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Re: P3D information



>Secondly, yeah, DSS dishes are small.  But they are DISHES and at 12 GHz,
>there is a LOT of gain in those things.  They are VERY directional antennas
>which means they have a LOT of gain.  If you are off a couple degrees with
>them you lose signal.  You can pack more oomph with a small microwave
>station simply because your wavelengths are smaller and therefore geometries
>of resonant structures are smaller.

Thank you for pretty much making my case for microwaves for me. I
don't know if you have a DSS dish or how long it took you to install
it, but despite its relatively narrow beamwidth it only took me a few
minutes with a wrench to align it.  I haven't touched it in 5 years.

Sure, amateur satellites like P3D move. But I'm sure the same people
who have devised all the rotor mounts, cabling schemes and computer
tracking systems for the large Oscar-class arrays now in use are
clever enough to do the same for much smaller antennas where the
reduced wind loading, weight torques and the like will more than
outweigh the tighter pointing accuracies required.

>But it's harder to generate power up there and more expensive.  Most guys

That's where the digital part comes in. I point out that analog TV
satellites also operate on the microwave bands, but the dishes used
with them are considerably larger than those used for the digital
satellites.

Some of the reduction in required dish size for DSS comes from the DSS
satellites having higher transponder powers, 240W vs 20-120W for
conventional Ku-band satellites in the fixed satellite service. But
the lion's share of the gain comes from using efficient digital
encoding, compression, error correction and modulation schemes.

By my reckoning the power gain from digital TV is about 20 dB. That
assumes digital video encoded at 2 megabits/sec and a FEC digital
modem that requires an Eb/No of 4 dB to operate, for a required C/No
of 67 dB. Analog FM operating at a 12 dB C/N ratio in a 36 MHz
transponder bandwidth requires 87.6 dB of C/No.

20 dB of either antenna gain or RF power is a big difference, wouldn't
you say?

Of course, the numbers are a little different for voice, especially
since the analog mode is AM/SSB, not FM. Speech encoders operating at
4800 bps have been available for some time, and in my opinion they
give much better voice quality than SSB even at high SNR. 4800 bps
operating over our same 4 dB Eb/No modem requires a C/No of 40.8 dB.

A C/No of 40.8 dB used with SSB in a 2400 Hz bandwidth gives an
average SNR of only 7 dB, which I don't think is very good. Increasing
that audio SNR to anything approaching telephone toll quality (20-30
dB) will take proportionately more RF power, while we're already
almost there with the digital voice.

And then we have the fact that SSB requires a completely linear
transmitter chain, while the digital mode can operate through a class
C amplifier if a constant envelope modulation scheme is chosen.

So if minimizing the cost of generating RF power (on any band) is your
goal, it sure seems like digital is the way to go.

Phil

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