A lot of people think this is a simple easy way of measuring distance, but its not.
Any variation in antenna angle, or objects in the path between TX and RX can cause very large variations in signal strength and thus the measured 'distance'.
If you want to experiment, there are quite a few SPI based UHF radio modules (not the ones you pictured) that will allow you to read the received signal strength (RSSI) directly as a digital number and also allow you to vary the power.