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Re: RE:Re: AIDC Downconverter Survey Results



I think a few numbers need correcting.

> 1.) There is a FIXED dynamic range to the satellite.  From the beacon to
the
> noise floor this is about 20 to 23 dB or so.

>From the MB to the noise floor is 33 db per the articles published by the
designers and measurements by several stations using various methods.

> 2.) The proscribed maximum power level is 10 dB below the beacon.

True.  Which allows potentially 23 db of dynamic range for QSOs.  However,
having the combined noise output of three receivers feeding the downlink
transmitter reduces that figure to near 18 db.

> 3.) In order to sufficiently copy an SSB signal, you really need a signal
> that has a 6 to 10 dB SNR.
> 4.) Given, 3, sufficient signals are then in the range of about 10 to 15
dB
> below the beacon!  WOW.

6-10 db above the noise floor would be 18-22 db below the beacon so a
station which hears the MB at 22 db won't hear these stations.

> Imagine that.  Obviously, there was a reason that
> the designers and control ops picked this point.  Does everyone think that
> the 10 dB point was picked arbitrarily?

Actually the original plan was that we would be operating using mostly the
S1 transmitter and the General Beacon. As with AO-10 and 13 before, the
accepted practice would have been to not be any stronger than the GB.
Unfortunately the GB is not available to the S2 tx so we are forced to use
the MB which transmits at a level 10db higher than the GB.

> There is a fallacy out there that the BBQ dish guys are the ones who are
> running the excessive power and clobbering the S2 ALC.  THIS IS JUST NOT
> TRUE.  The LOUD guys are all the ones who have the EME setups that use L
> band.  It's pretty damn near impossible to run excessively huge U band
> signals because of Leila.

True that the S2 ALC is primarily impacted by the L-band stations that are
mostly unrestrained by AGC.  But since LEILA can only get a certain number
of offenders at a time, and worse largely ignores some formats like CW and
SSTV, over powered U-band stations do have a great impact on the uplink
receiver sensitivity by driving the U-rx AGC up.

Consider the stations whose noise floor is only 20 db below the beacon.
They could be having a nice 10 db SNR QSO and also be operating correctly
10db below the MB.  Then suddenly someone starts up with some high powered
SSTV driving the AGC another 6db higher.  Now the two stations can barely
copy each other and are forced to increase their power to complete the QSO
so that even when the SSTV station stops the AGC stays up due to all the
other stations that increased their power.

Next consider two stations who can hear the fulll dynamic range of the
transponder.  At 10 db below the MB they have 18 db of SNR.  The same SSTV
station fires up and their SNR drops to 12 db and while they probably notice
the "fading", there's much less temptation to immeadiately reach for the
power control.  As a result once the single offending station stops, the AGC
can go back down.

> I have a BBQ dish and I have asked people to turn their power DOWN because
I
> could hear them plenty well.  I typically run about 150 to 250 W EIRP into
> the bird AND I HAVE NO PROBLEM HEARING MYSELF!  Stop saying that the guys
> with the BBQs are causing the problems.  IT JUST ISN'T TRUE!

I'm not saying it happens EVERYTIME, or that anyone shouldn't operate just
because they don't have an ideal set up.  I started with a 60cm dish and a
barefoot Drake myself but adding a Demi preamp and then moving to a 1m dish
has made an astounding difference.  I do think that people should be aware
of the limitations of their system and make allowances for it such as being
patient for a bit before firing up the power amps.

73,
Lee-KU4OS

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