Hi Markus,
That was exactly the quantitative info that I was seeking for
further progress. Thank-you very much for this and your advice.
Mike -- I saw some smudges, an "X" but little else since 0100 just
a bit above 136.172 kHz.
73
Joe VO1NA
On Sun, 5 Apr 2015, Markus Vester wrote:
Hi Joe,
sounds good! At 1.25 km you're still in the reactive nearfield, where signal
transfer occurs by capacitive coupling rather than Hertzian wave propagation.
In that regime, electric field strength is governed by an inverse cube law,
falling off rather steeply at 60 dB per range decade.
The SNR of your audible CW signal would probably have been on the order of 10
dB in 50 Hz. If you went to QRSS-3 you'd gain around 20 dB, which would get you
to 2.7 km. To reach the radiation field boundary (lambda/2pi = 5.77 km), you'd
need another 20 dB, or 100-fold symbol length (ie 300 s dots). The good news is
that beyond that point, it gets much easier to go further at 20 dB/decade.
For the very slow modes, it would probably be better to do away with the
receiver and connect the antenna directly to a laptop soundcard. Then you can
use SpecLab's sophisticated samplerate tracking functions and achieve ultimate
frequency stability.
On the transmit side, it's more important to design the antenna and coil for
voltage capability than for highest Q factor. It's relatively easy to overcome
losses by a stronger power amplifier. Maximum achievable ERP from a small
electrical antenna scales with (voltage withstanding * antenna capacitance *
effective height)^2 * frequency^4.
All the best,
Markus (DF6NM)
From: [email protected]
Sent: Sunday, April 05, 2015 12:33 AM
To: [email protected]
Subject: LF: VLF in Canada
Dear Group,
Last summer, my VLF CW signals were copied outside my back yard. Today,
using a Marconi XH-100 with a SRA-8 mixer & AD9850/Arduino LO and a
National LF-10 preselector, the sigs could be clearly heard outside my town
with a 20 metre wire aerial strung between the car and a road sign. For
TX, the 100m wire (~15 m high) was coupled with 425 mH to a variac (2:1) in
series with a 5:1 ferrite xfmr to the keyboard amp with the volume control
set to 5 (about 30 watts).
This is DX record on 8.277 kHz for this station -- 1.25 km! I wonder how
far away it could be copied with QRSS? Any TA attempt will have to wait
until a higher Q tuning coil appears. The DC resistance of the current one
is almost 1k and I think it has much series capacitance. It gets warm
and emits an stench of ozone when in use.
73
Joe VO1NA
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