Interference Definitions and Causes
It may seem somewhat academic at this point to define interference but the
bestunderstanding starts with a strong foundation in fundamentals.
Webster defines interference as: "4. radio.
a. A jumbling of radio signals, caused by the reception of
b. The signals or devices producing the incoherence.
Meanwhile the Federal Communications Commission (FCC) defines harmful
interference asinterference which endangers the functioning of a
radionavigation service or other safetyservices or seriously degrades,
obstructs or repeatedly interrupts a radio communicationservice operating in
accordance with the Radio Regulations.
From the amateur's standpoint, this means you cannot communicate. However,
rememberthat the amateur service is not a safety or essential service, some
moderation must beconsidered. e.g. hearing a distant co-channel repeater when
your local repeater is notactive, while being annoying, is not
"interference". Hearing adjacent channelsplatter while carrying on a
conversation on simplex or your local repeater whileaffecting the quality of
the conversation is not truly interference. If it makescommunication completely
impossible then it should be considered interference although itstill may not
be harmful or necessarily willful. Take note at this point, that many ofthe
noise sources to be defined below do not affect FM/PM type radio operation
except tocause desensing of the radio possibly masking the
Taking Webster quite literally, we first examine types of interference:
- Types of Interference
- Lightning: Crashing type momentary interference
- static electricity: Crashing, ripping, tearing sounds
- thermal shot: Momentary Impulse, raises noise floor of receiver
- solar radiation: raises noise floor of receiver
- There isn't much that can be done about these
- Electric power sources (60 Hz) leakage, arcing
- Neon signs (continual arc)
- florescent light fixtures (more arcing)
- computer clocks (disc drive controllers etc. ..(lovely
144 MHz sources)
- Radio sources:
- Co-channel : same frequency various power levels -
strongest signal captures receiver.
- Adjacent Channel: close in frequency with significant
power can cause desensing of receiver, pulling of AFC, even capture.
- Intermodulation: unrelated frequency mixes with another
signal generating a signal on or close to the receive frequency
desensing, capturing etc.
- Equipment malfunction: Cable connectors loose, corrode
causing rectification which in turn creates new signals. Amplifiers
driven into non-linearities causing spurs. Parasitic oscillation in
- As you can readily see, there are a multitude of problems
waiting for solutions. Each with few exceptions are solvable. It takes
patience, perseverance and the cooperation of others.
- The natural phenomena as defined above are not within our
capability to solve. It is the manmade signals that by and large can be
attenuated or controlled.
- Electric Power sources: these mostly emanate from leaky
insulators or cables belonging to the power company and with sufficient
coaxing, they will repair them. It may take considerable convincing on
your part but, once convinced they will repair them.
- Neon signs: These are more difficult to control because
of their design. Most sign companies will not be cooperative, however the
saving thing here is these typically do not interfere with FM/PM unless
you are in the immediate proximity.
- Florescent Light fixtures: These when working properly
are not a problem. However, when they fail, they can become annoying. If
they fail, the alternatives are repair or replacement. In that same vein
are the mercury vapor and sodium vapor lights used for outdoor lighting. These
utilize an arcing mechanism for startup and when they fail, this becomes
a great noise annoyance.
- Computer clocks etc.: This is a problem that is very
pervasive with the increased appearance of faster and faster computers.
Here, the best defense is Tempest shielded machines however these are not
available for everyone especially your neighbor. An alternative is start
with shielded/filtered power cords. On plastic cabinet computers, the use
of RFI sprays on the interior of the case with conductive fingers to the
metal chassis has been shown effective.. Monitors because of the harmonic
rich signals that float around inside require special attention including
RFI sprays, conductive fingers, filtered/shielded cables.
The biggest thing is don't let these problems beat you, they can be solved.
If youneed help, there are other amateurs out there that are ready, willing and
able to help.Just ask. PS: sometimes they also could use a hand, its called
sharing the load.
- Those are most of the manmade problems now for the
difficult ones: Interference from other amateurs and commercial sources.
There are two basic types unintentional or accidental e.g. keying up on
the repeater before turning up the volume control. This has happened to
many of us and those of you that haven't done it yet will sooner or later.
The other type is the harmful, willful or mischievous interference. This
is intentionally tying up a repeater or frequency to prevent its use by
other persons. A classic case was the 14.313 MHz fiasco of some few years
ago. Other incidents are frequently reported in QST, CQ, and 73 wherein
someone has issues false cries for help. These not only cost the taxpayer
in terms of manpower sent needlessly but they cost credibility to every
one with a legitimate need for help. Its the classic case of the "boy
who cried wolf" enough times, and no response will come.
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