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What is Transponder, definition please

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May 18, 2001
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What is Transponder?

Hi All,

I am always confused with the term "transponder", is it same as transceiver, anyone can made me a clear definition?



A Transponder is a Transmitter/Receiver. but!!!

It's not the kind used for normal two way communication.

The transponder is used for Air Traffic Control. These are the boxes that the ground radar transmit to, and the Transponder replies to the ground radar site with a code. the code is an identifying code that tell who you are and how high you are in the air.

I don't know how much more you need to know, So I'll keep this to the simplest reply to your question. but I hope this answers your question.


Hi WA,

Thanks to your answer. Your answer cause me falling deep in water. As you said, the transponder is a system-level term, but I always heard that it is in IC level.


Transponder in a satellite does the function of receiving, amplifying and transmitting. While receiving it downconverts and while transmitting it upconverts the frequency. Thus it is a repeater amplifier in the sky. One transponder generally has a bandwidth of 36 MHz!

What is Transponder?

Regarding to the answer from WA,

One of Transponder IC is "RFID". the code is an ID. Visit for more information.

I'm looking for RFID chip that.

1. Able to burry in animal body. So, it should cover with plastic or glass
2. Maximum distance from reader is "100cm"

I only found a chip from TI. It's cost about 3~4USD. Anybody know the cheaper solution? Please let me know.


I don't know what you mean by falling in deep water(I guess it's safety fall)?

But I'll try and help a little more. the origional transponders were use by militaries for identify friend or foe. (I've spent 15 years repairing them). Since then the system was converted to General/Commercial Avaition. It's the primary identification system use by Air Traffic Control (ATC).

As far as the IC level?

I think elektrom has hit nail on head. Today transponder is used in many applications. But a good rule of thumb for transponder? transponders reply to a transmitted signal with a coded reply that the transmitter understands.


I visited TI site and I think you are really looking for a "homing device" for your application. I think what you are looking for can be programmed for individual use per application? example. you have several wolves, you tag one with one code and tag another with a different code. This way you can track them individually.

However the TI site mentions the transponder chip as an anti-theft device for automobiles? maybe I didn't read about all of them?

As far as cost? 3-4 USD is pretty cheap. But only for small projects. If you plan on doing over 100, you can contact TI and see if they have bulk price. If they do, you may find a better price. Unfortunatelly, untill another manufacturer comes out with a cheaper version, we're at the "mercy of the manufacturer".

hope this helps all.

navigation transponder

A further use of transponders is for navigation. in the 1939-1945 war transponders were used in the RADAR bands. Airplanes would see these as bright spots on their viewing screens and be able to know their location as long as they knew where the transponders were located.

In modern times these are still used for going to a location. The transponder is put in a location that is known relative to the spot that is the final destination. Direction finding methods are used to find the angle to the transponder.

For tracking it is sometimes better to have beacons on the animal. These are transmitters that transmit on a small duty cycle.

you can imagine transponder as amplifier. something comes in, then amplify and then transmit (very similar to microphone, audio amplifier and speaker)-transmiting frequency could be different :idea: .


as far as I understand, transponder is a device which is able to receive some special signal then transmit some thing. I donot understand chip level transonder, it should be that transponder is made on chip level

maybe signal amplifier?

According to a lot of literature, transponder means receiving then transmitting. However, for the radio signal, you cannot just put an amplifier there. I may couse oscillating if you just amplify the original signal. I havent seen those real transponder yet. But for the HAM transponder, you will find the frequency conversion unit inside.

Thanks for all valuable reply. Seems that there are no exact definition of transponder in IC level.

Re: What is Transponder?

Transponder functions as a reptear. A signal might be generated at earth and uplinked to a satellite transponder which would amplify and reflect it back to earth for greater coverage (reception) such as broadcast of TV.

Re: What is Transponder?

transponder is a repeater

Re: What is Transponder?

Rayengine, be carefull with RFID the tag is often refered to as the transponder. In this case the IC is normally a two terminal device where the terminals go out to a printed antenna with a cap & resistor to form a resonant circuit.

When RF is applied, by an interegating transmitter the tag rectifys the voltage produced in the formed resonant circuit & self-powers itself. It then backscatters it's ID to the interegator by shorting the antenna terminal out. If this is what you want then buy a tag, there is no need to buy the IC.

The interegating tranmitter normally has a transponder stage in it, or what is often called this because the section of the circuit is transmitting & receiving from the same antenna at the same time, a constant RF transmission is required else the tag will power down, yet at the same time the interegator must receive the data from the tag.

This is passive technology, it's a bit different with active. whether what you want to do is possible depends on the frequency band & technology you use, it will be a lot harder to get 100cm when a tag is implanted, antenna efficiency drops very low.

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