What Is a Transponder?

J. Dellaporta

A transponder is an electronic device used to wirelessly receive and transmit electrical signals. Fittingly, its name is equally derived from the words "transmitter" and "responder." It was originally developed to be attached to an object that needed to be located, and some are still used in this manner today.

A transponder is a device that acts as both a transmitter and responder and is used to wirelessly receive and transmit electrical signals.
A transponder is a device that acts as both a transmitter and responder and is used to wirelessly receive and transmit electrical signals.

This device functions by receiving a signal, called an "interrogator" because it is effectively "asking" for information, then automatically conveying a radio wave at a predetermined frequency. In order to broadcast a signal on a different frequency than the one received, a frequency converter is built in. By receiving and transmitting on different frequencies, the two signals can be detected simultaneously.

A transponder key.
A transponder key.

The first use of a transponder was on an aircraft during World War II, as part of the Identify Friend or Foe (IFF) system. By answering secret interrogation frequencies, pilots could indicate to radar operators that they were friendly aircraft.

In aviation, transponders broadcast an aircraft's position and identity.
In aviation, transponders broadcast an aircraft's position and identity.

These devices are still common today in both military and commercial aviation. They receive a signal from the ground, and then automatically reply with an identification code for air traffic controllers, as well as altitude information. In aircraft applications, they are also configured to amplify the signal in order to make the plane more visible on radar.

Transponders on commercial aircraft transmit their carrier and flight identification numbers to air traffic control centers.
Transponders on commercial aircraft transmit their carrier and flight identification numbers to air traffic control centers.

They are also used to measure distance by calculating the elapsed time between the sending of the interrogator signal and the receipt of the transponder's signal. For example, sonar devices are used to mark underwater positions, calculate depth, and trace positions.

It may sound as if this is a technology that the average consumer never uses, but even if that was once the case, it no longer is. The modern commuter probably has at least one transmitter in his car, probably mounted on the windshield or dashboard. These are for roads that use electronic tolling systems that compute the amount of tolls to be paid and complete the transaction without requiring the driver to so much as lower his window. Some newer cars are also equipped with ones that operators can use to locate the vehicle in the event of an emergency. Cellular phones use a similar, albeit smaller, chip to send the phone's location if it used to call an emergency number.

Even casual television viewing often involves the use of these devices. A network can uplink its ground-based satellites to communications satellites orbiting the Earth, send multiple channels of digitally compressed video and audio to a single transponder aboard it, and local stations can then pick up the program and re-broadcast it locally by aiming the appropriate ground-based dish.

How Does a Transponder Work?

A wireless monitoring, communications, or control device that detects and responds to a signal is called a transponder. The word was coined as a combination of the words "transmit" and "respond," which perfectly defines the purpose of the device. Transponders operate through radio frequencies and can be either active or passive.

Active Transponders

Active transponders are powered utilizing an internal battery and can transmit data continually using their own power source.

  • Simple active transponders are frequently used in aircraft identification, location, and navigation systems. One example is a radio-frequency identification device (RFID) that sends out an encoded signal when a request is received from a control point. Input and output frequences are preassigned, and tracking of the transponder's output signal allows the transponder's position to be monitored constantly. This type of device can function successfully over long distances consisting of thousands of miles.
  • Sophisticated active transponders are found on space vehicles and communications satellites. These devices receive signals from a band of frequencies and simultaneously retransmit signals on a separate band. This type of transponder operates similarly to a land-based cellphone repeater, and is capable of functioning on an interplanetary scale.

Passive Transponders

Passive transponders utilize the radio frequency from a scanning antenna or reader as a power source and will only transmit data in response to a prompt from the reader/antenna. The transponder may be physically small, and the scanning reader may be located up to several feet away.

The magnetic label on a credit card is a good example of a passive transponder. The credit card strip must be decoded by an active sensor that transcribes the data that the transponder holds.

What Is a Transponder Number?

Transponder numbers are commonly used in the transportation industry, both in aircraft safety and highway administration.

Aircraft Transponder Numbers

Transponders were first used in relation to airplanes when the military used them to identify friendly aircraft through a coded signal picked up by military radar. This procedure was called IFF (Identification Friend or Foe).

Today's transponders on airplanes assist in identification on the air traffic control's radar. Systems for collision avoidance utilize transmissions from transponders to detect aircraft that are at risk of collision.

Air traffic control teams assign a transponder code to an aircraft using the term "squawk" or "squawking" followed by a four-digit code, which can be interpreted as "select transponder code."

Highway Transponder Numbers

Also known as a toll tag, a transponder number is used for vehicle operators to automatically transmit a radio signal to pay tolls instead of manually paying through a toll booth. The system reader is set up on the highway, which decodes the tag information and number of vehicle axles to calculate the correct toll and deduct the total from a debit account.

Some vehicles with metal oxide windshields may require an external toll tag on the license plate to be properly scanned.

What Is a Transponder Key?

Transponder, or "chipped" keys have become an industry standard for automobiles, and have been routinely used in most cars manufactured over the last two decades. These devices use microchips to send signals that are read by remote receivers. The manufacturer programs ach transponder key's microchip with a serial number unique to that particular vehicle. The receiver uses RFID to decode the proper serial number which allows the vehicle to be unlocked.

When the key is inserted, the receiver near the ignition picks up the signal. If the transponder's serial number matches the car's unique programmed number, the vehicle will start when the key is turned.

Enhanced Security

Transponder keys improve vehicle security in different ways.

  • The ability to "hot wire" a car or break its ignition lock is removed through the use of a transponder key. The ignition receiver requires recognition of the matching serial number to start the vehicle.
  • An immobilizer is generally at work with a transponder key to enhance security. Some devices will prevent the engine from cranking unless the transponder key is present, and others will prevent fuel injection to the motor so the car will not start.
  • The digital identity of a transponder key is much more difficult to replicate by a potential thief than the combination of physical cuts on a mechanical key, making it difficult for someone else's key to start your vehicle.

Prohibits Lock-outs

Before the use of transponder keys, it was common for people to lock themselves out of their vehicles, but transponder keys have helped to prevent this. Most vehicles will send an alert and halt the locking process if the key is still sensed inside the vehicle when you try to lock it.

Communications satellites carry transponders.
Communications satellites carry transponders.

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Discussion Comments


the key has a little slot in the top open it with a small flathead screwdriver. furthermore, usually you get a user's manual with your yaris and inside is the code and number of the key, a good locksmith will cut you a new key with the chip and everything. good luck.


The power for the transponder in your key is actually in the car itself. The rfid chip receives the power and then is able to send back a signal. Similar to security cards for parking garages and apartment buildings and is why sometimes you will have a split second delay while it receives the power. it's possible the chip has gone bad or some how has been erased and simply needs to be reprogrammed.


Please let me know the companies that are producers of a 'C' band transponder.


my car also has that transponder key. it gave problems recently where the car wouldn't start due to an immobiliser being triggered due to a weak signal from the key, but there are no batteries to power this key? I am now using the spare key.


immobiliser systems are actually RFID tags. the key fob has a small one in it. It has an antenna and a small microchip. When the system in the ignition sends an rf signal the rfid tag responds. The immobiliser keeps the vehicle running after about two seconds.


transponders use the signal to power themselves, I think they have what's called semi-conductors inside them and they have very special properties about them, this being one


How can I convert local tv rf output to a satellite transponder frequency?


Hi Wisegeek Editors,

My car has an engine immobilizer system. The car's manual says that when you insert the key in the ignition switch, the transponder chip in the key's head transmits an electronic code to the vehicle.

But, the key's head doesn't seem to have any screws to put the battery in should the battery run out. Or don't transponders use batteries? If it doesn't have batteries, how does it transmit an electronic code to the vehicle? Thank you very much!

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