[Microwave] Silly Rig Modification and Noise Temperature Ramble/Question

[Scott Townley]

My apologies up front, as this rambles a bit:

I have been looking at improvements/modifications to my FT-726 for AO-40
and MS work.  The two main published mods appear to be a front-end DGFET
replacement, and an IF amp boost.

As a starting point, I have measured my RX sensitivity (on 2m) as follows:
any comments as to why this is right/wrong/different would be appreciated.
Setup is siggen->HP VHF variable attenuator->rig->laptop.

I used the fixed audio output of the FT-726 to feed my laptop soundcard
running a program called Analyzer 2000 (ver. 5).  There are several slick
audio analysis programs available, but this is the only one I have seen
that measures SINAD directly (and you're not even restricted to a 1kHz
tone!).  
I measured 12dB SINAD at -124dBm input...so 0dB SINAD (or signal=total
noise power) at -124-12=-136dBm.
**NOTE 1** AGC cannot be defeated in FT-726R, but it takes about -100dBm
for any S-meter deflection, so it would appear that AGC is effectively
disabled at these signal levels
**NOTE 2** and to prove that to myself, a +/-3dB change in attenuator value
changed measured SINAD by +/-3dB, showing linearity of response.

So, total noise power here is going to be 290K from the siggen plus the
"unknown" input equivalent noise power of the rig-under-test (RUT?).  
-136dBm=Ttotal=827K (2.2kHz noise bandwidth assumed)
Ttotal=Tsiggen+Trut so Trut=827-290=537K (4.55dB NF)

This seems to be pretty good performance...

Now I have to wonder to myself, why modify?  I always use a mast-mount
preamp or some other low-noise gain element (e.g., downconverter for AO-40)
at the antenna, so with that noise temperture, even a mere 17dB of gain
knocks the rig noise contribution down to not quite 11K...for satellite
that's a lot less than you'll get from the downconverter and antenna, and
for MS as well (right?) so it seems pointless to mod the rig at all.

So...what have I screwed up here?

Thanks for your expertise!

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Scott Townley		
nx7u@arrl.net
Gilbert, AZ  DM43di
     
HF/VHF/UHF/Satellite MOBILE station
--> yet NEWER (August 2002) address http://members.cox.net/nx7u

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