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From: "mal hamilton" <g3kevmal@talktalk.net>
To: <rsgb_lf_group@blacksheep.org>
References: <000b01c90895$1b4f4920$4201a8c0@home>
Date: Thu, 28 Aug 2008 16:26:18 +0100
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Subject: LF: Re: Re: 531kHz, etc.
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Hello Jim.
Interesting to hear your observations. I suppose a request to OFCOM from 
radio amateurs for more RF power to help their signals through the megapower 
racket, would be looked at favourably.

de Mal/G3KEV

----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Wednesday, August 27, 2008 11:34 PM
Subject: LF: Re: 531kHz, etc.


> Dear Mal, LF Group,
>
> I'm afraid there isn't much chance of getting the 136kHz Loran problem
> sorted due to potential interference to commercial users - The main users 
> of
> this part of the spectrum seem to be the tele-switching stations like 
> DCF39,
> and the Datatrack beacons. The operators of these systems have to make 
> sure
> their signal levels will reliably overcome the QRN, not to mention the 
> local
> man-made QRM in buildings and vehicles where the receivers are located,
> hence their huge ERP. The presence of Loran sidebands is highly unlikely 
> to
> be a factor affecting the overall system reliability. So the Loran noise 
> is
> not really a problem to anyone apart from amateurs, who have, from other
> peoples viewpoint, a perverse insistence in trying to utilise signals that
> are only marginally and fleetingly above the natural band noise. This is a
> bit different from the potentially real safety issue of interference to
> Navtex broadcasts in the North Sea.
>
> The Loran transmitters have to conform to particular standards for signal
> quality, which they apparently do. The standard is based on the pulse 
> shape
> of the emitted signal, which has to be closely specified so that the
> receivers can extract the correct timing information, and thus get an
> accurate position fix.. The sideband levels at 136k are largely fixed by 
> the
> pulse shape. Modifying the TX signal to generate lower sideband levels at
> 136k may be feasible, but would have consequences throughout the system.
> Modifying a whole network of multi-100kW transmittersand possibly 
> thousands
> of receivers too would be a massively expensive undertaking, unlike the
> 531kHz broadcast TX, which just needed someone to go and turn the 
> modulation
> down a bit.
>
> I think it is more interesting to try and do something positive with 
> Loran,
> rather than just saying it shouldn't be as it is. DF6NM's propagation
> studies using Loran have been very interesting. Several methods to 
> overcome
> the QRM have been tried - some even work quite well! I have had quite good
> results with phased antennas with steerable nulls. It is frustrating
> sometimes, but also the challenge makes life more interesting.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>
>
>
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>