Why is the use of a pre-amplifier a more important aspect to think about than the use of a launch amplifier? This is a question Technical Director, Jeremy Kennedy, is asked all to frequently.

Firstly, we have to understand or at least accept that to provide consumers with a quality picture, you need to provide the Television or Set top box with an adequate signal level. The Television signal must be sufficiently free from noise, commonly referred to as C/N (Carrier to Noise).

As we know, if the signal level is too low from the aerial, no amplifier will achieve a clean, noise free picture, because the received C/N will already be too poor.

NF (Noise Figure) is the method used to measure the amount of noise that any amplifier or component adds to our 'clean' Television signals. The aerial is the best source of achieving more signal, as anything we place after this point will add noise and reduce the C/N ratio. Data sheets, or technical brochures, are available to indicate specifications for the NF of amplifiers.

So, the first point to mention is that the first amplifier has the most significant impact upon the final C/N, emphasising the fact that it should be chosen accurately to ensure it has a low NF.

Secondly, the location of this first amplifier is important because it can maintain the best C/N for system. Let me explain why.

The aerial’s job is to collect as much of the desired signal as possible and then transfer these signals onto cable with the least loss (an aerial with a BALUN does this the best). Once the signals are on the cable, the greater the cable run you have the lower the C/N will become.

This is why we mount masthead amplifiers 1 metre down the cable or use dipole amplifiers, which of course results in almost no loss between the Aerial and amplifier.

To conclude, in areas of weak signal, firstly use a good aerial with a BALUN. Secondly place a low noise pre-amplifier close to the aerial or use a Dipole Amplifier. Lastly, use double screened cables and screened outlets.

Following this practice will improve the quality of picture significantly and may well resolve DTT lock ups.

But I must also state that when you have very good signals from your aerial the C/N is normally good enough that none of the above will appear to make a difference, in other words you can get away with a lot and still provide a quality picture. Good Hunting!