For me just wanting to know is a good enough reason. Why not?

The answer is about 1". If I held the fob further away than that the key would of course turn, but not crank and start the engine. Closer and it will.

Now I want to find out how they do that.

 

How about just leave it alone

 

I told you. I want to know how it works.

 

This is the Ford PATS system that's been around about 2 decades. It's passive. Despite the integrated key unit having a battery, it's not needed to start the vehicle. The battery is used for the remote buttons for unlock etc. When the key is placed into the ignition, the car excites the transmitter in the key which then sends back a unique id. Read more about it here http://ricksfreeautorepairadvice.com...itheft-system/

Now, the pushbutton start is an active system. The car sends out a longer range command to wake up the key and waits for a response. This can work at greater distances... sometimes a little too far in the hands of a criminal with an amplifier. See
The active system does have a fallback in the case of a dead battery. There is a place somewhere on the vehicle where there is the same old exciter loop to validate it via PATS. In the case of the f150, there is a false bottom in one of the cup holders.

You sure you want to convert your key system to prox? In my opinion prox is less secure. You sacrifice security for some convenience. With the key'd PATS system, you need BOTH a valid mechanical cut and programmed transponder. With prox, you just need a laptop, no locksmithing skill required. And the 10 min delay to gain security access over obd is a thing of the past. It's all gone coded and is easily bypassed by brute forcing software which is available from non official places. The thief will be gone before you can even blink.

 

Thank you.

Energizing the coil of wire in the fob is the reason for the limited range.