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R: G4COM Noise Meter and system tweaking...



The G4COM Noise Meter was described in RADIO COMMUNICATION January 1976 and
it is a very old design presenting many limitations the most important of
wich is due to the fact that it works using the audio of  a  receiver.

Since the wideband of the audio is very narrow,the Noise Meter instrument is
continuously  vibrating a little bit up and down making it difficult to do
small Noise Figure adjustements.

I dont suggest to directly connect the output of  the above noise generator
head to the input of your 2.4 GHz  preamplifier or converter because the
output impedance of  the above noise head changes significantly from the
OFF to the ON state when  pulsed by the squared signal coming from cursor of
potentiometer RV3

The result of this mismatch is that your preamplifier or converter cannot be
really adjusted for the lowest Noise Figure because during the ON and
OFF  state  of your noise head the gain of your preamplifier or converter
connected to it changes as well  generating a false S/N measurement.

The metod to radiate noise from your head  connected to an antenna and
couple enough noise directly to your helix-feed  connected to your
preamplifier is very nice because the distance from the antenna  radiating
noise and the receiving helix -feed  acts as an attenuator reducing the
effect of the impedance changes from the OFF and ON head
conditions.

Unfortunately the power of the noise head at 2.4 GHz is very small
depending on your microwave diode used for it  but it is possible to bust it
building the very simple two stage MMICs wideband amplifier described by
WB5LUA on the ARRL UHF/Microwave Experimenters Manual Chapter 7
page 7-38 and 7-39  showing a gain of more than 15 dB at 2.4 GHz

Now connect your noise head to the input of this 15 dB amplifier and
connect to its output  a 6 turns helix for 2.4 GHz with a gain of  about
10 dBi or any other antenna available for 2.4 GHz

Put this antenna looking at your helix feed dish and some wavelenghts away
from it.

Remember that the free space attenuation at any frequency is  22 dB at a
distance of one waveleght from the antenna and that the attenuation
increases by 6 dB every time you double this distance.

With all of this in mind you can choose the maximum possible distance
between the antennas and  than tweak your preamplifier or converter for
 the lowest Noise Figure looking at the G4COM  Noise Meter indicator.

In this manner,having the helix feed directly connected to the input of
preamplifier or converter  do not disturb its adjustement for the lowest
Noise Figure because there are no gain variations due of impedance
changes between the OFF and ON state of the noise head.

Obviously,since the excess noise ratio ENR of  the noise system is unknow it
is impossible to get the Noise Figure of your preamplifier or converter in
dB but in any case,after the adjustement, the Noise Figure will be the
lowest possible to be obtained.

73 de i8CVS Domenico


----- Original Message -----
From: Russ Renaud <ae517@freenet.carleton.ca>
To: <amsat-bb@AMSAT.Org>
Sent: Tuesday, July 03, 2001 6:12 PM
Subject: [amsat-bb] G4COM Noise Meter and system tweaking...


> I'm just putting the finishing touches on my G4COM noise meter, and poking
> around on the WWW I found some EME articles at the PA3CSG web page
>
> http://www.qsl.net/pa3csg/Boek/BoekH2/content2.html
>
> and more specific to my noise meter:
>
> http://www.qsl.net/pa3csg/Boek/BoekH2/art2-32.html
>
> N6IZW kindly provided a microwave diode as a 2.4 GHz noise source to
> use in conjuction with my meter.
>
> How can I couple enough noise into my helix-fed dish to tweak my
> system?
>
> Thanks
>
>
>
>
> --
> 73 de va3rr/aa8lu
> http://www.qsl.net/va3rr
> Check out the new Ship Station Regs at:
> http://www.tc.gc.ca/Actsregs/csa-lmmc/csa63.html
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>



















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