I used to have good signal quality for all Freeview channels, but a year or two ago it started to degrade on TV3 & TV4. The problem got progressively worse, to the point where those two channels were unwatchable - especially in bad weather. I replaced my aerial and tinkered with other things but nothing helped, and I could find no answers online. It was very strange that the TV1 & TV2 signal quality was fine, because all four channels are transmitted at the same signal strength. The only difference is that they are transmitted at different frequencies.
Now I have fixed the problem by trimming a tree growing about 15m from my aerial. I don't know what the tree is called, but it is a spindly hedge type tree with small (~3cm) leaves. This tree had grown into the signal path over the last few years. I don't know why it was affecting the signal quality when other trees in the signal path are not. IE. my aerial is pointing directly into another tree a few meters closer, which has larger leaves (~9cm). Actually I'm surprised that I get a good signal at all because the Waiatarua Transmission Mast is hidden behind hills about 1km from my aerial, and also some thick bush about 70m away.
Anyway, my reception of all channels is perfect now, and I hope that my post will help others who are experiencing this weird problem.
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