Let's say you have a satellite communications system like XM. You've got an antenna that grabs RF energy radiated by a satellite, a coaxial cable carrying that signal from the antenna to a receiver, and the receiver itself with amplification and decoding and all that jazz. How much electrical energy does the antenna element grab and transmit into the receiver radio? I know it's a tiny amount; I would like to be able to compare it to amperages for other things so that I can describe how important it is to maintain signal quality and reduce any issues with the signal path. RF is not an area where I feel very strong as I never really dealt with it in my career thus far.
Are you looking for a specific number? Or just a ballpark?
It has a lot to do with broadcast power, antenna shape, distance, etc.
https://en.wikipedia.org/wiki/Free-space_path_loss
Ballpark is fine. XM was discussed recently on the job but other systems may be of interest later so a general idea is fine.
That is an interesting question. I can't help you on the receiver end, but it looks like the audio band transmitter on the satellite puts out about 75 watts. That's 36,000km away and covers a pretty broad area, so you can imagine the actual received amount is tiny. That compares to a terrestrial radio station which for a big one is in the 100,000+ watt range.
Quick napkin math on the XM #s above resulted in (1/12.5*10E18)*75watts
Someone double check that :P
Edit: I did not account for directivity, and surely the XM satellite is directional. No point in pointing it at anything but earth.
As a back of the napkin calc, about 30-36dB gain is all you're ever going to get out of any antenna, dish or otherwise. So, say 32 dB at the satellite max, and (roughly) every 3 dB is doubling effective power. So, 11 doubles on 75 Watts at the transmitter. And that's if they deployed a dish, which I don't think satellites use, but I'm no satellite expert. Hey, the Death Star had a dish. Maybe they used the same one?
