Transponder specs

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kujo806

Pre-takeoff checklist
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kujo806
As a RF engineer, I can't help my curiosity. After my plane's transponder went out, I was looking around at replacements and noticed that they are rated at 200W of transmit power. That seems ridiculously high for small aircraft. I had no idea they put out that much power. I am trying to figure out why they need such a powerful transmitter. The com radios are an order of magnitude lower than that TX power. I would think that at altitude and 1GHz, you would get good range at much less power. Now I know what instruments draw all that battery current!
 
Very short duration however. The spec is it that a Mode 3A/C has to put out 125W, I presume they juice it up by a bit to counter feed line/antenna issues. Mode S is less, 70W.
 
I believe the higher W output is required for flights in the flight levels. The breakpoint on altitude between 125W and 200W requirements may be as low as 15,000msl
 
Although the peak power of 200w is relatively high the average power is about 1 watt. Here are some of the reasons for the high peak power.

1. At 1090 MHz the path loss is 20 db greater than at 120 MHz.

2. The narrow pulse width of 0.45us is 30 dB less power density than audio.

3. At such low duty cycles the total power/sec transmitted is less than a 1watt

The average VHF COMM sensitivity is -110dBm while for L band transponder receivers is about -92dBm. The lower sensitivity is due to the bandwidth of 6MHz required. This bandwidth requirement is in part due to the frequency drift with old klystron cavity tube used in old transponders.

Old magnetron type radars at 10GHz used to run at 10KW. New radars run at 100 watts due to the narrow band receivers and solid state transmitters used.

Titanic CW wireless transmitter was rated at 5KW. Today HF SSB equipment at 100 watts have greater range.

Titanic wireless: http://marconigraph.com/titanic/wireless/mgy_wireless.html

RF is indeed an interesting field

José
 
BillTIZ said:
I believe the higher W output is required for flights in the flight levels. The breakpoint on altitude between 125W and 200W requirements may be as low as 15,000msl
Click to expand...

Nope, the spec is 125W everywhere.
 
The output power has to be high *because* the pulse is so short. The receiver doesn't have a long interval to integrate over.

As a backburner project I'm working on a DIY transponder. Sourcing transistors for the output stage is rather annoying.
 
Thanks a lot. This is great information. Where can I find a white paper or more detailed info on the pulse timing, modulation, etc?
 
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