Freeview quality - aerials - cabling

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grahamknight

Guest
Having read many threads from people concerned about Freeview picture quality, signal strength, aerials and so on I thought it might help to share my experience - having wrestled with, and now fixed, the problems.

1) It is absolutely essential to have a NEW (shiny parts, connections, good insulation, balanced feed from the folded dipole element etc) wideband aerial if you live more than even 10 miles from your nearest transmitter. The reason is that currently, DTT signals are very low strength compared with analogue to avoid interference with existing analogue services. So that really means - outside, high up, no obstructions and pointing in absolutely the right direction. Setting the direction is best done by a professional with professional equipment.
2) The comment above about signal strength from the transmitters means that you cannot rely on the strength of your analogue reception to tell you much about DTT reception. Analogue power can be 100+ KW from main transmitters, DTT power no more than 5 - 10KW (Sandy Heath for example has 1000 KW for analogue and 20KW for digital): and then there is a network of local low power transmitters that can be very confusing. You might get poor analogue because you aren't pointing at the transmitter you thought you were tuned to - but picking up decent DTT from a nearer local station. The BBC publishes transmitter locations and channels on their web site.
3) The cable and outlet sockets. Analogue TV is much less susceptible to moderate noise, interference, cable loss and impedance mismatch. The main problem with DTT is its susceptibility to pulse noise from thermostats, cookers, vacuum cleaners, motors, cars and so on. Whilst an analogue signal will only show show white streaks and speckles on the picture, a DTT signal will be severley corrupted - which means blocking, transient interference on sound - or no picture at all. The main culprit is old style sadle and clamp fittings and sockets which, given the balanced feed from the dipole, can induce interference on the coax inner from the outer screen. So - use the highest quality cable you can find, eliminate ALL joins with older bits of cable (like the one running down the inside of the wall by the TV) and dispense with the socket plate. Fit a Belling Lee or F plug straight onto the flying end of the cable and straight into the Freeview decoder (or mast head preamp power supply if you have one - see below).
4) Amplifiers. It is generally true that signal quality is more important than signal strength - hence the need for good screened sat quality cable. But if the signal is low you may still need to boost it. Only use the best quality masthead amplifier - get a new one designed for connection to the balanced dipole. Don't attempt to boost the signal at the TV end of the cable as you will amplify the noise as well; and avoid passive splitters which will just bring the strength down again - 3dB per split at least - and that might make the difference between picture and no picture.

By merely replacing the entire cable run I have increased the signal strength at the DTT decoder from 15 - 20 % to 30 - 40 % (depending on multiplex). Picture break up is - as far as I can tell - completely eliminated. In particular, MUX 2/64 QAM (ITV) was unwatchable but now OK. 64 QAM is very susceptible to pulse noise I think. This was after a new aerial and booster were fitted professionally 12 months ago - but without the replacement of ALL the existing cable. I was left with 5 metres of old coax inside the wall leading to the TV point and a sadle and clamp faceplate; results were almost unwatchable except when conditions were favourable.

This is quite a long post but hope that others will find it useful.
 

Sorter

Standard Member
Full of excellent info. I am checking out digital and am sure this will be a real help!
Thank you!
 

LV426

Administrator
Staff member
A wideband aerial is only necessary if a wideband aerial is necessary (!)

The correct type of aerial for any given transmitter is given here. In fact, if a specific (narrow) aerial is right for your transmitter, then it is preferable to use this rather than a wideband one as its sensitivity will be better, all other things being equal.

Otherwise, good advice.
 
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grahamknight

Guest
Nigel is quite right - a wideband aerial is only needed where the multiplex channels are widely spread or at the edges of the UHF band (in my case - Sandy- Ch40 through Ch67). You need to check the details for the transmitter you intend to use (many internet resources) and those should tell you the aerial group/type that you should use.
 

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