Envelope-to: dave@picks.force9.co.uk Delivery-date: Wed, 20 Jul 2005 20:20:39 +0100 Received: by pih-mxcore06.plus.net with spam-scanned (PlusNet MXCore v2.00) id 1DvK7S-0007wQ-Pi for dave@picks.force9.co.uk; Wed, 20 Jul 2005 20:20:39 +0100 Received: from post.thorcom.com ([193.82.116.20]) by pih-mxcore06.plus.net with esmtp (PlusNet MXCore v2.00) id 1DvK7S-0007vd-IL for dave@picks.force9.co.uk; Wed, 20 Jul 2005 20:20:38 +0100 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1DvK6X-00006g-Ge for rs_out_1@blacksheep.org; Wed, 20 Jul 2005 20:19:41 +0100 Received: from [193.82.116.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1DvK6W-00006X-U3 for rsgb_lf_group@blacksheep.org; Wed, 20 Jul 2005 20:19:40 +0100 Received: from mailout.zetnet.co.uk ([194.247.47.231]) by relay1.thorcom.net with esmtp (Exim 4.51) id 1DvKF4-0005HQ-Jy for rsgb_lf_group@blacksheep.org; Wed, 20 Jul 2005 20:28:32 +0100 Received: from irwell.zetnet.co.uk ([194.247.47.48] helo=zetnet.co.uk) by mailout.zetnet.co.uk with esmtp (Exim 4.50) id 1DvK6U-0001jC-Ta for rsgb_lf_group@blacksheep.org; Wed, 20 Jul 2005 20:19:38 +0100 Received: from john-b5a82ea1a4 (40-077.dialup.zetnet.co.uk [194.247.40.77]) by zetnet.co.uk (8.13.4/8.13.4/Debian-1) with ESMTP id j6KJJZ8O020351 for ; Wed, 20 Jul 2005 20:19:37 +0100 Message-ID: <200507202019290658.0651F0B9@mail.zetnet.co.uk> In-Reply-To: <001001c58ab2$0cd7acc0$dd088751@w4o8m9> References: <001001c58ab2$0cd7acc0$dd088751@w4o8m9> X-Mailer: Courier 3.50.00.09.1098 (http://www.rosecitysoftware.com) (K) Date: Wed, 20 Jul 2005 20:19:29 +0100 From: "John Rabson" To: rsgb_lf_group@blacksheep.org Mime-Version: 1.0 Subject: Re: LF: Combatting Loran Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-PN-SpamFiltered: by PlusNet MXCore (v2.00) Jim, Do you think this approach would be worth trying at the surface station of a cave radio link? Regards, John G3PAI *********** REPLY SEPARATOR *********** 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