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LEO station design and noise figures



I'm designing a LEO antenna system and would like some advice on the question of
system noise figure, particularly how important it is to gain small changes in
this figure.

This is what I have so far: an Orbit360 rotator which will turn a 4 element
'cheap yagis' 145 MHz beam and a perhaps 8 element 435 beam. I have bought
unused Andrew flex heliax for both. Since the runs are on the order of only 35',
I should clock in at .7 dB loss for both sides (I'm using 1/4" for the 2m run
and 1/2" for the 70cm one). The 70cm system comprises a Microwave Modules
transverter and a HF rig. 2m will be serviced by a FT-817 and an amp which puts
out 20w. 

I know the next step is to add preamps. For 2m I'm buying the Ramsey kit with an
rf relay. This is spec'd at 1 dB noise figure and I think about 17 dB gain.
Several sources on the web suggest that the FT-817 has a miserable 6-8 dB noise
figure in its first stage. Now it would be really nice, of course, if I could
install the preamp in the amplifier brick: reduced cost of connectors, avoiding
the jumpers up at the beam, etc. When I run the numbers using a spreadsheet that
performs the 3 stage calculations in fig. 11.3 of _The Radio Amateur's Satellite
Handbook_, I get a system noise figure of 1.43 dB for coax -> preamp ->
tranceiver and 1.36 dB for preamp -> coax -> tranceiver. A change of the same
size is calculated with a Hamtronics preamp in front of my MM transverter, which
is said to have a NF of about 3 dB.

The 2m receiver is for VO-52 and the future satellites that plan to use mode
U/V, such as KiwiSAT, Delfi-C3 and SATEDU.

I'm having trouble understanding what this system noise figure relates to. Is it
 above the typical noise at earth? Should I care about this figure change more
on 70cm than 144? It seems to me that the low price of high-quality coax is
possibly changing the situation for stations like mine, and I'd be grateful for
any advice.

73, Bruce VE9QRP
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