Multiple signals in the front ends of communications radios
intermodulating causing on-channel products that interfere with the
The first amplification stages, the front
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
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
Out of band emissions (OOBE) from transmission systems close
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.
Base Station Intermodulation products.
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.
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.
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.
General RF Noise
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