What is a Transponder?

A transponder (also TPDR, TR, XPNDR, XPDR) is an electronic device used in wireless communications, the word itself is shorthand for transmitter-responder.

This device is primarily used as a re-transmitter due to the fact that it receives a particular signal from a particular source, then it amplifies (strengthens) the signal before sending it to a predefined location. Transponders have an abnormally large number of applications in our daily lives. Some of the most common uses are: satellite television, satellite telephony, air traffic control and in automobiles. They are also embedded in cars to open gates automatically. We shall look at some of these applications later. First of all it is important to mention that transponders are of two general varieties which are active transponders and passive transponders.

Active transponder: These devices as the name implies, continually emit radio signals which are tracked and monitored. These can also be automatic devices which strengthen the received signals and relay them to another location.

These devices are so frequently used that we often fail to recognize them. For example, how do you think lap times of NASCAR and formula one cars are monitored so accurately? Well the answer lies in the transponders which cars have embedded in them. Each car has a unique ID code which is transmitted as the car moves. A special cable loop is dug into the ground at the start-finish lines. So when the cars zoom by the finish line, their IDs are recorded along with their lap times. These times are automatically displayed on the position board along with split times, laps remaining and so on.

Another important use of active transponders is in satellite communications. Normally there are hundreds of thousands of tiny transponders embedded in one satellite. These receive an incoming signal over a range of frequencies (band), measured in hertz and megahertz and retransmit these signals on a different band simultaneously. The incoming signal originating from a point on the earth (e.g. A broadcaster), is called the uplink and the outgoing signal back to the earth is called the downlink. The logic behind using satellites for this purpose is simple - as radio signals cannot curve along the curvature of the earth, they are sent in a straight line up and received down in a straight line. This reduces time of signal delivery and increases range.

Now we come to the passive transponder which although not as active as their counterparts still play a very important role. These transponders contain information which is used to identify particular objects. For example passive transponders are sometimes embedded in our credit cards and on magnetic labels in large stores. These are paired with active transponders which amplify and transcribe the information.