[amsat-bb] Re: IC-821H Frontend Overload?
domenico.i8cvs at tin.it
Fri Mar 11 07:13:18 PST 2011
Hi Mark, N8MH
Our friend Paul K6HR is worry about to overload the receiver front end
using in front of it a preamplifier and as per his question he want to know
if in front of his receiver IC 821H a preamplifier SP432 with 24 dB gain
is better or not then a SP432 model with a gain of only 15 dB
In his question Paul is not considering the Noise Figure but only the gain
of a preamplifier and this is not enough for a correct analysis.
In addition our friend Paul K6HR is considering preamplifiers model
SP 432 so that he need a preamplifier for 70 cm and not for 2 meters.
This is why I first converted the receiver IC 821 H sensitivity
specifications into a Noise Figure value of 4 dB and then I have explained
to him that following my calculations a preamplifier SP432VDG with
NF= 0.55 dB and a Gain of 16 dB into the frequency range 420-450 MHz
is recommended to increase the overall sensitivity without to overload
The model for 70 cm SP432VDG needed by Paul K6HR has a Noise Figure
of 0.55 dB and a gain of 16 dB as specified in the bottom line here:
----- Original Message -----
From: "Mark L. Hammond" <marklhammond at gmail.com>
To: "i8cvs" <domenico.i8cvs at tin.it>; "Paul Delaney - K6HR"
<paul.hamradio at verizon.net>; "Amsat - BBs" <amsat-bb at amsat.org>
Sent: Friday, March 11, 2011 12:11 AM
Subject: Re: [amsat-bb] Re: IC-821H Frontend Overload?
> 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
> 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 :)
> 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
> >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
> >> _______________________________________________
> >> Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the
> >> author.
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