Interference

Interference in the receiver of a radio communications device is the effect produced by an undesired signal, and it can be heard as degraded reception quality, repeated interruption, or abnormal operation of the receiving device.

Frequency sharing

Because emergency services and private companies need a large number of communications channels, the Department must often attribute the same radio frequency to several different users. If you receive communications from another station for several days or weeks, it could be because the frequency you are using has also been assigned to another user in your sector.

If communications between people who are sharing your frequency are unwelcome, you should equip your receiver with a coded squelch. The use of a tone squelch allows you to receive only those calls intended for your receiver by blocking the communications of other users sharing your frequency, unless of course you turn off the squelch to listen whether the channel is free before making your own calls.

Users sharing the same frequency who have receivers with the same tone squelch

Your receiver may use an inaudible tone squelch that can nevertheless be detected by the receiver. Generally, a separate tone squelch is assigned by the provider to each of the users on the same frequency. However, situations may arise in which the same tone is accidentally assigned to two users. If you do not wish to hear the communications for the other station, ask your provider to assign you a new tone squelch.

Transmitter noise, receiver desensitization or adjacent channel interference

Interference called "transmitter noise" is produced directly on the operating frequency of the affected receiver. This problem can usually be heard as a degradation in the reception of low level signals or by unusual and irregular performance in the normal coverage area for your mobile stations. To solve this kind of problem, a pass-band filter tuned to the transmission frequency generally needs to be added to each faulty transmitter.

It may also be possible for a new station installed nearby and operating at a different frequency from yours to be heard in your receiver. If the new transmitter is located very close to you and/or its transmission frequency is close to the one for your station, your receiver could be desensitized by these undesired signals, even if they are on a different frequency from yours. Because your device cannot reject these signals, you may have to use external filters to eliminate the problem. Contact your after-sales service representative or your provider to determine if your equipment is faulty in this situation.

Intermodulation

If you hear two or more radio communication systems at the same time, or if you hear voices and tone signals at the same time that result in garbled or incomplete messages, you may have an intermodulation problem. There are basically two types of intermodulation: one that is caused in your receiver and one that can be attributed to another station's transmitter.

In short, the intermodulation produced in a receiver is the result of two or more signals being mixed in that receiver. The only way to correct this type of problem is to increase the interference rejection capacity of the receiver by adding a filter that can eliminate the undesired signals.

Intermodulation produced by a transmitter is the result of at least two signals mixing in a transmitter other than your own. The only way to correct this type of problem is to find the faulty transmitter and install a filter on it.

If you are experiencing similar difficulties and would like to determine which of these two problems is the cause, contact your service representative or provider. They will be able to advise you accordingly. In the meanwhile, note the time and duration of the interference and the type of communication heard (music, squelch noise, several voices at the same time, etc.). This information is often essential when Communications Canada must intervene to determine the nature of the problem.

Faulty equipment

Does temporary interference occur on your channel in the form of a fluctuating signal, muffled voices or radio broadcasts? If so, the microphone button on one of your mobile stations may be in the transmit position. Mobile station operators must ensure that they do not leave the microphone on the seat of their vehicle where it could be covered by other objects, such as books, that could accidentally turn on the transmitter.

Just like your vehicle, your radio equipment needs regular maintenance to ensure optimal operation and to avoid failures that could paralyse your activities for an extended period of time. If you keep your equipment in good condition with regular checks, you will know that it is not the cause of any interference that might occur.

Prevention is the best medicine!
Interference in a radio communications device is the effect produced by an undesired signal, and it can be heard as degraded reception quality, repeated interruption, or abnormal operation of the device.

Intermodulation
Simply put, intermodulation is a mix of radio signals. There are basically two types of intermodulation in radio communications: one that is caused in the receiver and one that can be attributed to another station's transmitter.

Intermodulation produced in a receiver is the result of two or more signals (A & B) being mixed in that receiver. Several stations are on the air at the same time and the receiver is not selective enough. The only way to correct this type of problem is to increase the interference rejection capacity of the receiver by adding a filter that can eliminate the undesired signals and ensuring that the technician takes this into account when selecting frequencies, either by choosing a different one or by informing individuals who will have the interference to change their setup.

Intermodulation produced by a transmitter is due to at least two signals (A & B) being mixed into a third (c). The only way to correct this type of problem is to equip the faulty transmitter with a filter.

* In studies, in general, third and fifth order products should be taken more seriously because some of them are in the in-band.

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