But why is this needed at all? That can be determined once the unlock button is pressed.
Door unlock button pressed -> call out to fob via nearfield, fob responds, unlock occurs
Fob unlock button pressed -> call out to car, unlock occurs
In fact that is exactly what happens when I am walking to my car after work every day. I am 100 feet away, no where near this near-field bubble.
The whole idea is to not have to push a button. Consider a woman whose fob is in the murky depths of her purse. She doesn't have to go on an adventure finding it. She only has to walk up to the car and press a button or slide a hand through the door handle.
There's not much point to if if you have to manually activate it by pressing a button.
When you use the buttons, a similar thing happens but it's manually initiated rather than automatically. So here's a summary of the two systems:
In many keyless entry systems, pushing a button on the fob for remote locking/unlocking and the passive keyless entry (PKE) system use different communication frequencies and protocols. Here's how they typically differ:
1. Remote Lock/Unlock (Button Press):
Frequency Range: Typically operates on a frequency of 315 MHz (North America) or 433 MHz (Europe and other regions), depending on local regulations.
Transmission Mode: When you press a button, the fob sends a high-power signal to the car to lock, unlock, or perform other functions like opening the trunk or triggering the alarm.
Signal Type: It usually sends an encoded one-way communication, where the fob transmits a command, and the car receives it. In that signal, the fob identifies itself. If the identity is stored by the vehicle, it performs the function based on the button pressed.
2. Passive Keyless Entry (PKE):
Frequency Range: Operates on a combination of low-frequency (LF) signals (usually 125 kHz) for proximity detection and high-frequency (HF) signals (315/433 MHz or 868 MHz) for communication.
LF (125 kHz): The car emits a low-frequency signal to detect the presence of the fob.
HF (315/433 MHz): The fob responds to the car's query when detected.
Transmission Mode: This system involves bi-directional communication. The car and the fob exchange signals to authenticate and enable actions like unlocking the doors or starting the engine.
Why Different Frequencies?
Power Efficiency: Button-press signals are stronger and use more power since they must work over longer distances. PKE systems use low-power signals to conserve battery life.
Functionality: PKE systems require short-range, precise communication to ensure the car only unlocks when the fob is nearby, while remote button presses must work from a distance (e.g., across a parking lot).
Security: Using different frequencies for different functions makes it harder for malicious actors to intercept and misuse signals.