The functioning of the Transponder was therefore to move some property of the returned signal from that of the interrogation signal so that both could be detected simultaneously without the one swamping the other. The most common property to change is the transmission frequency meaning that the transponder might receive the interrogation frequency at one frequency, and respond on another frequency that is seperated sufficiently with regard to frequency so that both may be detected simultaneously.
Transponders were initially used in World War 2 on aircraft to identify the aircraft using IFF (Identify Friend or Foe), where friendly aircraft would respond to secret preprogrammed interrogation codes and indicate to the radar operators that they were friendly aircraft. Today Transponders are still used extensively on commercial aircraft to relay to the radar operators the height and identity of the aircraft on their radar displays.
Another important use for transponders has been in the measurement of distance. Here the interrogator sends a signal to the transponder, which immediately responds on another frequency. By measuring the time from the sending of the initial signal by the interrogator, to the receipt of the signal from the transponder, and calculating the effective double path travelled using the speed of light, the distance between the transponder and the interrogator can be determined. The accuracy of such systems is limited to fractions of a meter using electromagnetic propogation systems due to the limits in determining the transmission times with sufficient accuracy. (A system called Tellurometer invented in the 1960's improved this resolution over distances of 100's of kilometers to a few centimeters, but although this still used transponders, it was not based on the principle of time of flight).
Another major category of Transponders which is not the subject of this newspaper, is the use of transponders in radio relay systems such as fixed/mobile radio networks and satellite transmissions. The same principle applies in that the data is transmitted on a carrier frequency at one frequency, and rebroadcast on a carrier of another frequency, allowing the strong and weak signals to co-exist.
Transponder systems have recently started to become major players in the field of electronic identification. Within this application, it is necessary to make the transponders as cheap as possible, and to rather build the sophistication into the readers. This lack of sophistication generally means that changing the transmission frequency is no longer an option, as the frequency translation needs expensive and complex tuned circuitry. Instead the transponders have given up the ranging ability and rather time slice the communications channel with the interrogator. Here the interrogator (called a reader) sends an interrogation signal for a limited time. The transponder receives the signal and waits for its completion, and then responds on the same frequency with its identity and data code. (There are more complex methods but this covers the basics.)
The devices are sometimes called transponders and are also sometimes called tags, most probably because their end application eventually will be the tagging of goods.
Transponders vary in selling prices from $1000 US down to $0-20, depending on application and features.
Commonly available tags have an operating frequency in the range from 60kHz to 5.8GHz depending on application.
In operation one can generally say that there are three different types of technologies being implimented. They are:
If you want to contact the editor about additional information or questions,
send mail to The editor, Transponder News
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