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ADS-B Transponders are usually rated with 150W or 250W transmission power, however the electrical input at 14V is rated much lower, usually around 6 Watt. How can that be?
150 W transmission power with an efficiency of 70% would result in > 200 W input ~ 15 Ampere at 14V. Even if its not CW, there would still be a significant current peak.
The rated TX power is the maximum power emitted while transmitting. The electrical input power is much lower, because the design of the transponder is such that it accumulates energy from the input at relative lower power, to burst that out in one go on the output at high power.
The transmission of a ADS-B extended squitter has a duration of 120 microseconds. Since the modulation is pulse-position modulation, the transmitter actually only transmits for 58 microseconds per message. If the TX power is 250W, a single ADS-B message costs about 0.015 J (assuming 100% efficiency). The maximum squitter rate for ADS-B is 6.2 messages per second, equating to an average of approximately 0.1 Watt of transmitted energy. Other systems hosted by the transponder (Mode S and TCAS) are able to produce easily ten times more messages each. Mode A/C uses much fewer pulses but can have very high reply rates. This brings the average total transmitted power to over 2 W for congested airspace.
Internally in the transponder you will find big capacitor(s) that get charged by the aircraft's electical system, and discharged by the transmitter. They act as a buffer, preventing peak power surges in the aircraft system.
Transponders have a low duty cycle. It is limited to 4.5% but that typically only occurs in high interrogation airspace (an area with a large number of secondary surveillance radars and TCAS equipped aircraft.) A typical duty cycle is in the range of 1% to 2%.
ADS-B Mode S extended squitters are 120 usec long and consist of 120 half usec long pulses. So that's equivalent to 0.006% duty cycle for one reply per second. Between interrogation replies (which are about half the length of ADS-B outputs) and ADS-B squitters you're looking at typically 200 to 250 transmissions per sec resulting in a bit over 1% duty cycle.
Transponders typically use capacitors to store up power and a switching power supply to drive the transmitter. Even with an efficiency of 50% (not uncommon in many RF transmitters) you only need an average input of 3 Watts transmit 150W 1% of the time.. At the limit of 4% duty cycle, the power needed would be 12W (but only for a few seconds as the maximum duty cycle only has to be sustained for a few seconds.)
So, yes it is possible to have a transponder only need an average input power of 12W.
