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

[Mark L. Hammond]

I don't think this statement is correct:
"Since the NF of a  AR SP432VDA is 0.55 dB and the gain is 15 dB if the coax..."

The 2meter VDA has a noise figure great than 1, stated to be about 1.1 dB NF; the difference is more than just gain.

Thus the reason I suggest that he get the 2 meter VDG model with 0.55 dB NF  :) 

http://www.advancedreceiver.com/page7.html



Mark N8MH 

At 07:01 PM 3/10/2011 +0100, i8cvs 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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>
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