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Message-ID: <DFFD7B2AF9C74F0C9C1BFD90F8559389@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <004801cb04d6$e9740b80$0401a8c0@xphd97xgq27nyf> <CD190EC05BA84C1685A9A2F4A56DD284@JimPC> <004301cb0544$56965980$0201a8c0@Clemens04> <FF5414DBC74E45B69F5D9E01FDFB4A0C@White>
In-Reply-To: <FF5414DBC74E45B69F5D9E01FDFB4A0C@White>
Date: Sun, 6 Jun 2010 11:04:29 +0100
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Subject: LF: Re: Re: VLF 9 kcs: preamp and antenna noise
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Dear Clemens, Markus,

Yes, what Markus wrote is exactly what I mean...

In general, the SNR does depend on the source impedance. Also, the overall 
power gain of the preamp/receiver depends on the degree of mis-match at the 
input, so the fact that the output noise level stays the same in the Perseus 
does not imply that the SNR will also be the same.  But unless the noise 
figure of the preamp/receiver is quite low, this effect will often be masked 
by internal noise in practice. As Markus points out, with large RX antennas 
the source noise power at 9kHz is spectacularly high, and preamp noise is 
not usually a factor anyway. But for small whips and loops, preamp noise is 
important.

The figures Clemens quotes for the Perseus RX are curious... With 732Hz 
bandwidth and T = 300K, kTB would be -145.2dBm, so the -118.4dBm noise floor 
level indicates a noise figure of about 27dB, which seems quite high. The 
QST review quotes -127dBm noise floor in 2.4kHz bandwidth, which would imply 
about -132dBm noise floor in 732Hz bandwidth, so some discrepancy there.

On a crude practical level, if you listen to your 9kHz RX output and hear 
mostly crackling QRN, that is fine... if you hear mostly 50Hz buzzing and 
other man-made noises, you either have a problem with local noise being 
coupled to the preamp or receiver input, or need a better antenna 
location... If you hear mainly "white noise", your receive system is 
probably deaf!

Cheers, Jim Moritz
73 de M0BMU

----- Original Message ----- 
From: "Markus Vester" <markusvester@aol.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Sunday, June 06, 2010 8:38 AM
Subject: LF: Re: VLF 9 kcs: preamp and antenna noise


Clemens,

I believe what Jim was suggesting is a comparison of preamp noise output 
with the antenna connected versus a passive load, rather than between 
different passive load impedances. The intention is to demonstrate that the 
receiver noise contribution is neglegible against external noise...