Effect of Large Downrange Obstacles on OTA Signals

[El Gran Chico]

When I put my antenna up a while ago, my installer was concerned about a large condo building (34 stories) that is 2.7 km downrange (measured on Google Earth) dead center in the path to a transmitter over 100 km away.

It appears that the installer was correct. This station has been my "problem station" ever since. Last weekend I did an experiment by taking my spare antenna a block away (outside the shadow of the building) with a portable tv+STB and the station was solid, even when holding the antenna 4 feet above the ground (at the same time, my chimney mounted antenna at home wasn't able to get enough signal to lock). More evidence that supports what my installer was concerned about.

Is there a formula or rule of thumb that is a good indicator if a large object (I suspect this will usually be a building) in the direct path to a transmitter will be a problem or even fatal for reception?

By looking at some topographical maps, my antenna is about 35 meters above the elevation of the base of the building, so it's likely about 100 meters lower than the top of the building.

Without my installers professional insight, I would have (incorrectly) thought that 2.7 km would have been far enough away to not worry about it.