Along the same lines as a security camera, but for less $$ and hassle,
you could try a wildlife camera.
I know, it's not the same- it won't let you watch online or call you when someone is on your property, but we simply could not afford those kinds of cameras on our farm- as we have multiples covering more area.
Our camera is motion activated- if something moves in the field of vision, we have a record of it.
This includes proof that yes, my good neighbor, your dog IS visiting us daily and thinks she owns our property, which is why we had a dog fight 2 days ago.
And YES, (a different neighbor) I SAW that you visited my farm in your silver and green truck with license plates (123****) and left with one of my Christmas trees in the bed of your truck.
Yes, really these things happened and we have a record that will stand up in court if we chose to pursue that.
There are no wires or cords on our cameras, they are battery operated and yes, we have to bring the camera in and download the pics to see what occurred- not a problem for us.
You can strap it to a tree or anything else outside.
We have also put up signs with eyeballs on them that say "Security Camera watching 24/7- no hunting with rifles on this property"
and we have found that we have virtually no trespassing as opposed to past years where people ran their ATVs through our fields and opened our rear gates and let our livestock out.
It only takes one encounter for word to get out that you have eyes watching continuously.
And then people steer clear.