I recently installed two electric gate openers on a gated drive. The remotes work fine, but have a limited range (200 - 300 foot). How can they be modified to transmit a bit more power to open gates at 500 foot distance?
Increase the sensitivity of the receiving part of the system by designing proper antenna at the working frequency. Any ARRL book will be helpful. The remotes work from button batteries so increasing their transmitted signal is more work than it is worth. On the other hand if you are on "sci.electronics.design", I have seen gates operated by phone, so you ring from your mobile the number of the gate, from whatever distance, the gate opens, detects passage of car and closes. Or closes after some time delay when NO car passes. The ball is in your court;^)