Jim
Sensitivity of the Perseus drops off below 10 kHz - see attached spectrum shot taken earlier this
year. While the antenna in use at that time dropped off a few dB below 10 kHz the 'black hole' is
mainly due to Perseus.
http://www.w1vd.com/vlf.html
Jay W1VD WD2XNS WE2XGR
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Sunday, June 06, 2010 6:04 AM
Subject: LF: Re: Re: VLF 9 kcs: preamp and antenna noise
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" <[email protected]>
To: <[email protected]>
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...
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