3-40
of a desired signal intermodulates with the carrier of an undesired signal. Extremely strong, off-frequency
signals may even burn out the sensitive front-end stages of a receiver. Emi also can degrade overall
receiver performance in a less noticeable way. It does this by desensitizing the receiver front end. The
noise level is raised and effectively lowers the signal to noise ratio and thus the sensitivity. This causes a
decrease in desired signal amplification. For these reasons, shipboard receive systems are designed to
include protective circuitry between the antenna and receiver to filter out off-frequency signals. This
prevents or limits interference, desensitization, or burnout. Depending upon the system, these protective
devices may include filters, multicouplers, preselectors, and so forth. These devices can minimize
interference caused by inadequate frequency separation or poor physical isolation between transmit and
receive antennas.
Q35. What is the purpose of QMCS?
Q36. What is emi?
Q37. What are the two emi transmission methods?
ELECTROMAGNETIC RADIATION
Radio-frequency (rf) transmitting systems with high-power transmitting tubes and high-gain
antennas have increased the possibility of injury to personnel working in the vicinity.
An electromagnetic radiation hazard exists when electronic equipment generates a strong enough
electromagnetic field to fall in a category listed below:
Causes harmful or injurious effects to humans and wildlife
Induces or otherwise couples currents and/or voltages of magnitudes large enough to initiate
electroexplosive devices or other sensitive explosive components of weapons systems, ordnance,
or other explosive devices
Creates sparks large enough to ignite flammable mixtures or materials that must be handled in the
affected areas
These hazardous situations can be caused by a transmitter or antenna installation. These generate
electromagnetic radiation in the vicinity of personnel, ordnance, or fueling operations in excess of
established safe levels. Sometimes the existing electromagnetic radiation levels increase to a hazardous
level. When personnel, ordnance, or fueling evolutions are located in an area that can be illuminated by
electromagnetic radiation, hazardous situations may occur.
Electromagnetic radiation is hazardous to personnel in two ways. It can cause rf burns; and it can
cause biological, thermal, and neurological effects to personnel (RADHAZ). Because of the differences in
characteristics and safety precautions required for each of the two types, they will be discussed separately.
An rf burn hazard is a hazardous condition caused by the existence of radio frequency (rf) voltages in
places where they are not intended to be. Any ship with high-power hf transmitters is susceptible.
Potentially hazardous voltages have been found in many areas. Some of these areas are lifelines, vertical
ladders, ASROC launchers, gun mounts, rigging for underway replenishment, and boat davits. Another of
these areas is on aircraft tied down on carrier and helicopter flight decks.
Whether or not an induced voltage creates an rf burn hazard depends on whether personnel will
come into contact with the object being energized. Generally, only the voltage between an object and the
deck is important. The rf burn occurs when a person comes into contact with a source of rf voltage in a
