What are masthead and distribution amplifiers?
Masthead and distribution amplifiers are used in television systems to increase the level of signal received at a television set. Television masthead and distribution amplifiers can produce radio signal interference. Incorrect use or faulty amplifiers can affect broadcast television reception, mobile phone systems and two-way radio communications systems.
How do they cause radio signal interference?
There are three causes of interference. These are:
- overload (high level signals cause the amplifier to distort);
- mixing (multiple radio signal sources combine within the masthead amplifier); and
- oscillation (a result of signal feedback through ageing, damaged or faulty installation).
Who is responsible for any interference?
The person who owns or operates a masthead amplifier or distribution amplifier is responsible for any interfering signals radiated by the amplifier. An owner may not be aware that their television system includes an amplifier.
The Australian Communications and Media Authority (the ACMA) is responsible for investigating radio signal interference where it is suspected that a provision of the Radiocommunications Act 1992 is being breached. For instances of radio signal interference where the Act is not being breached, it is appropriate for a reputable antenna installer or technician to resolve the matter.
When the source of the interference has been identified as a faulty masthead amplifier or distribution amplifier, the owner or operator of the masthead amplifier will be required to arrange for the amplifier to be repaired or replaced.
How do I know if I have a masthead amplifier connected?
A masthead amplifier can usually be seen as a small grey or black plastic box attached to the pole supporting the television antenna. There will be wires running from the antenna into the box and a lead running down into the house to the television set. There are other devices that look similar to masthead amplifiers. The amplifier may also be out of sight in a roof cavity.
Amplifiers usually draw power through the antenna cable from a 240 volt plug pack, which is usually found near or behind the television set. The plug pack is wired to a device known as a power injector attached to the antenna cable going to the antenna.
If you have such a device, then it is almost certain that it is a masthead amplifier.
How do I know if I have a distribution amplifier connected?
Distribution amplifiers are generally used where many television outlets are available in the premises. They may be located in the roof cavity or within service ducts or cupboards. Distribution amplifiers are usually physically larger than masthead amplifiers and require a 240 volt power supply to function.
How can interference problems be corrected?
Cabling and amplifier faults are responsible for a large number of the problems caused by masthead and distribution amplifiers. Care needs to be taken with installation and maintenance of the cabling for the antenna and amplifier.
The following instructions are for use by people operating and installing the amplifiers.
|
Cause |
Example |
How it can be corrected |
|---|---|---|
|
Common installation errors |
Amplifier gain is set too high |
Use the minimum gain required for satisfactory operation. A setting of 15 dB is satisfactory in most situations. Usually this is a setting slightly less than the maximum setting. |
|
Unused outlets |
Turn off all amplifiers connected to unused antenna outlets. |
|
|
Mounting in close proximity to, or on, the antenna |
Locate the amplifier away from the antenna (at least 500 mm) and not between antennas. |
|
|
Poor cable connections |
Use masthead amplifiers with 'F' type co-axial connectors to ensure good shielding and signal isolation. |
|
|
Environmental considerations |
Amplifier subjected to high temperatures |
Avoid locating the amplifier in high temperature locations such as small, unventilated roof spaces. |
|
Corrosion at the cable termination point or on the exposed centre wire or outer braid |
Provide suitable moisture proofing for the connection point and any exposed wire. |
|
|
Corrosion of the balun components |
Ensure that the balun is installed appropriately to prevent entry of moisture. If it has an integral balun, ensure the antenna is mounted the correct way up. |
|
|
Corrosion of components and boards |
Install weather seal correctly and ensure that any drain holes are unblocked and the unit is mounted correctly, to the manufacturer's specifications. |
|
|
Bird attacks on plastic antenna insulators and balun housings |
Attacks are more prevalent when items are coloured bright orange. Black plastic components reduce the problem. If necessary, protect cables with conduit. |
|
|
Cabling |
Poor physical connection of the wires to the connection screws, particularly the outer braid |
Remove any deteriorated cable and re-attach securely. If necessary, use additional ties to prevent cable movement. |
|
Poor termination of connectors to the coaxial cable |
Follow the manufacturer's instructions and use the correct tools for terminating cables with connectors, or use a pre-terminated cable. |
|
|
Input or output cables have been coiled to accommodate excess |
Use cables of the correct length and avoid excess cabling cable length. |
|
|
Input and output cables have been tied together or are too close together |
Keep the input and output cables separate (do not tie them together). |
|
|
Cable is clamped or tied too tightly, resulting in damaged coaxial cable |
Use the minimum tightening required on all cable clamps and ties to prevent cable movement. Avoid over tightening. |
|
|
Cable kinked or bent too sharply, resulting in damaged coaxial cable |
Install without sharp bends or kinks in cables. |
|
|
Poor quality coaxial cable, connectors, combiner, or splitter |
Use good quality cable and 'F' type coaxial connectors. Use filter type combiners for combining signals from VHF and UHF antennas. Balun splitters are preferable to resistive splitters. |
Who can correct these problems?
A reputable antenna installer or technician can rectify any problems. It is recommended that old or deteriorated amplifiers, cable or connectors be replaced.
More information
For more information about masthead and distribution amplifiers and interference, contact the ACMA on 1300 850 115.
An Importance Notice for Installers is available on the ACMA website.
The ACMA's information booklet Better Television and Radio Reception is available from the ACMA offices and is on the ACMA website.
The ACMA has fact sheets on a range of topics.
Please note: this document is intended as a guide only and should not be relied on as legal advice or regarded as a substitute for legal advice in individual cases.
