CHAPTER
9
Testing for
Electromagnetic and
Radio Frequency
Interference
A
s boats become more complex, and we continue to install more and more electri-
cal and electronic gear on board, the potential for electromagnetic and radio
frequency interference (EMI and RFI) increases. When you combine this with the
space restrictions on boats, you’ll see the challenges installers face in finding suitable
locations for all the equipment carried by the average boat today.
Understanding Electromagnetic Interference
The ABYC electrical standards state that wire and cable runs need to be thought out when
they are being installed in close proximity to “magnetically sensitive devices,” such as the boat’s
compass. But the truth is that we need to consider far more than just the compass and mag-
netic sensitivity. For example, all microprocessors use clock oscillators that create a digital
square waveform (see Chapter 7). This waveform is not only loaded with distortion and har-
monics, it is affected by stray emissions as well. Devices such as fluxgate electronic sensors
(used as direction sensors on autopilots), satellite TVs, and satellite telephones are also
extremely sensitive to excessive interference. Tales abound of electronically controlled engines
displaying strange running conditions—such as running rough or excessive black exhaust
smoke—when certain, seemingly unrelated, electrical devices were activated. Certainly over
the years there have been cases where electronic navigation systems have been brought to their
knees due to RFI (Loran-C was particularly vulnerable), and I’ve even heard stories of lights
mysteriously turning on and off when someone keyed a single-sideband radio to transmit.
These strange symptoms are more easily understood in the light of two basic concepts:
1 All wire and cable that have electricity moving through them will have a magnetic
field surrounding them.
2 The strength of this magnetic field is directly proportional to the amount of electrical
current (amperage) flowing through the wire or cable; i.e., more amps = a stronger
magnetic field.
Copyright © 2007 by Edwin R. Sherman. Click here for terms of use.