[Lf] [Fwd: LF: Long integration times]

[Andre' Kesteloot]

An intersting set of ideas
Andre'
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Paul Keinanen wrote:

> On Sat, 29 Jan 2000 08:34:34 +0000, Stewart Bryant
> <stewart.bryant at virgin.net> wrote:
>
> >For some time I have been thinking of setting up a remote receiver
> >in a quite location linked back to a more conveniently located
> >transmitter location. This would be an interesting extension. There
> >are however some licensing issues.
> >
> >The obvious first stage would be to use a radio link, but that would
> >require licensing. Unfortunately the obvious band to use would be
> >70cms (quiet, low occupancy, reasonable range cheap equipment)
> >but it will be very difficult to get a permit, and then on top of that
> >an NoV will be required to relay the signals.
> >
> >The alternative would be to use the Internet or the telephone, but
> >ironically our administration which is ostensibly promoting the linking
> >of amateur radio and the Internet has just changed our license to
> >require government approval to connect our stations to an public
> >network.
>
> I have not seen the wording of your regulations, so this is just a
> guess, but did't you say you only wanted to move the receiver to a
> remote location. Assume that you have connected the receiver to a
> modified answering machine and you are calling your answering machine
> to listen to the signal and tune the receiver using the answering
> machine control functions. I don't understand how that remote receiver
> could be considered an amateur radio station (although the receiver
> can tune one or more amateur bands) since no transmitter is present at
> the remote location.
>
> There may be regulations against connected modified answering machines
> to the telephone network, but a computer with a receiver and auto
> answer modem could be constructed from type approved parts, but
> unfortunately, after introduction of the computer to a quiet place, it
> would no longer be a quiet place :-).
>
> >One approach that we could take is to record the sampled signals
> >for a long period with GPS timing markers, and post analyse. There
> >may be some stuff we can borrow from the SETI folks here.
>
> If you have two or more reception sites, you could do some aperture
> synthesis by postprocessing received data in a similar way as radio
> telescopes are used in VLBI. This requires accurate timing marks at
> each receiver site in order to generate various antenna patterns. A
> long time ago, there was a long discussion in some sci.space...
> newsgroup with the title "Amateur VLBI" or something similar about the
> feasibility of establishing a continent wide VLBI network to get some
> usable angular resolution for upper HF/VHF with small backyard
> antennas.
>
> The problem with only a few (2-3) antennas several wavelength distance
> from each other is that it will produce a combined radiation pattern
> with a lot of narrow "fingers", i.e. several very narrow peaks and
> nulls in the radiation pattern. With more stations, the undesired
> peaks can be suppressed and only a single strong and narrow peak will
> remain. Unfortunately this requires a lot of stations.
>
> In VLBI, the rotation of the Earth will constantly change the
> positions relative to the source and assuming the emission of the
> source does not change, the new relative position is considered a new
> synthesised station and with properly phase and amplitude settings the
> received signal is added into the correlator.
>
> For LF reception and beam synthesis, there is not much point in having
> very narrow beams, so several stations within a 10 km radius could be
> used to generate  a beam of about 5 degrees, with a radical reduction
> of (far field) noise, mainly QRN, coming from other direction, thus
> increasing the SNR. With such a small geographical area, synchro-
> nisation would not bee too difficult, even the power grid or a local
> TV station would be sufficient to provide common phase reference to
> all receivers. For larger geographical areas any geostationary TV-
> transponder visible on all ground stations could be used.
>
> With the received signal recorded on one channel of the sound card and
> the reference signal on the other channel, there is no need to
> synchronise the sampling rate at each station, thus existing hardware
> can be used.
>
> While most of these ideas above are not feasible in practice, I hope
> they will inspire someone else to find more practical solutions.
>
> Paul OH3LWR
>