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Types of Interference
Explanation: The first amplification stages, the front end, of communications type radios are wide (possibly 50 MHz or more) and very sensitive. If the radio is near (usually 1 km or less) several radio transmission sites such as cellular, Nextel, other communications sites, private two-way sites, etc., the signals from these sites mix in the non-linear front end of the communications radio. If the frequencies are just right, an intermodulation product will emerge on or near the desired reception frequency and cause interference. 2. Receiver overload. Explanation: If a receiver is near a transmitting system, the RF from that system will be too strong for the 1st amplification stage to handle. The transistors saturate and the receiver sensitivity decreases. Additional intermodulation products may be created within the receiver. Since many communications receivers have wideband front ends, they will be affected by strong out of band signals entering their antennas. This is not the same as out of band emissions from a nearby transmitter. 3. Out of band emissions (OOBE) from transmission systems close in frequency. Explanation: All transmitters emit radio frequency energy at frequencies other than the exactly desired frequencies. This is known generally as out of band emissions, or “OOBE”. Since these undesired signals may be on the desired receive frequencies, they can only be eliminated by filtering the offending transmitter or moving the offending transmitter to another location. 4. Base Station Intermodulation products. Explanation: Often there is more than one transmission system at a tower site or more than one frequency involved in a signal transmission site. Since all physical systems are non-linear, these frequencies can mix a cause other frequencies that could possibly have energy in the communications band. This is a common occurrence. It can usually be tracked down and filtered or other solutions put in place so that interference does not occur. To demonstrate how tricky this can be, I know of one case in which a rusty tower joint (a non-linear system) on an FM radio tower in Florida caused interference 40 miles away. The good news is that this type of interference can usually be found and fixed. 5. Skip Explanation: Skip is a phenomenon caused by the ionization of gases in the ionosphere. RF energy leaves the antenna and reflects off of this ionized layer back to earth at a great distance. This phenomenon is much more prevalent at frequencies below 30 or 40 MHz and is time dependent. However, it does occur at much higher frequencies from time to time. The interference usually occurs more at night and in the evenings. Thus, a two-way facility at 150 MHz in California could conceivable interfere with a 150 MHz system in Georgia. 6. Ducting Explanation: Ducting occurs when air of different temperatures and humidity forms layers in the lower atmosphere. RF energy is refracted and reflected from these layers and travels much farther than normal. An example would be when a 450 MHz system in Houston interferes with a 450 Mhz system in New Orleans. The two worst places for ducting are the California coast and the Gulf of Mexico. Ducting usually occurs when the atmosphere is still in the early mornings usually always ending by noon. This phenomenon can cause severe interference often blocking the desired signal just miles from the main transmitter site. 7. General RF Noise Explanation:
Devices other than radio equipment can cause radio frequency
energy. Typical of
items that can cause interference are arc welders, electric motors,
faulty spark plug wires, lightning, and even rusty tower bolts.
These types of problems can usually be tracked down and
eliminated. |
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