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Dear LF,
sometime back in 2000 I attempted to measure the
daytime noise background at 9 kHz (see http://www.qru.de/vlf-theorie.html).
I was using my regular LF antenna at my suburban QTH. The effective height
is ~ 9 m, calibrated on LF by comparison to a small loop,
and assuming that it would depend only little on
frequency. Using good old Spectrogram with moderate averaging and placing
the cursor between the visible impulsive spherics, I arrived at
a background noise of about 5 dBuV/m referenced to 1 Hz (equivalent to
142 dB kTo).
This seems lower than most of the figures quoted
recently, including Jim's measurement. There may have been some issues with my
calibration, but I don't think my figure would have been off by tens of
dBs. Anyway I will try to repeat the measurement this weekend. Part of the discrepancy could be that published figures
inevitably include the energy of the distict spherics, which may well
contribute the dominant part of the average noise power. Empiricaly I found
that some noise limiting or blanking was essential to maximize SNR for
narrow bandwidth reception.
Based on this figure for noise, a first
calculation for Stefan's 100 m kite antenna predicted a reception range
well over 200 km,. using QRSS-30 at 21 mHz! So the dreamers may
be in for a nice sunday surprise...
Best 73,
Markus (DF6NM)
Sent: Thursday, February 25, 2010 7:50 PM
Subject: LF: Re: 9kHz noise level
Dear LF Group,
As
a quick experiment to estimate noise level at 9kHz, I coupled a sig gen
into
my broadband RX loop antenna using a "current transformer in reverse"
arrangement to induce a known EMF into the loop. I used Spectrum Lab to
monitor the RX output, set up for QRSS3 reception (0.46Hz noise bandwidth).
I reduced the signal level to give what I imagine would be an "M" copy
trace, and from the induced EMF and loop area worked out the equivalent
field strength - this was 35uV/m. It is a bit difficult to say what the SNR
is, due to the constantly varying impulsive noise, but 6dB is probably not a
million miles from the mark, which would imply a noise field strength about
25uV/m per sqrtHz. I don't know if the noise level on VLF this evening is
particularly high or low. The Alpha beacons are clearly audible
here.
Cheers, Jim Moritz
73 de M0BMU
----- Original
Message -----
From: "Andy Talbot" <[email protected]>
To:
<[email protected]>
Sent:
Thursday, February 25, 2010 11:22 AM
Subject: Re: LF: AW: 9
Dreamers
> To get the V/m value from this, first calculate the
effective area of a
> 9kHz
> isotropic antenna :
> At 9kHz,
lambda = 33333m, G = 4.pi.A/lambda^2, G = 1,
so A =
> 88*10^6 m^2
>
> 1mW of noise per Hz
bandwidth, received in this aperture means noise
> density
> Nd =
is 1.1E-11 W/m^2
> Field strength E volts/metre, == SQRT(Nd.
377)
> = 65uV/m in 1 Hz bandwidth, or 65uV/ (ROOT
Hz)
>
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