Need to come up with a scheme in which the distance of the slave device by X meters from the master causes the circuit to be turned off in the slave device?

Question

Do it yourself

There is an interesting technical challenge. How would you implement it?
There are X people in space (in the forest).
One of them (the master) has a radio transmitter.
The rest (running away) with radio receivers.
The point is that when escaping from the zone of the radio signal, some equipment is turned off for the evaders.
An example - the evader left the master at a distance of 900 meters and his flashlight turned off.
The goal is to protect rental equipment from theft
. All that my experience allows is a tx-rx bundle from model radio equipment ....
generate some kind of PPM in TX, and when it changes, the receiving side will turn on the relay.
How would you solve such a problem?
Thanks