Radio transceiver interference and noise
A discussion of various sources of noise and interference, things that degrade your ability to listen to other stations on your radio transceiver. Electromagnetic interference in the form of radio frequency interference (RFI) can degrade or seriously disrupt your receiver's ability to listen on one or more frequencies. RFI may be on one frequency, across predictable harmonics of a frequency, or broad across many or all bands.
Ham radio codes for noise fall into two categories, that which is man made "QRM" and that which is natural "QRN." Lightning is an example of QRN. RFI from man made devices is QRM and is regulated by the FCC. The FCC no longer investigates RFI complaints to telephone, TV or entertainment systems. You can call their 800 number (888-CALL-FCC) and listen to the FCC's RFI message. The FCC does regulate many other sources such as the power utility companies.
There are different types of Radio Frequency Noise .
Contents
Category Types of RFI
intermod
Intermod is short for intermodulation. Intermodulation is a source of interference that arises from the combination of two or more signals at different frequencies in some kind of non-linear system. For example, you could hear strong interference at 14.0 MHz if there are two strong signals at 86.0 and 100.0 MHz that combine in a rectifying circuit, such as an oxidized fence connection or a gutter.
Intermodulation differs from other forms of interference in that it is created in the wireless system itself, not directly by some external source. Interference due to intermodulation is caused by strong signals which are generally not near that of the receiver listening frequency. Instead, these strong signals overload some circuit in the receiver, causing the circuit to internally generate harmonics of the strong signals. These harmonics then combine, or mix, in the receiver to create a new frequency that was not present at the receiver input. The newly-created frequency, called an "intermodulation product," then interferes with the radio receiver in much the same way as other sources of interference.
power line noise
Power line noise is a type of electrical interference. Power lines or associated hardware improperly generate unwanted radio signals that override or compete with desired radio signals. Disruption of radio communications, such as amateur radio, can occur. Loss of critical communications, such as police, fire, military and other similar users of the radio spectrum can result in even more serious consequences.
Power line noise is typically due to a spark or arcing across some power-line related hardware. A breakdown and ionization of air occurs, and current flows between two conductors in a gap. Alternatively, although possible, Corona discharge is rarely the source of power line noise. Corona discharge has to do with the breakdown of air near a conductor and is very limited in range.
see: http://www.arrl.org/power-line-noise
spurious emission
LED traffic lights are a major source of RFI in the VHF range. RFI emissions from electronic equipment in businesses, including security systems, intercoms, wireless pagers, and so on can cause noticeable RFI when your radio receiver is near these places. The interfering signals might have been transmitted intentionally, or unintentionally as the result of some defect or undesired characteristic of the source. It is not necessary that the interfering signal be exactly on the same frequency as the receiver is tuned to be troublesome. Strong RF signals that are near the receiver listening frequency can affect the operation of the wireless receiver, causing reception problems.
broadcast harmonics
Harmonics from FM radio stations and communications transmitters is a type of RFI. Powerful FM station transmitters usually have a small amount of output at twice their operating frequency on the second harmonic frequency.
FCC Legal
§ 15.13 Incidental radiators. - Manufacturers of these devices shall employ good engineering practices to minimize the risk of harmful interference.
harmful interference - Any emission, radiation or induction that endangers the functioning of a radio navigation service or of other safety services or seriously degrades, obstructs or repeatedly interrupts a radio communications service operating in accordance with this chapter.
Relaxed Standards
Why the FCC doesn't seem to give a darn about incidental radiators anymore.
- Public service and government communication has moved to higher frequencies UHF and above using digital modes which are less susceptible to RFI generated by incidental radiators.
- FCC operation has shifted from engineering practices logic to legal financial commercial pursuits. The FCC would rather auction spectrum to the cellular telcoms rather than consider what is best for all spectrum users.
- Amateur radio is an extremely low priority to the FCC
- Traditional business land mobile radio has been replaced in the age of digital mobile phone communication.
- The FCC has been downsized with reduced staff and fewer field offices
- Influx of a magnitude more cheaply made Chinese import electronics overwhelm the FCC type acceptance office.
Common Sources of QRM RFI at the QTH
Electronic or radio frequency, RF noise can be generated in a number of ways by different mechanisms by common household devices.
Some examples:
- Personal Computer
- Plasma Television
- HVAC System
- Home Security System
- LED Lights
- Outdoor Arc Lights
