Envelope-to: dave@picks.force9.co.uk
Delivery-date: Tue, 24 May 2005 18:06:57 +0100
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From: "Stewart Nelson" <sn@scgroup.com>
To: rsgb_lf_group@blacksheep.org
References: <140.449929e1.2fc47c18@aol.com>
Date: Tue, 24 May 2005 10:07:24 -0700
MIME-Version: 1.0
Subject: Re: LF: reception with multiple antennas
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Hi Pat,

> This is interesting stuff! Alan, G3NYK and I have had many discussions over
> the past few years along the lines of a transmitting equivalent of this idea.
> The basic premise is that it should be possible to effectively "synthesize" a
>  large array from generators at a number of widely spaced locations. We have
> done  little more than just discuss the ideas but maybe it could be an
> interesting  area of experimentation and development.

Yes, the transmitting equivalent is a powerful idea, both literally and
figuratively.  While an array of four receivers could provide a maximum
S/N improvement of 6 dB, four properly phased transmitters of equal power
could boost the signal at a far-field target by a whopping 12 dB!

Unfortunately, I haven't been able to think of a way to do this without
added hardware, since it is clearly hopeless to see LORAN lines when your
TX signal is in the passband.

Here are a few suggestions that shouldn't take much hardware; perhaps
one of them can be refined into something practical:

Feed a sample of the signal from an SSB transmitter, along with 1 PPS
from a GPS receiver, into a separate LF receiver.  Software would analyze
the output and servo the Tx audio input accordingly.  During QRSS etc.
key-up intervals, you would only reduce the output by, say, 20 dB,
so tracking could continue.

Build a simple SSB exciter by generating a ~120 kHz signal (must be
reasonably stable but need not be accurate), and mixing it with ~16 kHz
audio from a sound card (sample rate 48 kHz or more).  The image at
~104 kHz is filtered out and the desired ~136 kHz is sent to the PA.
A separate receiver, tuned to 104 kHz, can pick up lots of LORAN and
the relatively weak image.  Software does the rest.

If you have an existing SSB transmitter that derives all frequencies
from a single reference, divide that reference down to an audio
frequency, and feed that into a sound card, along with output from
a receiver tuned to LORAN.

None of the above schemes provides absolute phase control, but it
should be easy for a receiver in the neighborhood to compare the
phases and determine the offsets needed.

73,

Stewart KK7KA