CBWI February 1997 MODERN COAXIAL LIGHTNING ARRESTORS: CUSHCRAFT VS I.C.E.
MODERN COAXIAL LIGHTNING ARRESTORS: CUSHCRAFT VS I.C.E.
This is a comparison report between coaxial lightning arrestor units manufactured by Cushcraft Corp. and Industrial Communication Engineers, Ltd. Both companies make a variety of such protective devices and are sold worldwide.
The Cushcraft "Blitz Bug" design and the I.C.E. design are both protected by patents issued by the U.S. Bureau of Patents and Trademarks in Washington, D.C. Cushcraft manufactures two different arrestor units that are basically the same principle but utilize different methods.
The first and most basic is the "Blitz Bug" patented by Mr. Cushman around 1960. In this device both outer coaxial conductor (shield) and center conductor pass directly through the unit.
Three metal fastener screws are drilled and tapped into the outer metallic conductor and driven in to a close proximity to the center conductor.
With the outer conductor at ground connection potential a voltage spike exceeding about 1,500 volts that develops between the center conductor and ground arcs across the space between the center conductor and the tips of the embedded screws.
There are no other parts in the unit. The second and more modern unit uses nearly the same philosophy but uses a gas discharge assembly between the center conductor and an external insulated ground terminal fitting protruding through the case.
The gas discharge unit (GDU) has a rated breakdown voltage in the 400-1000 volt range to permit the transmission of an RF voltage waveform through the unit without creating a sufficient voltage potential referenced to ground to ignite the device.
When a voltage greater than the breakdown voltage of the GDU appears across the center conductor referenced to ground the gas unit ignites, creating a temporary low resistance path to ground, thus neutralizing the potential.
While these arrangements may offer suitable protection in a few cases they both suffer from numerous limitations that we believe to be serious.
1. The case of the more modem unit that is connected to the coaxial cable outer conductor passes across the unit and m provision is made for grounding the case directly to earth neutral. In lightning strike applications of both direct hits and indirectly (inductively) coupled events various measurement studies have shown that as much as 80% of the incoming surge flows down the shield of the cable.
The unfortunate result is that a large amount of the strike simply passes across the arrestor chassis and reaches station equipment frames, dividing between many destructive paths seeking ground. 'Me case of the earlier "Blitz Bug" design encourages connection of the shield conductor to ground, even providing a terminal to do so.
2. Both units use pass-through center conductors. Although the gas discharge assembly and the arc gap (Blitz Bug) both ignite when their respective breakdown voltages are reached many hundreds or thousands of volts are presented to the radio equipment before the arrestor action occurs.
In either case when used with solid state radio gear it means that the equipment will nearly always be damaged or destroyed before the arrestor activates to neutralize the incoming surge wavefront.
3. Use of gas discharge units or arc screws as a sole-source mechanism for neutralizing lightning currents delivered by heavy coaxial cable line conductors is controversial. Gas units have only a small dissipative power rating, seldom exceeding 1 watt. While the devices can handle large jolts of thousands of amperes of current, they can perform that service only if the entire impact event lasts only a few microseconds.
Lighting currents, especially slowed down by time constants due to the inductance of transmission line conductors are much slower to rise, endure, and dissipate. The result is frequent rupture and failure of the GDU, requiring down time and parts replacement.
In the case of arc screws each "hit" causes some of the screw tip to be burned away so that the next jolt must be even larger to start an arc. Additionally, it is difficult to determine in either case the actual condition of a GDU or the arc screws in actual field service after they have been used for a time.
GDUs often fracture and break apart while arc screws scar and often weld themselves to the case. In both cases it is assumed, of course, that the internal resistance of the radio equipment that can take input jolts of the magnitude and service without damage or destruction.
4. In both designs m constant drain method is employed to leak static development from cables. A coaxial line often acts like a large capacitor, storing an electrical charge that can only leak off the line through antenna joint connections or through the insulated dielectric region between the conductors.
When this occurs it nearly always causes receiver "hash" noise during electrical activity.
The I.C.E. design, shown below on the right side, took these characteristics into account during development and testing.
Our arrangement uses a central high voltage rated blocking capacitor which allows the free flow of RF energy through the arrestor device but blocks DC voltage and low frequency AC voltage.
The heavy inductor on the antenna side of the unit is the primary neutralizing agent. Voltage development is quickly shunted to ground through the DC shorting nature of the inductor/RF choke.
If large currents of a fast-rising nature are presented to the arrestor in such a way that a back- MF develops across the inductor then the companion paralleled gas discharge unit ignites, but its only workload is to collapse the short-lived magnetic field of the inductor.
The result is an arrestor that is constantly active, requires non pre- determined voltage to activate, and whose GDU workload is so low that it will probably last forever.
The added resistance on the equipment side of the arrestor was inserted to provide a similar drain function on the user side, shunting away tiny currents that may appear from capacitor dielectric leakage during an impact event. Schematics for all three are below: