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@hotmail.com To: amsat-bb@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@tin.it To: "Steve Raas" sraas@optonline.net, "AMSAT-BB" amsat-bb@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
following
simple test will tell you if your preamplifier connected to the actual antenna is working or not for you:
{snip}