Return-Path: Received: (qmail 16235 invoked from network); 25 Feb 2000 13:01:39 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by dimple.core.plus.net.uk with SMTP; 25 Feb 2000 13:01:39 -0000 Received: from majordom by post.thorcom.com with local (Exim 3.02 #1) id 12OKGd-0007gV-00 for rsgb_lf_group-outgoing@blacksheep.org; Fri, 25 Feb 2000 12:54:47 +0000 Received: from helios.herts.ac.uk ([147.197.200.2]) by post.thorcom.com with esmtp (Exim 3.02 #1) id 12OKGZ-0007gQ-00 for rsgb_lf_group@blacksheep.org; Fri, 25 Feb 2000 12:54:44 +0000 Received: from [147.197.200.44] (helo=gemini) by helios.herts.ac.uk with esmtp (Exim 3.11 #1) id 12OKDh-00012R-00 for rsgb_lf_group@blacksheep.org; Fri, 25 Feb 2000 12:51:45 +0000 X-Priority: 3 X-MSMail-Priority: Normal Message-ID: <29412.200002251259@gemini> From: "James Moritz" X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Organization: University of Hertfordshire To: rsgb_lf_group@blacksheep.org Date: Fri, 25 Feb 2000 12:59:50 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 8bit Subject: LF: 73kHz on Friday? / Field strength X-Mailer: Pegasus Mail for Win32 (v3.11) Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Dear LF Group, Last Friday I was QRV on 73kHz between about 2100 and 2300 utc - didn't hear/see anyone else though. This was a pity because the Rugby TX was transmitting only two unmodulated carriers, spaced by 85Hz, which considerably reduces the amount of QRM at 71.8kHz. This was also the case the week before, and also when I listened briefly last night. Obviously, it isn't guaranteed when these windows of opportunity will occur, but it seems worth having a go. Anyway, I intend to be on 73kHz again tonight from about 2000 onwards - see you there? Also last Friday, I measured G3XDV's field strength at the University of Hertfordshire Telecomms Lab, on 135.9kHz. Using the HLA6120 loop antenna, and HP8591 EMC analyser, the equivalent electric field strength was 42.9dBuV/m (140uV/m). The distance between the University and Mike's QTH is 7.5km, from the Ordnance Survey map. Mike's ERP from these measurements works out to 22mW. I also simulated Mike's antenna using EZNEC, using the details on his web site, in order to obtain the radiation resistance. This comes out to about 50milliohms, compared with 64milliohms as calculated using the simple vertical monopole formula. The exact value of Rrad depends on the loading coil inductance, spacing between wires & masts etc., but the differences are not major, leading to perhaps 1dB variation in results. Using 50milliohms, and an antenna current of 1.5A as quoted by Mike, the calculated radiated power is 113mW, and the ERP 203mW. This should give a field strength at 7.5km of 421uV/m. There is therefore a discrepancy of -9.6dB between expected and actual ERP - even greater than the -6.4dB difference that was found for the M0BMU measurements a few weeks ago (calculated E = 408uV/m, measured E = 195uV/m). It was suggested at that time that part of the difference might be due to the short distance between transmitter and receiver not being enough to achieve 'far field' conditions, however, Mike's QTH is nearly twice as far as mine, so would be expected to show less loss from this cause. I have been searching for information on LF propagation - some interesting results were obtained by Dr. J.H. Causebrook in the late 70's, who was working for the BBC at the time. He started from the observation that the ground wave field strength of medium wave broadcast stations was often much lower than expected in built-up areas. He did some detailed measurements on the signals from Brookmans Park (small world....) in the London area, which confirmed this. He showed that the usual ground wave propagation data for 'poor ground', 'good ground', and sea-water were not adequate to describe propagation in urban areas. He developed a theoretical model in which low density buildings, lamp posts, and so on were modelled as grounded, lossy monopoles. Heavily built up areas were described by a high conductivity layer (the buildings) on top of the lower conductivity ground. This approach was persuaded to give quite good agreement with the observed results. This work is not directly useful to LF amateurs, since the frequency range is different, and the analysis does not yield any simple formula for the signal losses. However, it does show that in urban or suburban areas, LF amateurs can expect to see increased losses in signal strength than occurs in open countryside over the same distance. The path of G3XDV's signal to the receive site goes through Welwyn Garden City and Hatfield town most of the way, whereas my signal goes mostly through a 'semi-rural' environment. This could account for the increased loss in Mike's signal. Another factor is that the amateur's transmitting antenna is often in an urban area too, which is not often the case for professional LF users. Since the 'near field' of an LF antenna must extend at least for hundreds of metres, the radiation from the antenna must be greatly influenced by the surrounding scenery, especially when the features in that scenery include many things that are bigger than the antenna itself. So the conclusion so far is that, if you live in an urban area, your ERP is probably a lot lower than you would expect from simple calculations for a given antenna and power. This is in rough agreement with the PA0SE & DK8KW results. For ultimate DX results, operate /P from a small, flat, uninhabited island surrounded by sea! Obviously, more measurements are required to get a better picture, so my project for this summer is to put together a portable, calibrated field-strength measurement set. The HLA6120/HP8591EM setup is a bit too bulky to drag round the countryside, so I am working on a calibrated ferrite rod antenna of the type used by PA0SE, for use with an old Rycom selective voltmeter I have. Hope to see someone on 73kHz... Cheers, Jim Moritz 73 de M0BMU