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Date: Wed, 20 Jul 2005 20:19:29 +0100
From: "John Rabson" <word.factory@zetnet.co.uk>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Combatting Loran
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Jim,

Do you think this approach would be worth trying at the surface station of a cave radio link?

Regards,
John G3PAI

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On 17/07/2005 at 10:27 James Moritz wrote:

>Dear LF Group,
>
>Since the new Rugby Loran renders all except local 136k amateur signals
>inaudible at my QTH, I have been trying some experiments to see what can
>be done to improve the situation. I have used loop antennas to null the
>noise from Lessay coming from a rougly SW direction for a long time, but
>the direction to Rugby is roughly NW, so using a loop it is only possible
>to null one Loran at a time - plus, with a null in a NW direction, the
>other null is SE, which reduces many European signals too.
>
>What is required is an antenna with two nulls which can be "steered" so
>that the nulls have an arbitrary difference in angle between them, rather
>than being fixed at 180degrees. A unidirectional cardioid pattern with a
>single null can be obtained by summing equal signal levels from a loop
>antenna and a vertical, as used in D/F. If the signal from the vertical is
>attenuated, you get a pattern intermediate between a cardioid and a
>figure-of-eight, i.e. an asymetrical "8" with one lobe smaller than the
>other, and the null angles skewed away from 180 degrees, by an amount
>which depends on the relative signal levels from the two antennas.
>
>To implement this, I used one of the "bandpass loops" (see G3YXM's
>"features" page). In principle, the TX vertical antenna could be used, but
>at my QTH this picks up a lot of mains noise, plus it has a narrow
>bandwidth which would cause the amplitude and phase to vary a lot across
>the band, so I built a seperate "bandpass vertical", with a double-tuned
>input similar to the loop, and a bandwidth of about 15kHz. Each antenna
>has a preamp, and the signals are combined using an RC amplitude/phase
>adjusting network not unlike a "noise canceller".
>
>To set this up, I aligned the loop so its maximum direction was mid-way
>between the bearings to Lessay and Rugby - about E-W at my QTH. Then I
>adjusted the signal levels from each antenna individually so that the
>Loran noise was about the same. Then both signals were added together ,
>and the phase control adjusted for minimum Loran. Then amplitude, phase
>and the position of the antenna were iteratively tweaked for the best
>results.
>
>The results are better than I expected - see the attached SpecLab spectra
>- the audible Loran chatter is virtually eliminated , and only very slight
>tweaking is required from one end of the band to the other to maintain the
>null. As a bonus, it also nulls the "Watford Whistle" power line carrier
>on about 136.645kHz, which is always annoying here. I was able to copy
>DL1DQZ, DK7SU and F6BWO this morning, which with nulling disabled were
>completely inaudible. It proved important to de-tune the TX vertical to
>prevent it "sucking out" signal from the RX vertical - this is much more
>pronounced than with the RX loop antenna. I think the antenna pattern
>should give good coverage of Europe and Scandinavia and beyond, although
>not very good for the western UK and GI/EI, or North America. But with the
>very narrow band modes used for transatlantic DX, the Loran noise is less
>of a problem. So once I get the current lash-up sorted out, with some T/R
>switching, I should be back in business again :-)
>
>Cheers, Jim Moritz
>73 de M0BMU