A tree has very little effect on VHF-low, but a significant effect on VHF-high. But the big problem is UHF. A tree with leaves blocks about 90% of a UHF signal. The space behind the tree is an overlap of the signal going through the tree and the signal diffracting around the tree. Such overlapping fields have an alternating pattern of strong and weak spots separated by only a few feet. An antenna in a strong spot might work nicely until the wind blows, deforming the tree and moving the spots. Thus, for DTV stations, you are likely to see dropouts when the wind blows. Even in a good-signal neighborhood it is inadvisable to put a UHF antenna behind a tree.
If the tree loses its leaves in the fall, reception behind it will improve dramatically. Many people get a TV for Christmas, and erect an antenna for it in January, and then wonder why it quit working in May. It
Trees as obstructions
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