Modern Lightning Protection For Radio
Facilities: RF Entry Ports
Lightning is one of Nature's
most destructive forces. It has the power of a good-sized explosive and cannot be
avoided if you're connected to antennas that are high and in the clear. And it's
not just lightning. On a recent evening our 160 Meter dipole (260 foot wire span)
strung between towers at 180' here at the I.C.E. factory exhibited several hundred
volts of charge from a light rain shower enough to shock one of the technicians working
with the cable outside. During an electrical storm with overhead discharges many
thousands of volts have been measured on this wire, respective to earth ground terminals.
In installations employing coaxial feedlines the measures used to protect station
equipment are simple but critically important. Here is a list of observations and
our recommendations ...
- Always bring coaxial cables to ground level before entering equipment area.
Never bring coaxial lines into the building at an elevated height directly. Lightning
currents induced into the cables will be forced through the equipment chassis
on the way to ground, and that's what causes extensive damage. Even if your equipment
is on the second floor, always bring coax to ground level first and insert appropriate
lightning protection, then route the cable to the station gear.
- Absolutely, positively ground coaxial cable shields with as short an earth terminal
connection as possible. Use a commercial shield grounding block if possible,
or fashion your own. In most cases as much as 80% of an induced or direct lightning
blast comes in on the shield. This is because of the external exposed nature
of the shield and its larger metallic mass. Always make sure that grounding of
the shields occurs before the cable enters the building or reaches equipment
chassis. Multiple shield grounding (such as once at the tower base and once more
before building entry) is an excellent, low cost idea.
- Use lightning arrestors on lines that feed sensitive electronics. But beware:
Don't use so-called lightning arrestors that offer nothing more than a
device to ground. These units are DC-passive and only activate when the potential
voltage between conductors reaches hundreds of volts. By that time in most cases
the radio gear has already been damaged before the arrestor attacks, leaving
you with an arrestor that did mostly nothing and a damaged rig. Also, gas discharge
tubes are very low power rated, typically only about 1 watt dissipation. They
may be "rated" for 20,000 amps or more, but only if a lightning bolt
starts and ends in a few billionths of a second. Few bolts ever do, and blasts
that are slowed down coming through transmission lines almost never do. That's
why gas discharge arrestors require repair or replacement so often. They're overpriced
and offer little, if any, protection from induced voltages.
If arrestors are used always specify a blocking-type arrestor - a unit that
has no DC continuity from input port to output port and some means to handle
current besides or in addition to a gas tube.
- Establish a bulkhead grounding system near the radio equipment where the distance from the
bulkhead to coax entry is short - preferably less than a foot. Use this bulkhead
for lightning protection as well as RF neutral for interference filters and similar
items. The bulkhead can be a bar, metal sheet or just heavy wire. But remember
- the length of ground leads is far more significant to good grounding performance
than the specific materials or even wire size used.