[amsat-bb] Re: Pre amp question

i8cvs domenico.i8cvs at tin.it
Thu Dec 14 06:01:00 PST 2006

Hi Greg, KO6TH

If the AGC cannot be switched OFF put it to slow and reduce the RF gain
to a minimum necessary to read about 100 mV in the voltmeter connected
to the audio jack and this because you don't want to drive the AGC

For this type of measurement with AGC ON the IF of your receiver must
work with a gain wich is well below the gain necessary to drive the AGC

In this situation the AC voltage across the AC voltmeter is low but it can
be elevated to about 3 volt using an audio transformer with impedance ratio
5/5000 ohm recovered for example from an old receiver using a 6V6 for
the audio amplifier.

Connect the 5 ohm secondary winding to the low impedance audio of your
receiver and connect the 5000 ohm primary winding to your voltmeter via
a 10 k resistor with a 100 n capacitor across the AC voltmeter to damp the
pointer indication.

If the above transformer is not available then a small power supply
transformer 9/220 volt works equally well.

If your AC voltmeter has a scale directly calibrated in dB you can directly
read the ratio V2/V1 expressed in dB as I do with my popular analog Volt-
Ohm-Milliammeter model ICE 680R calibrated for -10dB ---0dB---+22dB
with 0 dB = 1mW over 600 ohm

Before to make any measurement be sure that having reduced the RF gain
to a minimum necessary your S meter do not move at all tuning the band
particularly when the preamplifier is ON

Good references to better understand about this type of measurements are:

Don Lund WA0IQN, Using Sun Noise, QST 4/1968

D.W.Bray K2LMG and P.H. Kirchner W2YBP "Antenna Pattern from the Sun
QST 7/1960

P.Schuch N6TX and P.Wilson W4HHK,"Calibrating the Signal Generator
in the Sky" QST 11/1992 , pag-42

W.Atchison, Calculating System Performance Using Solar Flux Data"
Proceedings of Microwave Update 1991, pag 61

D.Shaffer W8MIF ,Microwave System Calibration Using the Sun and the Moon,
ARRL UHF/Microwave Experimenter Manual, pag-60

J.Kraus W8JK, Radio Astronomy, Cap. VIII Mc Graw-Hill Book Company 1966

73" de

i8CVS Domenico

----- Original Message -----
From: "Greg D." <ko6th_greg at hotmail.com>
To: <amsat-bb at amsat.org>
Sent: Thursday, December 14, 2006 4:48 AM
Subject: [amsat-bb] Re: Pre amp question

> Is this test useful on a receiver where the AGC cannot be turned off?  My
> FT-736 has only fast/medium/slow.  No off.  I would expect a reduction in
> accuracy, but is it even worth doing?
> Greg  KO6TH
> ----Original Message Follows----
> From: "i8cvs" <domenico.i8cvs at tin.it>
> To: "Steve Raas" <sraas at optonline.net>, "AMSAT-BB" <amsat-bb at amsat.org>
> Subject: [amsat-bb] Re: Pre amp question
> Date: Wed, 13 Dec 2006 20:55:50 +0100
> Hi Steve, N2JDQ
> If your receiver has the capability to switch OFF the AGC than the
> simple test will tell you if your preamplifier connected to the actual
> antenna is working or not for you:
> {snip}

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