Topic: Police wire transmission distance?

Does anyone know how far a small police "wire" can transmit? I have a scene where someone riding in a car is wearing a small wire (ideally somewhere around the size of a tie clip), and I want the police to be able to listen in from another car following the first at a distance (e.g., a few hundred feet). There's no opportunity to add a powerful transmitter to the car being followed, so the wire has to be powerful enough to do the transmission by itself. I can increase the size of a wire somewhat, if needed, as long as it can be hidden in baggy clothing.

I've been unable to find any distance estimates online.

Thanks
Dirk

Re: Police wire transmission distance?

Increasing the size of the antenna and making it directional will compensate somewhat for a weak transmitter.  If directional, the receiver should have an indicator showing signal strength so they can adjust the aim, or pick up a signal,  when nobody is speaking.

If someone is building transmitters and receivers on one or two chips each (which requires a certain mix of tech/equipment and design skills--which national spy agencies surely have) the transmitter can be a little larger than a bluetooth earpiece.  (Most of that extra bulk will be battery.)  It depends also on the frequency band used.  The band used by bluetooth is more easily blocked by walls and metal than the AM band.  If you want this UHF/EHF to work in cars, the part that emits the signal should probably be in a collar or shoulder.

Look up Code Division Multiplexing, which cellphone and Bluetooth are based on.