RF Transmitters

Radio Frequency (RF) Transmitters

Transmitter Operation:

• Pulsating AC current is run through radio transmitter antenna wires.  The pulsating AC current pulsates in unison with the sound or signal being broadcast. 

• The pulsating AC current creates a pulsating magnetic field which, like on a bar magnet, the field strength is very strong right next to the transmitter and gets weaker further away from the transmitter. 

• The higher the power used to transmit, the farther away the signal can still be picked up by the antenna through electromagnetic induction.

• Each radio transmitter has it's own broadcast frequency, this means that there are dozens, if not hundreds or thousands of different frequencies of radio signals being broadcast at all times in any one location.  Wireless phones use one of three different frequencies to broadcast and receive signals.  Analog signals use 800 MHz, digital cellular signals use 900 MHz, and digital PCS signals use 1.8GHz.  Your wireless phone may use one or more of these frequencies as needed.  Digitizing a signal allows multiple users to use the same frequency. 

RF Transmitters are used on many of today's automobiles, examples of transmitters are:

• Keyless Entry transmitters (key fobs) - When you press a button on a transmitter, the transmitter sends a RF signal to a receiver. The receiver interprets the signal and activates the requested function over the serial data line. Most keyless entry systems use rolling code technology.  Rolling code technology prevents anyone from recording the message sent from the transmitter and using the message in order to gain entry to the vehicle.

• Ignition Key Transponder - A transponder is a radio transceiver that automatically transmits a signal of its own when it receives a predetermined exciter signal from elsewhere.  The PK3 key has a transponder embedded in the plastic head of the key. The exciter is an antenna which is made up of wire coils which surround the ignition lock cylinder. The powertrain control module (PCM) enables engine starting when it receives a valid (learned) password from the theft deterrent control module.

• Tire Pressure Monitoring (TPM) transmitters -  The tire pressure monitor system warns the driver when a significant loss, or gain of tire pressure occurs in any of the 4 tires.  The system uses the remote keyless entry receiver, and a radio frequency (RF) transmitting pressure sensor inside each wheel/tire assembly.  When the vehicle is stationary for more then 15 minutes the sensors go into stationary mode. In this mode the sensors transmit once every 60 minutes to minimizes sensor battery consumption. As vehicle speed increases to 20 mph, centrifugal force closes the sensors internal roll switch, causing the sensor to go into drive mode. In this mode the sensor transmits once every 60 seconds.

• Built-in Cellular Communication Systems - Many of today's automobiles have built-in cellular communication systems. These systems can use analog or digital signals. The system is used to call for help when necessary or to make everyday telephone calls from your car. The On-Star system is an example of this type of system. See www.onstar.com for more information.   The antenna shown below receives and transmits RF signals for cellular communication.