Line of sight. Pretty simple idea actually. Antennas work best when there is nothing between them and the thing that’s doing all the transmitting.
RF signals do a decent job traveling through trees and walls, but not such a good job traveling through hills. Hills are a lot more substantial and have iron and other minerals in them that deflect RF signals. These signals are attenuated (in other words they lose strength) every time they travel through anything, even open air. That’s part of the reason that an indoor antenna doesn’t work as well as an outdoor one. The walls attenuate the signal.
If you’re worried about getting good reception from a TV station 10 miles away, you don’t have to worry about line of sight. What about if you are 50 miles from the broadcast towers, or over 22,000 miles from a satellite? Signals also lose power over distance so in a case like this the best bet is to make sure you have a clear “line of sight.”
Poor line of sight can cause you to lose signal, especially when the weather gets bad. (Snow and rain also attenuate the signal.) While you need an expensive meter to figure out the exact line of sight, you can do a pretty good job with just a cheap compass or smartphone app.
For antenna line of sight, start with a site like TV Fool to find out the compass headings for your local stations. Most likely they are near the closest major city. Then stand where the antenna is (get on the roof if you have an antenna up there) and point yourself in those directions. Do you see blue sky, or do you see something else? Blue sky good, something else bad. If the something else is far in the distance it may not be as big of a problem as something nearby.
For DIRECTV satellites, you can use a program like Google Earth to figure out the approximate compass heading to get to Dallas, Texas. DIRECTV’s primary satellite is at 101 degrees west longitude and Dallas is pretty close to that. Point yourself in that direction and look about halfway up the sky. That should give you an idea if you can see the satellite. Look a little to the left and right as well.
In very few markets DIRECTV subscribers need to see the satellite at 119 degrees, so do the same thing but instead of pointing toward Dallas, point toward San Diego.
DISH subscribers have a little tougher time because DISH has two fleets in the sky but in general if the dish can see the sky with no trees, it will be fine.