Manifestations of Interference, Causes of Interference, and Solutions to Interference

Radio Frequency Interference can manifest itself in many ways, below are several common ones and their possible causes and cures.

1. General background noise, usually with a “hissing” sound that causes the desired signal to be hard to understand.

Ø     Cause:  The most common is a weak desired receive signal.  The hissing noise is usually transistor and resistor thermal noise. 

Ø     Solution:  If the equipment is operating properly, this noise cannot be eliminated in an analog system. 

2. General background noise, usually with a “grinding” sound that causes the desired signal to be hard to understand.

Ø     Cause:  The grinding sound is usually caused by data.  The analog receiver is being interfered with by a data system.  The data can and often is intermittent.  The offending signal is usually on channel or on a frequency very near the channel.

Ø     Solution:  A spectrum analyzer with a good directional antenna can often be used to track down the source of data interference.  Sometimes it can be very local.  It is often possible to get permission to briefly turn off the suspected data transmitter in order to determine the source of the problem. 

3. Data noise or analog voice that interferes with the desired signal generally in the morning.

Ø     Cause:  Usually ducting is the cause of this type of interference.  It happens in the morning and last for several hours but generally goes away by late morning.

Ø     Solution:  Since the cause is generally an on-channel signal from a licensed system hundreds of miles away, it cannot be remedied.  If it is a continuing problem, the offending system can usually be identified and it may be possible for one user to change frequencies. 

4. Data noise or analog voice that interferes with the desired signal generally at night.

Ø     Cause:  Usually skip is the cause of this type of interference.  This type of interference is rare at frequencies above 100 MH but does sometimes occur.

Ø     Solution:  Since the cause is generally an on-channel signal from a licensed system hundreds or even thousands of miles away, it cannot be remedied.  If it is a continuing problem, the offending system can usually be identified and it may be possible for one user to change frequencies. 

5. Interfering voice or data can be heard in a repeater at a particular site.  Occurs occasionally.  Sometimes it may be hard to identify the voice, data, general noise, or combinations of these sounds.

Ø     Cause:  Normally this type of problem is caused by intermodulation products at the site or at a nearby site perhaps within a 5 mile distance of the offended site.

Ø     Solution:  This type of interference can usually be tracked down with a directional antenna and spectrum analyzer.  Often it can be traced by actually listening to the interference and identifying a call sign.  Once the offending transmitter or transmitters are identified, filters placed in the output of these transmitter will usually eliminate the interference. 

6. Interfering voice or data can be heard in a mobile or portable unit when located at a particular place or area.  Sometimes it may be hard to identify the voice, data, general noise, or combinations of these sounds.

Ø     Cause:  This can be caused by by intermodulation products at the site or at a nearby site as in 5 above.  Or, this can be caused by intermodulation within the receiver front end.   It is easy to tell which type of problem is causing the interference.  If you take a spectrum analyzer to the location and you can see the interference on the spectrum analyzer, then it is caused by intermodulation products coming from a nearby transmitting facility.  Usually the facility will contain several types of two-way or other commercial radio transmitting equipment.  If the noise or signal cannot be identified using a spectrum analyzer, then the interference is not actually being transmitted but is cause within the receiver front end.

Ø     Solution:  This type of interference generally is not easily eliminated.  You could move some of the transmitting equipment to another location so that the receiver does not get a strong signal from one or more of the offending transmitters, or you can redesign the receiver.  Neither solution is often the optimal one.