Transponder - Bandwidth

[pavannanduri]

Even though this question sounds simple. this question is bugging me for quite some time.

why should a transponder have only some tens of MHz (like 36, 27 etc)

what makes it difficult to employed wide band transponders?

we often do employ wideband receivers inside the transponder channel but we never restrict ourselves to a single wideband transponder? why is it so?

and lastly what the name transponder for a mere down converter (with amplifiers) [ i mean a simple basic transponder] ?

 

[rreddy]

Hi, I dont know all that much about transponders. I would guess that it is better for a transponder to require less bandwidth to perform its receive/transmit. Hence for a given frequency sprectrum, there may be a greater number of transponder channels present.

In general, designing systems that need to operate over a wider bandwidth is more difficult. The design now needs to work accross a greater range of frequencues whereas with a narrower bandwidth, the specifications need to be met accross a more limited bandwidth.

 

[jiripolivka]

Hi, I dont know all that much about transponders. I would guess that it is better for a transponder to require less bandwidth to perform its receive/transmit. Hence for a given frequency sprectrum, there may be a greater number of transponder channels present.

In general, designing systems that need to operate over a wider bandwidth is more difficult. The design now needs to work accross a greater range of frequencues whereas with a narrower bandwidth, the specifications need to be met accross a more limited bandwidth.

You should first study a textbook on satellite communication to get the basic ideas on how transponders are designed, and why .

Historically, first there were "bent-pipe" transponders, with one 27-30 MHz bandwidth to transmit by frequency modulation. as amplifiers, TWTs were used, with a high gain but a limited power output. To reduce intermodulation and distortion mostly if multichannel telephony was transmitted, the output power had to be reduced by "back-off".

Later, multi-channel transponders were designed, also for FM, and using switches, transponder life was extended by switching to spare TWTs after some died.

Latest, instead of "bent-pipe", signal processing transponders are used, with QPSK and digital multiplexes, spot-beams, etc. Again the new transponders often do not cover a huge territory with a single antenna but they can move the spot-beams rapidly, so certain signal blocks are sent to a defined location, other blocks somewhere else.

Satellite communication is not so easy a "radio" but a very complex system, gradually growing in complexity.

---------- Post added at 18:41 ---------- Previous post was at 18:36 ----------

To add the explanation to what is a "transponder": In satellite communication, you send a limited RF power over 35800 km to and from a geostationary orbit. Uplink, say at 6 GHz, sends up to several kW to a satellite but there it is received at ~ -90 dBm. You cannot transmit back the downlink on the same frequency; this is why here 4 GHz band is used, to prevent oscillation and crosstalk. Using so different receive and transmit frequencies allows to use good filters for isolation. The satellite transmitter transmits back at 4 GHz with 20-40 W, or, +33...+43 dBm, so isolation must be > 130 dB. Similarly at 14/11 and 18/12 GHz, etc.
The device aboard a satellite transponds the spectrum from 6 GHz band to 4 GHz band, so it is named a transponder.