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" marklhammond@gmail.com To: "i8cvs" domenico.i8cvs@tin.it; "Paul Delaney - K6HR" paul.hamradio@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 20NF = 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@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.hamradio@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