Jim
Looking at your observations its going to be a struggle using convential QRS
speeds to cover several hundred kilometres, which was my aim, also a few
Kilowatts
looks necessary rather than a few hundred watts.
This 9 Kcs adventure needs some serious thought if one wants to cover any
distance. I wonder if we could get a slot around 73 Kcs again or maybe
around 30 - 50 Kcs.
73 de mal/g3kev
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Friday, September 10, 2010 10:18 PM
Subject: LF: Re: UK NoVs
> 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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