airplane. The dark band around the loop is an insulating insert in the metal shield so that the shield will not offer a continuous path to current flow. (Courtesy Bell Telephone System.)