1. Field of the Invention
This invention relates to a device to neutralize acquired static and voltage impulse transients which occur naturally such as from lightning or from man made sources. The device is used to protect valuable radio and television transmitting and receiving sets and systems from damage due to these transients.
2. Description of the Prior Art
Unlike earlier vacuum tube systems, modern semiconductor based radio and television equipment is particularly susceptible to damage from lightning strikes and other induced voltage transients. Semiconductors are relatively low voltage low power devices with very limited capacity to withstand high voltage. A direct hit by lightning on an antenna is not required, and even a close strike can cause sufficient voltage to be induced to cause semiconductor damage due to surges of DC voltage above the capacity of the semiconductor. Similarly, accidental contact of an antenna with a voltage source such as power lines or induced EMP voltage can cause severe voltage surges in a radio or TV system.
Ordinarily, the semiconductors experience only normal operating voltage levels, typically 6 to 12 volts, provided by a battery or power supply. A lightning strike close to a transmission or receiving antenna or other induced voltage may produce sufficient transient voltage surge to damage semiconductors in the system requiring the replacement of the damaged semiconductor causing severe financial loss and disruption of broadcasting capability. A direct hit on the antenna can be even more devastating, destroying the entire system and perhaps other facilities as well. Further, less severe induced voltages from wind, rain, snow, lightning that ordinarily produce noise or static in the system can be a significant problem for an RF frequency system even if not severe enough to cause semiconductor damage.
Lightning or surge protection devices heretofore known in the art have suffered various disadvantages. For example, early devices utilized a discharge unit comprising a small air gap or spark gap between closely placed conductors connected between the 15 signal conducting line and ground so that if there was a lightning strike, the air gap device arced over discharging the voltage surge to ground. Such devices are inherently inadequate because a relatively high voltage is required to ionize the air gap producing a firing lag time, often permitting greater voltage surges before firing than the semiconductors could withstand. Further, once the unit has fired, a plasma arc path is formed that tends to continue for a period of time after conclusion of the lightning strike resulting in a continued signal loss. Such air gap units can also withstand only a few lightning strikes before the conductors are damaged or oxidized such that their effectiveness is reduced requiring frequent replacement to maintain system safety.
With the development of the gas discharge tube, the air gap devices were eliminated and replaced with surge protection devices using gas discharge tubes that do not deteriorate from frequent use. Examples of such devices are illustrated in U.S. Pat. Nos. 4,554,608, 4,409,637, and 4,359,764 all by the same inventor, Block.
However, such gas tube discharge devices have also been found to be inadequate for a variety of reasons. First, if the breakdown voltage of the gas discharge tube is sufficiently low to assure that voltage surges will not damage semiconductors in the system, peak voltages in the transmitted radio frequency signal may be sufficient to trigger the discharge tube resulting in momentary interruption of signal. Further, since the breakdown voltage of the tube must be sufficiently above the signal voltage to prevent the aforesaid signal interruption problem, minor voltage transients such as storm or equipment induced DC static are transmitted or received and are not immediately eliminated because they are not of sufficient magnitude to trigger the gas tube. Such minor transients must be drained by the dielectric of the transmission line which can take a considerable length of time producing prolonged static and reception noise.
Various solid state devices have been used for lightning surge suppression such as varistors, e.g., U.S. Pat. No. 3,863,111--Martzloff, and avalanche type diodes, e.g., U.S. Pat. No. 3,777,219--Winters. However, such devices have not been satisfactory because such solid state devices have low power capacity making them susceptible to same voltage surge damage they are to eliminate, and they have inherent capacitance which tends to load an RF system reducing signal levels unless corrective measures are taken. Thus, such devices are typically used in conjunction with other higher power devices such as gas tubes and air gap units that will protect the solid state device against high power damage or in systems where the expected power level is limited to a level below the rated capacity of the device.
The present invention eliminates the above deficiencies in the prior art Transients as low as 0.01 volts DC that would produce static or noise in prior systems, are effectively eliminated, but the device is not susceptible to damage from repeated lightning strikes. Further, since the invention uses an inductive coil and does not rely on solid state devices, it can safely dissipate very high induced power levels.