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Re: AO-40 spreadsheet






>1. Antenna: yes, very important, but there are only so many things >you can 
>do. 21 and 24 dBi BBQ style parabolic section dishes are >pretty common.

Bigger is better until you can't point accurately enough anymore.

>
>2. MOST CRITICAL: Noise Figure of the Preamp (with very low loss in >the 
>coax between the antenna and the preamp). Gain of preamp IS >important. 
>Without the gain of the preamp, you don't set the noise >figure out at the 
>preamp properly.  10 or 12 dB gain , just isn't >good enough.

...snip..

My comments about the SSB electronics unit having probably two LNA's in them 
is that the 0.8 dBNF system and 30 dB gain are consistent with a low NF like 
this.  The Transystems, Drakes (with external preamp), etc. have around 1.5 
dBNF system due to the fact that they only have ONE low noise stage, and a 
higher NF stage right after it.

What is the difference between 0.8 dBNF system and 1.5 dBNF system?  
According to the spreadsheet, a minimum discernable signal difference of 3 
dB.  3dB is again equal to an area twice the size on a dish (1/4 
radii/diameter increase)-- so both factors are important.  The larger dish 
also will make the beamwidth narrower..

Anyway-- most PHEMTS are from 12-16 dB of gain at minimum NF at 2.4 GHz-- 
not enough unless a low NF stage (<3 dB or so) follows.  That is a good 
motivation for lowering the NF in the Drakes, maybe figuring out how to put 
another LNA in the Transystems, etc.

This the reason why the SSB units seem to hear better-- they do-- but we can 
improve our more reasonably priced Transystems and Drakes by adding more low 
noise gain up front.

Fred W0FMS

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