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A continuous signal would flatten the battery very quickly. I can't see the handsfree system working like that. I'm not sure how the handsfree card works, but the two possibilities that occur to me are:
1) The card periodically sends short pulses - e.g. a 100ms pulse every 5 seconds. That would conserve power, but still leave the card unnecessarily running itself down by transmitting when away from the car. It also raises issues with having a radio transmitter constantly running in your pocket. You know how paranoid people are about that sort of thing these days.
2) The card works on a 'challenge, response' basis and sends a signal to the car only after receiving a signal from it. This would mean that it would only need to operate a radio receiver for most of the time, and would optimise battery life.
So I suspect it works the second way, though I have no way of proving it. I have a different handsfree card (my three buttons are lock, unlock, and one that turns on the 'see you home headlights' feature.) On top of that, I don't have a spectrum analyser to tinker with, so I'm afraid it's a bit of a non-starter.
Currently in Horatio's garage:
2010 Laguna III Initiale.