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Operation of a total ship system in this unique shipboard environment presents a challenge to all
concerned. You must always consider the effects that motion, temperature variations, and exposure to
adverse elements will have on the performance of the total ship system. This is particularly true on those
system components that are mounted topside.
On board ship, you will find much attention is given to keeping the topside cosmetically and
mechanically shipshape. It is equally important to keep it electronically shipshape. Minor mechanical
problems, such as loose connections, broken bond straps, or rusty junctions can cause serious
communications problems. These sources of electromagnetic radiations reduce receiver performance and
are known as ELECTROMAGNETIC INTERFERENCE (emi). Sources of emi can be divided into the
following broad categories:
Functional. Emi can originate from any source designed to generate electromagnetic energy and
which may create interference as a normal part of its operation. The interference may be
unintentional or caused by other on board or adjacent platform systems. This interference also
may be intentional or caused by electronic countermeasures (ECM).
Incidental. Emi can originate from man-made sources. These are sources not designed
specifically to generate electromagnetic energy but which do in fact cause interference. Examples
of incidental emi sources include power lines, motors, and switches.
Natural. Emi can be caused by natural phenomena, such as electrical storms, rain particles, and
solar and interstellar radiation. It is recognized by the following audible noises:
– Intermittent impulses of high intensity that are caused by nearby electrical storms
– Steady rattling or cracking caused by distant electrical storms
– Continuous noise of precipitation static caused by electrically charged rain drops
– A steady hiss at high frequencies caused by interstellar noise
Hull-generated. Emi can be caused by the interaction of radiated signals with elements of the hull
and rigging of a ship. (The functional signals themselves do not cause interference.)
The following are two general methods by which emi is transmitted:
Conduction. Undesired energy from one equipment is coupled to interconnecting cables or
components of another equipment. This energy is conducted via the wiring in the shielded enclosure
that protects sensitive circuits. You will find proper design, adequate isolation, and shielding of
cables and equipment can control this problem.
Radiation. Energy is beamed directly from the transmitting antenna, or source, to the victim
receiving antenna. When this interference is picked up by a receiver, you have two solutions.
Interfering energy can be eliminated at the source or you can filter, or blank it out at the victim
equipment. Filtering is far less desirable. Interference may be on the same frequency as the desired
signal and will not be eliminated without affecting the reception of all desired signals.
Most unprotected shipboard receivers are susceptible to emi over a frequency range much wider than
their normal bandpass. Off-frequency rejection rarely excludes strong, adjacent-channel signals. These
signals enter the receiver and degrade receiver performance by being processed along with the desired,
tuned signal. Usually, the presence of emi will be apparent to you. It has a bad effect. Upon the desired
signal quality, such as that in CROSS-MODULATION where a spurious response occurs when the carrier
