|Modern Coaxial Lightning
Alpha Delta Vs. I.C.E.
This is a comparison report between coaxial lightning arrestor units manufactured
by Alpha Delta Corp. and Industrial Communication Engineers, Ltd. Both companies
make a variety of such protective devices and are sold worldwide. The I.C.E. design
described in this report is protected by a patent issued by the U.S. Bureau of Patents
and Trademarks in Washington, D.C.,
Alpha Delta's primary configuration is a one part system consisting of a gas discharge
breakdown unit connected in a shielded enclosure between the coaxial center conductor
and an insulated, external ground terminal fitting protruding through the case.
The gas discharge unit (GDU) has a rated breakdown voltage in the 400-1000 volt
range to allow the transmission of an-RF waveform through the unit without creating
a sufficient voltage potential referenced to ground to ignite the conductor referenced
to ground the gas unit ignites, creating a temporary low resistance path to ground,
thus neutralizing the potential.
While this arrangement may be suitable protection in a few cases it suffers from
numerous limitations that we believe to be serious. Among them:
- The case of the unit that is connected to the coaxial cable outer conductor
passes throughout the unit and no provision is made for grounding the case directly
to earth ground. In lightning strike applications, both direct hits and indirectly
(inductively) coupled events, measurement studies have shown that as much as
80% of the incoming surge flows down the exposed 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.
- The unit uses a pass-through center conductor. Although the gas discharge assembly
will ignite when the breakdown potential is reached many hundreds of volts are
presented to the input of the radio equipment before the arrestor action occurs.
In modem solid state terms it means that the radio will nearly always be damaged
or destroyed before the arrestor activates to neutralize the income electromagnetic
- The use of a gas discharge unit 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, I 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. Lightning 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. Additionally, it's difficult to determine the
condition of a GDU in service, notably after they have taken a few suspected
"hits." They don't always go short circuit, but they sometimes fracture
- No constant drain method is employed to leak static development from cables.
A coaxial line often acts like a large capacitor, storing electrical charge which
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-EMF 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 no predetermined voltage to activate, and whose GDU workload is so low
that is will probably last forever. To date not a single replacement gas unit has
been sold by us. The added resistance on the equipment side of the arrestor was
inserted to provide a similar drain function on the user side of the arrestor, shunting
away tiny currents that may appear from capacitor dielectric leakage during an impact
Schematic diagrams of the two designs appear below: