Adequate grounding means different things to different engineers. To
the electrical engineer it means compliance with the National Electric
Code (NEC). To the RF engineer it means proper grounding and lightning
protection for all pieces of equipment connected with RF power
transmission. To the audio engineer it means correct grounding to
eliminate the possibility of hum or instability developing in the audio
signal. Overall it also means compliance with those portions of the NEC
that apply in particular circumstances, as well as individual state and
local ordinances. In addition, the engineer is concerned with questions
of audio quality and hum. Overriding all the considerations of audio
quality and noise in the signal is the necessity for providing a safe
system that is free from electrocution or burns from primary power ac,
RF power or dangerous ac voltages developing in the audio equipment
wiring and equipment.
I will not touch much on power engineering at this point because
more and more it is becoming beyond the purview of radio engineers
whose power installations usually have to be completed by
state-approved and licensed contractors, and most radio engineers are
not state licensed to perform such work. Sometimes this leads to
problems when new installations are made that are subject to inspection
by non-radio minded inspectors before completely installed, as well as
afterwards. It seems that as the world becomes more and more regulated
and licensed individual freedom of design and application becomes more
circumscribed by political, or local union interference.
A secure and reliable ground system is a critical part of any RF
installation. Photos by Ben Weiss, CPBE.
A typical argument with electrical inspectors frequently is ac power
connections inside equipment cabinets. Sometimes there is a wide
divergence in opinions and requirements for the location of electrical
outlets inside a cabinet. This can often be a sticking point in an
inspection in a large city — and sometimes even in small ones.
Another argument is often the actual grounding location, and its
interaction with other radio grounds.
It is best to bring the primary ac power in underground. Sometimes
the power company owns the transformer and sometimes the broadcaster
has to pay for it. In cases where the station is at the end of a short,
dedicated line it may pay to own the transformer. This enables the
broadcaster to have some control over which users are connected to his
line. In any case a surge suppressor should be installed directly at
the point of entrance into the transmitter building and connected
directly to the system ground.
An adequate ground system is essential for any radio station. In the
case of AM stations this is provided through the antenna ground system.
However, sometimes this large and efficient ground system is some
distance from the studio/transmitter building where the primary power
ground is located. Of the station components located in the area of the
antenna ground system, few are connected to the ac power line. There is
usually one, however, and that one can sometimes cause problems and
introduce hum in an AM signal. I am referring to the tower lighting
Seeing the light
The tower lighting system is tied to the antenna ground system, and
it is powered by an underground cable from the transmitter building.
From time to time we hear of signal interference produced by leaking or
damaged components in the tower lighting system.
Examples of a poor tower ground (top) and a good tower ground. Not only
is the conductor in the poor example too small, it is not attached.
Photos by Ben Weiss, CPBE.
In the case of FM stations the antenna tower grounding is,
unfortunately, sometimes less than adequate. Unlike AM transmitters, a
large and efficient ground system is not required for FM stations,
which rely on line-of-sight coverage. As a result many FM towers are
grounded by a few grounding rods at the tower base. This is usually
inadequate, and it is not unusual for lightning strikes to damage
transmitters and associated equipment.
The damage caused by a lightning strike could be prevented by
properly grounding the tower and transmission line. A few standard
grounding rods connected by pieces of small-gauge copper wire will not
dissipate the high current produced by a lightning strike. Before
vaporizing, the small gauge copper wire will present a high impedance
to the lightning that will seek an easier path, which usually turns out
to be transmitting equipment. In many of the smaller FM stations, far
more consideration needs to be given to FM tower grounding.
It is in the transmitter building and ATUs that grounding becomes of
the greatest radio engineering importance. It is not just for
appearance that manufacturers of phasors and ATUs lay the expensive
4-inch copper strap on top of the cabinet floors and walls to
interconnect inductances and capacitors. Not only does the copper strap
ensure a perfect connection between components, but its presence tends
to reduce stray RF voltages from developing in cabinet sections.
The presence of a 4-inch copper strap interconnecting tower bases to
the transmitter output is also important. It is not unusual in small
stations, especially non-directional installations, to find that the
only connection between the transmitter ground and the ATU is the outer
conductor of the coaxial transmission line. Theoretically, this outer
conductor will suffice to carry the currents between the transmitter
and the radiating system. However, the 4-inch copper strap is needed to
stabilize the radiating system and ensures an adequate connection to
and from the ground system.
A continuation of this 4-inch copper strap should extend to the base
of all equipment cabinets. Sometimes in high AM RF fields, a 2-inch
copper strap is brought inside each cabinet to provide even better
grounding connections. It is common to find a copper screen forming a
Faraday shield around the transmitter, and some audio facilities are
required in high-power installations located close to the antenna.
Sometimes problems arise when attempting to ground cable shields
where an automatic ground is already in existence through a third
grounding pin on an equipment cable. Only one end of a cable shield
should be grounded. If both ends are grounded, a current will flow
through the shield. If the potential difference between the two ground
connections is high enough it is possible for hum to be produced.
In areas where high RF voltages are present, audio (and video)
signals produce a wide variety of interference ranging from whistles to
buzzes. RF interference to unshielded or ungrounded video equipment
usually produces the familiar herringbone interference pattern. When
investigating sources of RF interference to studio equipment it is
important to confirm that the connection known as ground really is a
good ground connection. This is especially true if a water pipe has
been used as a ground. Hot water pipes do not make suitable grounds. It
is essential to be sure that a water pipe ground is connected to ground
through a metallic pipe and not plastic. The water in the plastic pipe
will not substitute for a good metallic connection.
E-mail Battison at email@example.com.