[Paul R.]

1) Add lighting.

2) Add 2nd sign (also well lit): “If you can read this, you are in range.”

OR

2-alt) Add 2nd sign (also well lit): “Video Surveillance Active”

Optional: Add video surveillance.

[Paul R.]

“1” may be subject to local codes, esp. if close to street or road, depending on how repressive your area is. But, election signs might be exempt?