The below info was kindly provided to me recently & it explains very well the importance of setting up Freeview|HD correctly in the first instance to avoid intermittent reception or "no signal" problems later down the line...hope you'll find this as helpful as I!

Digital Reception Margin and the Digital ‘Cliff  Edge’

Unlike analogue TV signals that can still be viewed under weak received signal strength conditions, (even though they are fuzzy and noisy) or in the presence of interference, Digital Tv pictures and sound will either be perfect, in the process of breaking up (pixilation where the image appears to be coarse blocks) or non-existent!

Digital Tv reception exhibits a very rapid change from being excellent to disappearing altogether; this phenomenon is usually referred to as the digital cliff or threshold. It is clearly important for reliable service to ensure that the receiver is not operating too close to the cliff edge or the picture may suddenly fail.

The digital ‘channels’ are grouped into sets and transmitted on different frequencies: Tv1, Tv2, on one of them , Tv3 and  Four on another and Maori Tv, Choice etc. on the third.

All the channels are transmitted at the same power, however the transmission path can affect each frequency differently. Where the signal reaching your receiver is weak, one or more of the incoming frequencies may be weaker than the others. This is why sometimes one set of channels appears to be received perfectly while another does not work well at all.

A weak signal may not be apparent until some change in conditions tips it over the Cliff Edge to pixilation or failure. Such a change in conditions could be due the weather, tree foliage, other electrical devices in the vicinity (local interference), aerial wiring/connection movement due to wind or other effects. The antenna could also have been blown off-beam and is now pointing in slightly the wrong direction.

Having sufficient margin will ensure that the receiver is operating well clear of the cliff edge.

The margin is a measure of the amount by which the signal can be reduced from its normal operating level to the point at which the picture suddenly fails. Signal levels at the receiver will vary for a number of reasons, so it is therefore important for reliable installations not to have very small values of margin. The recommended minimum margin is 3 to 6dB.

To ensure sufficient margin when setting up an aerial installation, a simple ‘pad’ or attenuator can be introduced into the aerial feed to test for this. The pad will reduce the signal level by the nominated amount.  If there is still good pictures and sound when a 3 or 6 dB pad is in the aerial feed; then once the pad is removed the viewer can be assures of sufficient margin. However if the picture breaks up or fails during this test then a ‘better’ signal is required, typically a larger antenna. 

Note: care should be exercised with this test, because if the system is limited, for example, by intermodulation caused in a masthead preamplifier instead of the receiver's internal noise, the attenuator will reduce the intermodulation as well as the signal, giving a misleading result.