Interference Definitions And Causes

By Jonathan Imberi

It may seem somewhat academic at this point to define interference but the best understanding starts with a strong foundation in fundamentals. 

Webster defines interference as:
a. A jumbling of radio signals, caused by the reception of undesired ones.
b. The signals or devices producing the incoherence. 

Meanwhile the Federal Communications Commission (FCC) defines harmful interference as interference, which endangers the functioning of a radio navigation service or other safety services, or seriously degrades, obstructs or repeatedly interrupts a radio communication service operating in accordance with the Radio Regulations. 

From the amateur's standpoint, this means you cannot communicate. However, remember that the amateur service is not a safety or essential service, some moderation must be considered. e.g. hearing a distant co-channel repeater when your local repeater is not active, while being annoying, is not "interference". Hearing adjacent channel splatter while carrying on a conversation on simplex or your local repeater while affecting the quality of the conversation is not truly interference. If it makes communication completely impossible then it should be considered interference although it still may not be harmful or necessarily willful. 

Take note at this point, that many of the noise sources to be defined below do not affect FM/PM type radio operation except to cause desensing of the radio possibly masking the desired signal. 

Taking Webster quite literally, we first examine types of interference: 

Types of Interference 

Natural: 
  • 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 sources. 

Manmade: Non-Radio: 
  • 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 that in turn creates new signals. Amplifiers driven into non-linearities causing spurs. Parasitic oscillation in amplifiers. 

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. 

Non-Radio 
  • 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 you need help, there are other amateurs out there that are ready, willing and able to help. Just ask. Remember, sometimes they also could use a hand - it's 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.