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Dear Jim, LF,
yes I'm aware of the fact that the shielding from
trees etc is more significant at lower frequency. Their ohmic
conductance becomes a better shunt in comparison with
decreasing capacitive admittance, somewhat similar to a C-R highpass
equivalent circuit. There used to be two beautiful 15 m high fir trees
in the vicinity of our house. At 137 kHz, I measured a ~
15% increase in effective height when the trees were deeply frozen,
but the effect on 9 kHz may have been more severe. A couple of years ago our
neighbours had these trees chopped down, good for LF but otherwise
sad.
In April 2003, I attempted to transmitt an
8.97 kHz carrier, radiating about 1 microwatt from my normal LF
antenna (220 pF at ~ 9m eff. height). I drove around and stopped in
different places, putting up a 6m fishing pole with a wire, connected
to a resonant circuit and the laptop soundcard. Each time I took a short
Spectrogram full-band screenshot, along with a narrowband capture from a
special Argo version, patched for 22 kHz samplerate. An assembly of the
screenshots is at http://freenet-homepage.de/df6nm/8970_ALL.gif.
Maximum detection range was 6 km, just marginally outside the reactive
nearfield. No noiseblanking was attempted at the time.
If you look at the Spectrogram strips, you can see
that the first (1.6 km) and third (6.0 km) images have a much lower
absolute receive level. At first I thought something was wrong with the
receive antenna, until I realized that this was purely due to these sites
being in a forested area.
I have now rigged up SpecLab again
for VLF reception. The Russian Alpha beacons seem to
be usefiul calibration markers, the nearest one is currently about 20 dB
SNR here in a 42 Hz FFT. Does anybody in the group have information about
their EMRP, or has someone attempted to measure their fieldstrength in
Europe?
Best 73,
Markus (DF6NM)
----- Original Message -----
Sent: Friday, February 26, 2010 6:51
PM
Subject: Re: LF: Re: 9kHz noise
level
Dear Markus, LF Group,
I checked the noise level again during the
daytime, and the noise level was about 6dB lower than last night, i.e. about
13uV/m per sqrtHz
One possibility for the discrepancy between results
is that the effective height of your antenna is reduced at VLF compared to the
LF calibration point. In my transmitting vertical field strength measuring
sessions on 500k, 136k and earlier 73k, the Heff of the same antenna works out
consistently less as the frequency is reduced. I think this is due to
increased "site loss" due to surrounding trees at the lower frequencies.
H-field sensing loop antennas can be expected to be less affected. Of
course, if there are no trees near your antenna, that theory is blown out of
the water...
Cheers, Jim Moritz
73 de
M0BMU
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