[amsat-bb] Re: AMSAT Broadband Preamp Specs

i8cvs domenico.i8cvs at tin.it
Thu Oct 3 20:06:46 PDT 2013

----- Original Message -----
From: "Paul Stoetzer" <n8hm at arrl.net>
Cc: "Alan" <wa4sca at gmail.com>; <amsat-bb at amsat.org>
Sent: Thursday, October 03, 2013 6:02 PM
Subject: [amsat-bb] Re: AMSAT Broadband Preamp Specs

> Alan,
> Thanks for the "real world" example. My receiver is an Icom IC-R10,
> which the specifications say has a 0.25 uV sensitivity on 144 MHz and
> 0.32 uV on 435. I operate portable, so I'm not using the preamp for the
> high gain to overcome cable loss, but to overcome the relatively high
> noise figure of the receiver. The preamp works great for FO-29. When I
> have a clear horizon, I can hear it down to 1 degree or so with an Arrow
> antenna. My problem is the horizon (and a military spread spectrum
> radiolocation system that causes "clicking" noises all over the 70cm
> band here in Washington).
> 73,
> Paul, N8HM

Hi Paul, N8HM

Just for your information the sensitivity of the ICOM IC-R10 in CW 
and SSB is :

144 MHz: 0.25 uV for a S/N ratio of 10 dB
435 MHz: 0.32 uV for a S/N ratio of 10 dB

To convert the above sensitivity in Noise Figure (NF) you can use the
folloving formula:
                                              -6   2
                                ( Vi x 10    )   x 20
NF    = 10 log     [ ------------------------ ] +174  
    dB              10          BW  x  S/N


Vi = input signal in uV applied over the 50 ohm RX input
BW = Band Wide in Hz (2400 Hz for SSB)
S/N = Signal to Noise ratio in power ratio = 10 

Back to arithmetic and using a scientific pocket calculator you
get the following results:

144 MHz  NF = 11.2 dB
435MHz   NF = 13.3 dB 

The above NF are very high and to overcome only part of
your problems ignoring all interference in your QTH that
requires sharp notch filtering you need front end preamplifiers
with at least the following characteristic:

144 MHz : NF= 0.6 dB and gain G = 20 dB with a very high
third order intercept point IP3 and very high 1 dB compression
point like a  preamplifiers of the Norton type circuit.

435 MHz : NF = 0.5 dB and gain G 20 dB possibly with
cavity input circuit like that described in my article publisched
recently into the AMSAT Journal March/April 2013 

I hope this helps.

73" de i8CVS Domenico

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