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Re: AO-40 spreadsheet




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

A 24dB linear BBQ should be equivalent to 21dBic.

>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 test sheet #'s are only valid for the original 2.5 GHz band, and even 
then seem optimistic.  I modified mine using the IF mods, and extended the 
front end filter strips- and got to 6.0 dBNF/23 dB Gain measured at the Weak 
Signal Banquet at Dayton.  This is what the mods predicted exactly according 
to the web site I got them from.
If you only changed the xtal and did the IF mods you are looking at more 
like 8 dB NF and less gain.

>
>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
-- there is a fairly significant difference between these two amps--
>get very little change in the visual picture of what signals I can >hear.

Run the spreadsheet for a more typical # for the DEM preamp (W1BFN's 
measured at Dayton) -- 0.58 dBNF/16.95 dB Gain into the 1.5 dB Conifer, and 
then into your 8dbNF/25 dB Gain Drake-- you'll then see what we mean.

There isn't much difference between a 0.7 dBNF amp and a 1.5 dBNF amp with 
low gain when driving into the 8dBNF Drake-- yup-- that is because there 
isn't enough "low-noise gain" in front of the noisy Drake.

>(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..
>
Nope, you're not getting it.  The gain itself is meaningless-- the gain at a 
low noise figure early in the receive chain is what is important.  
(subsequent noisy stages exponentially contribute less and less to the 
system NF)

Here is a simple experiment-- pretend your bloody rich and bought a SSB 
downconverter-- put in 0.8 db and 30 dB of gain into your 10 dB NF rig on 
the spreadsheet.  Compare the MDS (minimum discernable signal) compared to 
your present system.  I'd guess you'd pick up 5-7 dB..

It should make sense.

Yes, antenna gain is good-- but the low noise front end is just as good..

Fred W0FMS

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