Aerial amplifier

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Types

Good quality masthead aerial amplifier (shown with its protective cover raised)

There are 3 main types of TV aerial amplifier:

  • plug-in
  • masthead
  • loftbox

Plug-in

Plug-in amps have a mains plug. These are easy to use where there's a mains socket.

Masthead

A mains plug is not convenient for lofts and rooftops, where there are usually no sockets, and for these locations masthead amps are generally used. A masthead amp is divided into 2 units, the amplifier and a separate power supply.

The amplifier is mounted as near the aerial as possible for best signal quality. Often a masthead amp is mounted directly under the aerial. Masthead amps come in weatherproof containers.

The power supply is inserted into the co-ax aerial lead somewhere further down the line. It sends 12v (usually) up the wire to the masthead amp. Thus the one aerial wire carries both tv signal and 12v power.

All centre pin connections on plugs and sockets between power unit (psu) and masthead amp should be soldered, crimped or screwed. Failure to do this results in oxidation over time, and sometimes loss of power to the amp, which then ceases working.

Where splitters, diplexers or combiners are used between amp and psu, they need to be 'dc pass' either on all ports, or on the ports used in the run from amp to psu. If they're not dc pass, the power won't get through to the amp.

The power supplies sold for use with masthead amps have low current limited supplies, so any aerial wiring defect doesn't create a fire risk. A standard wallwart is not a workable replacement.

Loftbox

A loftbox contains an aerial ampifier, power supply and splitter in one unit. Mounted in the loft, they do the whole job in one go. Loftboxes often also incorporate a second input to allow the output from digital decoder boxes, DVDs etc to be fed back into the house-wide aerial distribution system, and thus be viewed on a TV anywhere in the house.

Loftboxes are convenient if one meets your required specs, and power is conveniently available in the loft, but neither is always the case.

Where to put the amplifier

It matters where the amp goes. As the signal travels down the lead from the aerial, both signal level and S/N ratio degrade. Amplifiers can restore signal level, but can not undo loss of S/N ratio that has already occurred, and this is a key spec for picture quality.

So for best results the amp is always put close to the aerial. The closer it is to the aerial, the less uncorrectable degradation occurs. The further it is from the aerial, the more the signal degrades before amplification.

Masthead amps can be mounted right under the aerial, and this is frequently done for best results. Indoor amps should be mounted in the loft.

Aerial amps placed near the TV set do have some effect, as a good aerial amp has a better noise figure than your average TV input stage. But the result is much better if the amp is put near the aerial. The better S/N ratio means a clearer picture and less interference, and more reliable digital reception.


Specs

A good aerial amp will have

  • Adequate gain - enough to overcome cable and distribution losses, and raise a weak signal to an appropriate level. This does not always equate to the highest gain available.
  • Lower gain units are also available and suitable for some installations. This may especially be the case where some pre-amplification is required prior to a loft box.
  • 1.7 - 4dB noise figure (the lower the better)
  • Variable gain amps should have interstage gain control rather than using a built-in input attenuator. The latter types give real world performance far below their quoted specs on all settings but highest gain.

Fire

Electrical appliances do cause fires, and a mains appliance in the loft is out of sight, and usually mounted on or close to a lot of bare woodwork. With 69,000 house fires a year in the UK, a percentage of which are electrical, masthead amps may be a bit safer than plug-ins in the loft. Make sure that equipment in a loft is:

  • Protected by the correct fuse - use a small a fuse as is appropriate - 1A is more than adequate.
  • Do not allow equipment or its power supply units / transformers to be covered by loft insulation. Pay particular attention if you are increasing your insulation to reduce energy usage.
  • Mounting on non combustible surfaces can help - e.g. a spare ceramic wall or floor tile.

Sometimes people put the amp in a biscuit tin. This contains any fire, reduces peak surface temp and reduces oxygen supply. Fold cut edges over to protect cables from sharp edges.

See Also