From: "Paul Keinanen" <keinanen@sci.fi>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Long integration times
Date: Sat, 29 Jan 2000 15:26:50 +0200
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On Sat, 29 Jan 2000 08:34:34 +0000, Stewart Bryant
<stewart.bryant@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