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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)
Sent: Sunday, April 05, 2015 12:33 AM
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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