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[ale] Wandering OT: Re: Car PC's and internet radio?



Sean -

I'm going to cede to Dale's point w.r.t. whether or not the satellite
radios actually transmit; like I'd said, I'd want to look inside one to
be able to tell for sure - apparently he has done just that.
>
> Jeff,
> I am not a ham or any other sort of "expert" in radio. But I do know a
> little bit about the subject.
>   
Likewise; EE degree (fat lot of good, that).
> Consider your cell phone and its battery. It can transmit a high frequency
> signal only a few miles at most to reach the nearest cell tower.  If you 
> wanted to use your cell phone on a boat cruising along the coast, you would
> need a signal amplifier and a much better and taller antenna.  West Marine
> probably can sell you the kit, but it aint cheap.
> For a satellite radio receiver to also transmit a signal back to the bird
> 22k miles above you would need a lot more transmit power than your cell
> phone has and a much better antenna, which would (for your own safety) have
> to be mounted _outside_ the car.
>   
Last year, a German team of hams were able to obtain data from /Voyager
I/'s 20-watt transmitter at a range of 98AU using a 20-meter dish.  The
point of my saying this is that much has to do with what it is you're
trying to transmit.  The /Voyager/ data rate is going to be dog-slow
(1.4Kbit/s), and I expect that they're using something akin to Barker
code compression to greatly stretch the decode threshold.  As I've said,
a hypothetical XM uplink would be very little data - maybe 1K or less -
that could be transmitted in maybe one second (I don't know how long it
takes the radios to actually switch channels once you've pushed the
buttons or whatever). 

Moving the range from geosync distance to /Voyager I/ distance results
in a 112dB drop in received power.  So, if a 20-meter dish can get
~1Kb/s data from a 20-watt transmitter at 98AU, can I lower the gain of
the transmit/receive antenna system  (from ~4m dish and 20m dish to
near-omni and [rectally-extracted number] 1m dish) and lower the power
of the transmitter from 20W to, say 20mW (30dB difference) and still not
have soaked up the difference?

In other words, given the German team's accomplishment, it seems that I
ought to be able to do the same thing (~1Kb/s data rate) at about
1/420,000th the distance (i.e., geosync) with 1/1000th the transmit
power and about 1/160,000,000th the combined antenna system gain. 
Somehow I think that I'm within the realm of handheld battery-powered
devices there. 

So, satellite radios may not be transmitting *now*, as Dale said, but it
seems a lot more plausible after having run some numbers.