Envelope-to: dave@picks.force9.co.uk
Delivery-date: Wed, 25 May 2005 13:51:16 +0100
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From: "Alan Melia" <Alan.Melia@btinternet.com>
To: rsgb_lf_group@blacksheep.org
References: <429470CF.10995.5AB276@localhost>
Date: Wed, 25 May 2005 13:48:19 +0100
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Subject: LF: Re: Reception with multiple antennas
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Hi all this is an interesting problem, and I have done some crude
calculations to try and understand the effects and some causes of fading.
Often after a geomag. storm there can be multipath which leads to dips in
excess of 20dB in the received signal. I suspect that the multiple aerial
would not be a help in these conditions where the phase is often changing at
around 8 degrees per minute.

As I understand it, two sites receiving the same signal and correcting for
phase difference would lead to a 6dB increase in the required signal. If the
noise is assumed to be non-coherent combining the singals should lead to
just a 3dB increase in noise so one should see an increse in S/N of 3dB for
every doubling of the aerial size.

Now to answer Mike's other query, previous experience on fading suggest that
there is a "footprint" for the incoming signal around 50kms in diameter
where stations receive with similar fading patterns. Although the phases
will be different at different locations and will change as the ionospheric
"refection" layer moves, all the stations inside the footprint will see
sensibly the same changes in phase. ( the parts of the ionosphere "used" by
each station will be sensibly moving up and down the same amount) This is a
phase difference that can be optimised for all the stations initially for a
given target path, then the phase difference should stay sensibly constant
over practical usable periods. We have noticed a lag in the fading of QRSS
stations at different locations, most dramartically with John WD2XES and
Warren WD2XGJ (who are about 50 miles apart) over the winter when there is
often a 20 minute difference on the time that they fade down, but this can
remain almost constant for several hours (and several fade cycles).

This suggests there may be the need for a "calibration" phase, but this is
not too different from the situation with WOLF, though it may be difficult
to optimise the initial phase when you dont know what you are looking for.
This suggests to me that it may be necessary for the tranmitting station to
transmit a "training" signal, in moden parlance, but I suppose this is just
what the interleaved code does in WOLF. One can certainly see a
phase"locking" in WOLF well before the variable signal becone decodable.

I find this a fascinating possibility as it might allow me to probe the
ionosphere in a more detailed way. I had been hoping to be able to monitor
phase changes during darkness to define how the path lengths changed and try
to understand the interaction between absorption and cancellation in
determining the signal levels at extreme distances.  Remote reception of
Loran signals was a possibility but I am not sufficiently expert in software
to write the necessary code, and we did discuss the possible use of DCF39
Wolf DL4YHF some years ago, but there are different problems there.

What is interesting is that if you cant "hear" a signal you dont know how
much to improve the equipment by, so you are handicapped. We used CFH in the
early days to estimate the possibilities of Transatlantic reception at 1w
ERP. Since those days LF horizons have expanded immensly

Cheers de Alan G3NYK

----- Original Message -----
From: "Mike Dennison" <mike.dennison@ntlworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: 25 May 2005 12:34
Subject: LF: Reception with multiple antennas


> I have been following this interesting discussion, but I have some
> perhaps naive observations which could do with some expert answers.
>
> Combining the output from two stations will give double the noise and
> double the signal - so no advantage there. I presume the fix is to
> make sure the two outputs are in phase, thus producing an improvement
> as the signals will be coherent but the noise will not.
>
> Plainly, there will be phase differences between the two receiving
> stations because of the different path lengths, and I suppose there
> would need to be compensation for this at the combining station.
>
> I can see this being useful for point to point ground-wave links,
> where the relative phase between the two received signals can be
> predicted and is constant. But how can it work when the incoming
> signal is from an unpredictable direction, and the path length (and
> hence phase) of a sky-wave signal is varying all of the time?
>
> At best you will have diversity reception (as described by Mal), but
> how do you keep two sky-wave signals in synch?
>
> Mike, G3XDV
> ==========
>
>