Electrical potential
Hi DevNull,
a friend of mine had internet in main house/2 bedroom "granny flat" in back, about 70 to 100 yards.
I looked into main house switch >> ethernet cable >> back house switch, but, even in a low lightning area (California--lightning every two years or so), everything I read said that even this simple setup could result in significant electrical potential differential, and was a bad idea.
We looked at fiber optics: too expensive.
Line of sight is unobstructed, so, set up a wireless bridge and that works fine.
For OP: "100 yards through woods" is a bit different of course.
Anyway, stringing Cat5E or Cat6: I would have a professional do that--also, as you know devnill, that 100 yards is right at the limit for ethernet cable length.
I think wireless, with dedicated units that have decent and movable antennae is the best way to go.
Note: might want to look into the "break even" point, ie.
cost of adding this connection to grandma's house versus
monthly cost of getting her own internet connection, ie. DSL
Depending on where you live, there can be a ten dollar a month DSL connection as I recall, and, after all is said and done with wireless, it could be a DSL line's speed, even low end, will be faster than "piggybacking" on the grandkid's connection.
Here is DSL for ten dollah:
http://dslfor10.com/
I assume grandma is not playing BF3 or COD4.
HTH