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 undesiredones.
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 desiredsignal.
Taking Webster quite literally, we first examine types of interference:
There isn't much that can be done about these sources. Manmade:
- 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
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 (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 amplifiers.
- 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.
- GO to: Home Page or TOP of Page