Re: [amsat-bb] AO-40 spreadsheet

[Greg Dolkas <greg@xxxxxxxxxxxxxxxx>]

Hello Hasan,

> Greg, please take this in the spirit it is offered. It's not personal, but
> technical.
> 

No problem - I appreciate the information!

> I think you may be misapplying, if not misinterpreting, the 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.

I have no idea what mine is rated at; for the exercise, I used 20dbi.

> 
> 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. The noise figure
> following the preamp, which is a composite of coax loss and the noise figure
> of the radio,  is divided by the GAIN OF THE preceding  PREAMP...if that
> value isn't high enough, you won't "overcome" the poor noise figure of the
> radio and its preceding coax out to the preamp/converter.

ok, so I start with 20dbi for the antenna, and 3db/16db gain for the
Drake.  The test sheet that came with it showed these numbers for the
unmodified unit.  I modified it by changing the crystal for a 2100mhz
LO which puts AO-40 in the middle of its unmodified IF band.  I hope
the NF and gain of the modified unit will be similar to original.

The Conifer preamp is rated at 1.5db noise, and 10db gain.  If I
try an experiment of "buying" a DEM preamp (.7db noise 16db gain), I
get very little change in the visual picture of what signals I can hear.
(I could have the 1.5db number wrong - it might be as good as 1.0, but
that doesn't seem to affect the picture much.)

If instead I apply the 6db gain into the antenna, I get a big change.
That is how I come to the conclusion.  Of course, 6db in antenna gain
is harder to get than 6db in preamp gain..

> 

{snip}

> Here's the sweet, and readily attainable spot, Greg:
>
>
> Antenna > 21 dBi Linear....BBQ antennas with this performance are not
> absurdly expensive. 

Already got it.

>
> Coax Loss From Antenna to Preamp:  no more than .5 db, hopefully just couple
> tenths of a dB.

About 3' of RG-8.  Could probably improve this.  Would there be a problem
mounting the Preamp between the dipole feed and the grill?  The feed is
sealed to the coax.

>
> Preamp/Downconverter: NF < 1 dB, Gain 30 dB if a downconverter.15 dB if a
> preamp  feeding a downconverter. Preamp cost: 150 bucks. NF: .6 dB, Gain >
> 20 dB
>
> In other words, if you are using just a downconverter, the NF should be less
> than 1 dB and it's gain should be 25 to 30 dB.
>
> If you are using a preamp before the downconverter, then the preamp should
> have a NF less than 1 dB and a gain of at least 15 dB. The downconverter
> will have a NF of 3 to 9 dB if it's a surplus unit, and a gain of maybe 15
> more dB.

Ok, so the preamp I have is 1.5db gain of 10, and the Drake is 3, gain of
16 (I think these are the right numbers).  So, I'm close, but the preamp
is a little light.  DEM's unit would seem about right, but the difference
isn't going to be huge, right?

I guess the frustrating part of all of this is that I don't see anything
that I can do to make a major difference in my reception.  But, the good
news is that there are a couple of small things the I can do which will
help.  The system is basically working, but I'm no where near the "very
happy" state.  On a good pass I can hear myself just above the noise,
and "meet LEILA" if I try to make it better.  Nothing like the booming
S5+ signals others report.

Thanks again for your help,

Greg.
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