Path Loss in Remote Keyless Entry Systems

In a remote keyless entry (RKE) system a car is unlocked from a distance by a radio code sent from a key fob to a receiver in the car. RKE systems usually operate at frequencies in the unlicensed ISM band that includes 315MHz and 433.92MHz. With the advent of remote starting features and two-way RKE with verification, designers want to increase the usable range of these short-range devices. Replacing SAW resonator transmitters with high-frequency, tolerance phase-locked-loop transmitters allows receivers with narrower bandwidth to be used. RFIC receivers take advantage of these narrower bandwidths to improve sensitivity, which in turn increases the usable range. Once the transmitted power and the receiver sensitivity are established, the path loss of the signal from transmitter to receiver is the main determination of usable range. This application note describes how path loss is affected by the ground bounce of the radio signal and gives a simplified approximate equation for path loss. Graphs predict path loss as a function of distance in an empty parking lot. Finally, the note offers guidelines for predicting the effects of multipath signals and blockage.