[amsat-bb] Re: Turn off AGC when receiving BPSK-1000
karn at philkarn.net
Thu Aug 18 20:30:39 PDT 2011
On 8/18/11 5:47 AM, Bob Bruninga wrote:
> The problem with LEO satellites is that a nadir facing antenna does give
> great gain directly overhead ground stations, but only for about the center
> 2 minutes of only the one direct overhead pass a day. The problem with
> facing antennas "down" on a LEO satellite is that 90% of the users are not
> "under it", but to the side of it.
You don't have to use a narrow beam antenna with maximum gain on axis.
You can always design it with a bowl-shaped pattern that increases gain
toward the edges and lowers it in the middle.
The GPS L-band transmitting antennas are a good example; even at their
much higher altitudes, the earth appears fairly large and a simple lobe
would provide uneven illumination across the earth.
A stabilized satellite pointing an antenna at a fixed angle to the local
vertical would eliminate the deep, slow, periodic fading that we see at
present due to constant changes in polarization and/or the antenna lobes
sweeping past earth. Without these fades, coherent demodulation would be
practical, picking up several dB of performance even before you consider
the considerable increase in average signal strength. On a microwave
band with sufficient bandwidth, efficient coding would make possible
much higher user data rates and/or operate with considerably less DC
power. Data rates of several megabits/sec would be entirely practical
from a small satellite to a small dish on the ground. That means HDTV in
near real time, or a steady stream of high resolution still photographs.
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