title_body: What blocks an FPV video signal? I am interested in long-range range FPV flying. What things block the video signal of a 5.8gHz video transmitter the most?
upvoted_answer: Just about anything will block/interfere with radio signaling if there's enough of it in the path between the transmitter and receiver or in the immediate vicinity of them. (sometimes referrred to as the Fresnel Zone) However, these are some of the most notorious offenders:
  
  
  Tight spaces with a lot of multipath interference
  Concrete and brick structures
  Bodies of water
  Soil and earth
  Large structures
  Forests and large amounts of trees
  Lots of human bodies
  etc.
  
  
  One could go on forever listing out different materials, but these are some of the more common items to encounter for FPV.
  
  In general, the rule is to maintain as much line-of-sight as possible between the transmitter and receiver. This is true because obstacles will force the radio waves to take indirect paths, bouncing repeatedly off of stuff until it hits the receiver antenna. Multipath interference comes from this, because often many different (multi)paths will converge on the receiver at different times and cause interference with each other, distorting the image that your goggles/FPV receiver will pick up.