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 his receiver. 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: http://www.advancedreceiver.com/page7.html 73" de i8CVS Domenico ----- Original Message ----- From: "Mark L. Hammond" <marklhamm...@gmail.com> To: "i8cvs" <domenico.i8...@tin.it>; "Paul Delaney - K6HR" <paul.hamra...@verizon.net>; "Amsat - BBs" <amsat-bb@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 > 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.hamra...@verizon.net> > >To: <amsat-bb@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.hamra...@verizon.net > >> http://k6hr.dyndns.org:8080 > >> _______________________________________________ > >> Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the > >> author. > >> Not an AMSAT-NA member? Join now to support the amateur satellite > >> program! > >> Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
