Dear Jim, Mal, LF Group
G3KEV wrote...
Jim
Do you expect to reach me with this sort of erp? I am QRV when you are
ready. I expect you to hear me with a few hundred watts
...As a feasibility study, one can make order-of-magnitude estimates of
required ERP fairly easily. The noise level at 9kHz in quiet winter-time
conditions at a reasonably quiet QTH during DK7FC's previous tests was
roughly 8uV/m per sqrt(Hz) by my estimate. The distance between G3KEV and
G7NKS is roughly 200km. If we assume G7NKS actually achieves 5uW ERP, field
strength at G3KEV will be E = 7*sqrt(Perp) / d, around 80nV/m, assuming
ground wave losses are neglegible. This is 40dB below the noise floor in 1Hz
bandwidth, so the RX bandwidth would have to be reduced to 100 microhertz to
get 0dB SNR, or say 50 uHz to get a few dB positive SNR. I think this would
be feasible to do; it would require frequency stability of the order of
parts in 10^9 at both TX and RX, so both Mal and Jim would have to make some
improvements!
So can Mal really expect to be "heard with a few hundred watts"? Mal is
always a bit vague about his antenna, but often mentions 3 x 150m wires
supported by a 100ft mast, and unless he has a whole collection of other
100ft masts to support the other ends, one would assume this means sloping
wires and an effective height in the region of 15m. Capacitive reactance at
9kHz would be roughly 6 kilohms, allowing 3A or so of antenna current with a
reasonable voltage of 20kV. With a relatively big antenna like this, one
would hope for a fairly low loss resistance, provided a very low-loss
loading coil can be made. Let's optimistically assume 50ohms total loss
resistance - to get 3A Iant, 450W TX power would be needed, which is
stretching the "few hundred watts" a bit, but never mind. Radiation
resistance at 9kHz would be 320 micro-ohms, and with Iant = 3A, ERP works
out to about 5mW. This is comparable with the ERP from DK7FC's VLF
experiments. 200km away at G7NKS, field strength would be about 2.5uV/m. I
reckon if you actually want an audible CW signal, it has to be above 0dB SNR
in a normal CW filter bandwidth, say 250Hz. The band noise in 250Hz would be
about 130uV/m, so 2.5uV/m is 34dB below an audible signal level. So even
though G3KEV might have an ERP 1000 times greater as an optimistic estimate,
it would still be far too weak to be actually heard at G7NKS. QRSS30 should
be quite easy, though.
So, the verdict is that it is probably feasible for G7NKS to produce a
detectable 9kHz signal over a distance of a few hundred km, but extreme
narrow bandwidths, integrating periods of several hours and high frequency
stability would be needed to do it. It should be possible for Mal to produce
a 9kHz signal that can be detected at a similar range using techniques
familiar from LF amateur radio, but it certainly won't be audible at that
distance.
Cheers, Jim Moritz
73 de M0BMU
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