After your post on what you used, I went and looked them up and found this site. http://www.gadgetshack.com/wireless-driveway-alarms/
What I focused on was the detection range of the detector itself. The range to the receiver seemed to be way longer than needed (for me 150yds).
Lowest seems to be 30ft via the Chamberlain (what I use) and a few others on here have used. The highest was 50ft. I never did find a spec for the Miltronics one you have.
I think what's telling is the detection field as stated is like 40' on one one of them. Depending on where you have it, that's not that big of space given the size of the average hog hunting arena. (30-50ft distance with a 40' view)
Per the other post, I think you're missing more than you know by relying on those so much, I know I was. You need something to scan with.
Click this --> Leupold scope