|Modern Coaxial Lightning Arrestors:
Polyphaser Vs. I.C.E.
This is a comparison report between coaxial cable lightning arrestor units manufactured
by Polyphaser Corp. and Industrial Communication Engineers, Ltd. Both companies
manufacture a wide variety of such protective devices and are sold worldwide. Each
of the designs described in this report is protected. by patents issued by the U.S.-
Bureau of Patents and Trademarks in Washington, D.C.
Although there are some subtle variations in the product line, Polyphaser's basic
coaxial line layout is basically a two component system. As shown in the schematic
below, a high-voltage rated capacitor is used as a central blocking device to permit
the unimpeded flow of RF currents through the arrestor while blocking DC voltages
and low frequency AC voltages from passing through the arrestor while blocking DC
voltages and low frequency AC voltages from passing through the device to reach
station equipment. A gas discharge assembly having a breakdown voltage rating in
the 400-1,000 volt range is used for transmitting services so that when a difference
of potential between the conductors reaches this amount on the antenna side of the
polarized unit the gas discharge unit ignites, shunting the voltage surge to ground.
While this is certainly a workable arrangement and the Polyphaser units are well
built, we concluded in our engineering studies that there were significant limitations
to the design. Among them:
- No constant drain mechanism is provided in the Polyphaser design. A coaxial
line acts often like a large capacitor, storing electrical charge that can only
leak off the line through antenna joint connections or through the dielectric,
nearly always causing receiver "hash" noise during electrical activity.
- The use of a gas discharge unit as a sole-source mechanism for neutralizing
lightning currents delivered by heavy coaxial 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. Lightning currents, especially slowed down by time constants due
to the inductance of transmission lines are much slower to begin, endure, and
end. The result is rupture and failure of gas discharge units, requiring frequent
replacement and down time.
- It is very difficult to determine the condition of a gas discharge unit, especially
after it has taken a few "hits". They don't always go short circuit.
The I.C.E. design, also shown below, took these characteristics into account during
development and testing. We also use a central high voltage blocking capacitor,
but with a large discharge inductor on the antenna side as a primary neutralizing
agent. Any 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 magnetic field of the inductor. The result is an arrestor whose gas
unit undertakes such a low workload that is will probably last forever. To date
no replacement gas units have 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. I.C.E. uses a four part system.