What I learned when installing my antenna is that DVB-T has some funny quirks. Digital tends to be all or nothing so a marginal signal can either look perfect or pixelated depending on small changes. The other thing is that UHF reflects in odd ways, which can also affect reception. We aren't surrounded by houses but there was a big hill in the way, much higher than the aerial could go. Yet I found a sweet spot on a high mast that gave us perfect reception. Higher or lower deteriorated the signal, but the height that worked, a few meters above the roof, worked perfectly.