Aerial, Amps, Boosters, Splitters etc.

What can an aerial do for you?

A well designed aerial is the beginning of any quality installation. If you don't start with a clean strong signal then everything you do afterwards is compromised.

Improve signal strength.
A weak signal will look snowy and there may be a loss of colour or break up of Teletext. In order to overcome this you need an aerial which can "focus" more signal onto the dipole, this is known as Forward Gain. The higher the gain, the greater the increase in signal.

Improve Channel 5.
If your reception of Channel 5 is poor you need to know the cause before it can be resolved. In many cases it is because the new signal is outside the tuned range of your old aerial. Antiference have extended the performance of their aerials to cover a range of frequencies to suit your location.

Reduce ghosting.
Ghosting results when reflected signals are received as well as the primary signal. To overcome this, the performance of the aerial has to be limited in all directions except straight ahead. There are two measures of this, a) Acceptance angle which will get narrower to exclude signals from the side and b) Front-to-Back ratio which will increase with the aerials ability to reject signal from behind.

Why does digital demand more of your aerial?

The new DTT signals are broadcast in the UHF range just as analogue TV is, but in order to allocate six new frequencies the demands on your aerial have increased.

Fewer transmitters
DTT will be broadcast from just 81 sites which compares with over 1100 for analogue, in many cases you may be further from your nearest digital transmitter than you were from your old analogue transmitter. The signal may therefore be relatively weaker when it reaches you.

Wider frequency ranges
The new frequencies have often had to be allocated across a wider range than analogue and in certain areas you may find your old aerial cannot receive all six multiplexes (a multiplex is a signal carrying more than one programme). Wideband aerials have been designed to cover the whole of the UHF range.

Greater use of frequencies
Another feature of DTT is that the same frequency may be used by adjacent transmitters. If you are located between these transmitters you need an aerial that can receive the desired signal but reject the unwanted one. If both were received one may cancel the other leaving you with no picture at all, so aerials need a higher Front-to-Back ratio.

Reflected signals
In the same way that signals from competing transmitters can prevent the signal from being decoded, reflected signals can have the same effect. Reflections can occur at two points, outside when signals bounce off buildings or hill sides or in the system where there is a mis-match of impedance. To counter this, Antiference aerials have both tuned reflectors and impedance matched dipoles.

Amplifying and Distributing Signals

A signal booster will increase the strength of the signal delivered to your set, but cannot overcome problems arising from the aerial, like ghosting. A good quality Antiference aerial is the best start to any multi room installation.

Adding the choice of video or a new digital in every room is relatively simple by connecting your booster to the RF (coaxial) output of your decoder, or video, as shown to the right. (You may need to retune the RF output of your decoder or video to clear channels.)

Using the high amplification of the PB261 and a series of Antiference taps and splitters, systems of up to 20 outlets can be fed.
Illustrators assumes 5m of double screened coaxial cable 5540) between units for the calculation of signal loss.

The OX range of multipoint amplifiers can support systems from 2 points for TV only and up to 8 points for TV and FM as shown.

In the example illustrated, use CS1000 outdoor or FS204 F-type splitter for the output leg with power supply, other legs can use the same or CS2000 surface and CS300 plug-in splitters.
For best results, install the amplifier about 1-2m from the aerial. If you are amplifying a distant signal and there may be stronger signals in the area, use a channel grouped amplifier (UP2501).

Antiference produce two masthead amplifiers with a second input that can handle FM signals.
The UP3302 only amplifies UHF, whilst the UP3501 has amplification on both inputss.
Use a diplexer (810 flush, 823 surface or UF23 plug-in) to separate the signals where needed.

Digital Reception Issues

1. Signal Strength
Digital signals are broadcast at significantly lower power than analogue. However, because the decoder can recognise the binary signal and seperate it from noise, the signal required at the receiver is also lower.

Typical threshold for quality analogue TV: 60dBuV
Typical threshold to decode digital TV: 35dBuV

Falling below the threshold for analogue TV would cause a gradual decline in picture quality, but with digital, if you fall below the threshold the picture becomes unwatchable. Therefore to allow for signal loss over time, seasonal and weather changes allow a 6dB margin above the threshold for digital signals.

Suggestions to increase signal level:
Use an Extragain aerial
Use a low noise amplifier with moderate gain.

Note: Because of the presence of powerful analogue signals, be careful not to over amplify the signal and overload the decoder.
If possible use a high gain aerial to improve signal before amplifying.

2. Signal Quality
In order to be able to decode the digital signal, the decoder has to be able to recognise the signal amongst the background noise. Therefore the level of noise in relation to signal is critical:

Minimum Carrier-to-noise Ratio for DTT >26dB
Maximum Bit Error Rate 2E-4 (2x10-4)

Generally, to improve the Signal Quality:
Use an impedance matched Antiference aerial, mounted in a high position out of doors.
Use an aerial with a high Front/Back ratio and narrow acceptable angle (DX8 and Extragain).
Use double screened (5540) cable, without links or breaks between aerial and decoder.
Avoid routing cable near other electrical devices.
Avoid using amplifiers where possible.
Choose low noise amps where necessary.