> The biggest load in a communications satellite.... > is almost always the downlink RF power amplifier(s)...
True for some, not true for others. The locations of ham operators are only about 10% of the earths surface. 90% of transmitter power can be saved if the transmitter is not left on 100% of the time. An APRS satellite takes advantage of this by knowing not only are there users only 10% of the time, but even over a saturated ham area, the TX/RX duty cycle cannot exceed 40% or so. The result is that the transmitter is only on for 4% of the orbit. This means we can run a 5W transmitter to make the packets hearable on an HT with a stock antenna, but that the transmitter AVERAGE power is only 200 mW (less power than the receiver). And by putting the downlink on 2m instead of UHF, there is another 9 dB advantage to OMNI anennas. Add that to the 10 dB advantage of running a 5W transmitter instead of 0.5W and you can see that an APRS cubesat can have almost 19 dB downlink advantage over the typical UHF 0.5W downlink. (theoretically)... Many other factors of course are involved, but we need a mix of satellites and modes, but the above is why I like AX.25 for short bursty texting and data for a large number of people to share in a short period of time. Bob, WB4APR _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
