[amsat-bb] Re: IC-821H Frontend Overload?

[Andrew Rich]

Very good explanation 

Sent from my iPhone
Andrew Rich

On 11/03/2011, at 4:01, "i8cvs" <domenico.i8cvs at tin.it> wrote:

> Hi Paul, K6HR
> 
> The SSB and CW sensitivity of an IC 821H in 70 cm is less than 0.11uV
> for a Signal to Noise ratio S/N= 10 dB
> 
> Converting the above data into Noise Figure NF in dB we get:
>                                                 -6  2
>                                 ( Vin x 10    )   x 20
> NF    =  10 log     [  ------------------------ ] + 174
>    dB               10            BW x S/N
> 
> Where:
> 
> Vin = input signal = 0.11 uV (microvolt)
> BW = Band Width for SSB = 2400 Hz
> S/N = Signal to Noise ratio = 10 dB i.e. 10 time in power ratio
> 
>                                                     -6   2
>                                   ( 0.11 x 10     )    x 20
> NF    =  10 log       [  ---------------------------  ] + 174  =  4 dB
>    dB                10                2400 x 10
> 
> Showing a NF= 4 dB the IC 821 H seems to be a very good sensitive receiver
> but if you want to improve the overall Noise Figure using an antenna mounted
> preamplifier the Noise Figure of it must be less than 1 dB and infact the
> Noise Figure of a AR SP432VDG is NF= 0.55 dB and the only change between
> two models is only the gain because model VDG has a gain of 24 dB while the
> model VDA has a gain of 15 dB
> 
> The important thing here adding an antenna mounted preamplifier to a
> receiver is not to overload the receiver with a very high noise coming from
> the preamplifier gain.
> 
> Addind RF stages in cascade the rule of thumb is the following one:
> 
> "When adding a preamplifier if the noise increases by 10 dB than the overall
> noise of the receiving system is not deteriorated "
> 
> Since the NF of a  AR SP432VDA is 0.55 dB and the gain is 15 dB if the coax
> cable between the output of your preamplifier and receiver input is not too
> long and lossy then you will not overload the IC 821 H because the S meter
> reading with the preamplifier ON will be only about two S units of more
> noise but the overall Noise Figure of your receiver system will be in the
> range between 0.55 dB and 1 dB so that you will get a consistent improvement
> in sensitivity.
> 
> About the above matter also please read the following letters :
> 
> http://www.db0anf.de/app/bbs/messages/show-AMSATBB4673
> 
> 73" de
> 
> i8CVS Domenico
> 
> ----- Original Message -----
> From: "Paul Delaney - K6HR" <paul.hamradio at verizon.net>
> To: <amsat-bb at amsat.org>
> Sent: Wednesday, March 09, 2011 7:11 PM
> Subject: [amsat-bb] IC-821H Frontend Overload?
> 
> 
>> I'm setting up for satellite operation and considering the AR2 SP432VDG
> 24db preamp. A friend suggests this will likely overload the frontend of the
> radio (an 821H) The antennas are M2 eggbeaters. Would the SP432VDA 15db be a
> better choice?
>> 
>> Can anyone using these preamps give me some feedback on the 15db vs. 24db
> preamps and which might be the best way to go considering the radio and
> antennas I have?
>> 
>> Paul Delaney
>> paul.hamradio at verizon.net
>> http://k6hr.dyndns.org:8080
>> _______________________________________________
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