transmitter to receiver alert system

Hello I am new to this site and I am happy to have found it. I am looking into designing an alert system that will alert me when my pug spunky has left our rv site when my wife and I travel. We keep him outside an a long leash but he has a tendency to go into to our neighbors site. So I would like to design a tag that he would wear that contains the transmitter and I would have a receiver that would emit a beeping alarm when he goes beyond a certain distance say 15 feet. Has anyone done something similar to this project that can give me some design ideas? It would seem like a pretty simple design compared to what I have seen on this site. Thank you.

This is a lot harder than you imagine. Determining position or distance with an accuracy of a few feet is difficult.

The best and simplest method is to set up a "pet fence", not to shock him but to trigger an alarm. I don't know the details about how they work, but some Googling should help. That requires that you set up the perimeter fence everywhere you go.

The signal strength of a transmitter goes down as it moved farther away, but walking behind a tree, a car, under a picnic table or even laying down with the tag under his body will probably reduce the signal more than moving 10-15 feet.

I can't think of a good way to do this that wouldn't have a lot of errors. Sorry.

Acoustic feed back could be used, based on the speed of sound, like with obstacle avoidance (ping) sensors. Then a trigger signal can sent by the base station, whereupon the remote station emits a ping, and the base station determines the distance from the delay between trigger and received ping. The trigger signal also can be sent by RF, so that e.g. the remote station emits both the trigger (RF) and ping (US) at regular intervals (1 minute?), and can sleep in between to save power.

But beware, the sound frequency should be above what a dog can hear!