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Re: Sensitivity and noise floor (theory question)



At 10:40 PM 9/13/2003 -0500, DC wrote:
>This is a general radio theory question.
>
>My AO-40 downconverter has a NF of 1.7db.  What exactly does that mean?
>My 2m rig has a sensitivity of .18 micro Volts (12db SINAD).  How is 
>this sensitivy related to the noise floor of the reciever in the rig?  
>Can I calculate the noise floor?  Finally would I benefit from a 1.5 db 
>nf preamp in front of this reciever? 
>
>Thanks for your knowledge here.  Let me know if there are any good 
>sources to learn more about this...
>-David Carr
>KD5QGR

David you already had a couple detailed answers, so far.

In general the important characterisitics for receiving AO-40 on 2.4 GHz are:
1)  Antenna gain.  A min of 24 dBi is desired if you plan to operate SSB.
This implies a dish of 3-foot.  Smaller dishes can be used but you will not
hear the signals of many stations well enough for use of SSB.  CW can be
used with a 21 DBi dish (2-foot).  If you are using anything other than a
dish (say  helix or yagi) you will be disappointed.
2)  Receiver noise figure.  You should try for an overall NF < 1.0 dB.  The
first stage of your receiving system normally sets the lower level of NF
but succeeding stages add to it as well.  It is paramount that the first
stage be directly mounted at the dish feed.  Coax cable losses at 2.4 GHz
directly add to the NF in dB.  Some "BBQ" style dishes require using a
short "pigtail" of coax (that is OK but does effect your sensitivity).
Your 1.7 dB NF will not be significantly reduced with a 1.5 dB NF preamp.
I would try to get one of NF< 1.0 dB and gain >15 dB.

with these two requirements met you will hear AO-40 well.

3)  Your receiver sensitivity is fine if your preamp and convertor have
sufficiently low NF and adequate gain ahead of it.  Remember that FM is not
approved for use on AO-40 (only CW/SSB/SSTV...).
4)  Noise floor of AO-40 refers to the sensitivity of the receiver on-board
AO-40.  It outputs noise to the downlink transmitter when signals are below
this level (or there are no signals on that frequency).  You can see this
level if your downlink receiving system is good enough (you will see a
slight rise in your receiver noise when you swing your downlink antenna by
AO-40's position).  Achieving this level of senstivity is the goal for
ultimate AO-40 hearing.



73, Ed - AL7EB


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