Interference

There are many sources of potential RF noise generation that can interfere with broadcast radio and television receivers. These sources of man made noise include:

  • Power tools, Computers, Arc Welders, Power lines etc.

Interference can be caused to:

  • Telephones – mobile and fixed, Television, Radio reception,
  • Audio equipment, Computers, Vehicle electronic equipment, Blasting devices

There are sources of natural noise that can cause interference such as lightning, electrostatic build build-up etc.

Resolving interference

Interference can generally be resolved by introducing technical solutions, however these can be complex and a qualified person should be sought to resolve these problems.

Electronic equipment can operate within an electromagnetic field without interference. This is called Electromagnetic Compatibility (EMC) or radio frequency immunity.

EMC issues are likely to cause neighbourhood disputes. To resolve a dispute:

  • Discuss the problem with the person concerned, obtain dates and times of interference
  • Try and identify the source of interference
  • Understand the need for diplomacy
  • Seek advice and where necessary involve the ACMA

Interference as a result of EMC issues is dependent on:

  • Transmitted power
  • Frequency of operation
  • Type of emission from the transmitter i.e. AM, SSB, FM etc
  • Distance the transmitter is from the affected equipment – selection and location of antennas can significantly reduce the likelihood of interference.

Radio Frequency (RF) Earthing

  • An RF earth connection in an amateur station is to provide a path to minimise RF ground currents entering the mains earth system and causing interference
  • The image below shows a typical copper earthing strip used as an RF earth (note this is separate from the mains earth)

The image below shows separate RF and Mains Earths. Each piece of equipment is connected from its chassis directly to the RF earthing bar (do not daisy chain the earths).

Sources of radio interference

Amateur stations can cause interference to other radio communication services and television services.

The cause of the interference from an amateur station is often as a result of incorrect operation of amateur transmitting equipment.

Potential reasons for interference from an amateur station is:

  • Over modulation – excessive microphone gain
  • Incorrectly tuned antennas – high SWR
  • Breakthrough on FM or SSB – close proximity of transmit antenna to receiving antenna
  • Mains fed interference – signals fed by the 230 volt mains supply

Filters

  • Filters can be used to reduce the likelihood of interference
  • Other filters can be used in the power supply or interconnections between the equipment
  • Filters must be fitted as close to the affected device as possible
Low Pass Filter

Low pass filter in the output of a transmitter, or an ATU as a filter

Simple “choke” filter

  • Ferrite rods with wire wound onto them can be used to make effective RF filters
  • These filters are sometimes call coils or chokes

Toroids can also be used as RF filters

  • For the practical you will be required to make an RF filter using a toroid or a ferrite rod
  • The RF is blocked by the filter

Various clamp-on magnetic cores or rings with wrapped insulated wires. EMI filter inductors prevent high-frequency electronic noise

Harmful Interference

Harmful interference is when interference is caused to a licensed radio communication service and may effect navigation beacons, emergency services etc.

Interference to domestic TV and radio is not considered harmful but is objectionable.

An amateur station must not cause harmful interference to radio communications including:

  • Other users
  • Other services

It may be necessary to shut down your station for a time to identify the interference source but this is not mandatory, unless the interference is deemed to be harmful, in which case transmission must cease until the problem is resolved.