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

[i8cvs]

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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