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Re: AO-40 spreadsheet



Fred, W0FMS and Hasan, NØAN have done a fine job responding to Greg Dolkas'
perceptions on this topic.

Understanding how NF affects receiving capabilities above VHF frequencies
is a somewhat mysterious subject for many.  The significance of obtaining a
low system NF becomes much greater as you move above 1 GHz.  At lower
frequencies high sky temperatures limit what a receiver can detect [MDS].
Beginning around 100 MHz sky temps drop to <250K making low NF important
for weak signal detection.  Above 1 GHz sky temp drops below 20K.  The use
of an antenna mounted low noise preamp is standard practise at 2m for
operating on AO-10/13.  It is also the case for moonbounce at 144 MHz+.  I
wonder why it was considered not "required" for microwave satellite
operation by people getting on 2.4 GHz.

Well, Gene Marcus' spreadsheet has made understanding the factors for good
reception much easier to get a feel for.  My recommendation for Greg and
others that are trying to get a handle on how it all fits together, is to
"play" with the numbers until you see how ant gain, ant temp, preamp gain,
preamp NF, cable losses, following stage NF, etc. play in the total sytem
performance {MDS, NF}.  To really see how each parameter affects things,
set all the  parameters to a particular system setup, then just change the
value of that ONE parameter and watch the effect.

For example enter the numbers for a Drake [or TranSys] with no preamp.  Try
changing the NF of the Drake.
reset the Drake NF to stock.  Change its gain.  Reset the Drake gain and NF
to stock, and add a preamp.
vary the preamp NF.  Reset the preamp NF to your begining value, now vary
its gain.  I think you'll begin to see what others have been pointing out:
"a good receiving system is made up of the combination of the factors...Not
just one or two".

My eventual system for 2.4G satellite operation:
Ant gain ~25 dBic [33  inch offset feed dish, helix feed]
no cable loss to the preamp [~0.1 dB]
Preamp 0.7 dBNF, 35 dB gain [MKU-232A]
cable loss to converter 3 dB
Drake 6 dBNF, ~15 dB gain [If coils/caps removed, only]
cable loss to receiver 1 dB [35-foot]
Receiver 14 dBNF [FT-847]
Receiver BW 2200 Hz

This setup will have a system NF ~0.8 dB [59K] and a MDS -147 dBm and
should produce a S/N of 28 dB on the S2 beacon with zero squint.  This
should just about get me down to the transponder noise floor? ...I hope :-)

Ed

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