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LF: Combatting Loran

To: [email protected]
Subject: LF: Combatting Loran
From: "James Moritz" <[email protected]>
Date: Sun, 17 Jul 2005 10:27:54 +0100
Delivery-date: Sun, 17 Jul 2005 10:32:59 +0100
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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

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