Return-Path: Received: from rly-dc06.mx.aol.com (rly-dc06.mail.aol.com [172.19.136.35]) by air-dc03.mail.aol.com (v123.4) with ESMTP id MAILINDC034-b254a19dfad349; Sun, 24 May 2009 20:01:03 -0400 Received: from post.thorcom.com (post.thorcom.com [193.82.116.20]) by rly-dc06.mx.aol.com (v123.4) with ESMTP id MAILRELAYINDC061-b254a19dfad349; Sun, 24 May 2009 20:00:48 -0400 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1M8Nba-0002S0-9r for rs_out_1@blacksheep.org; Mon, 25 May 2009 00:59:50 +0100 Received: from [193.82.116.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1M8NbZ-0002Rr-Ko for rsgb_lf_group@blacksheep.org; Mon, 25 May 2009 00:59:49 +0100 Received: from smtp802.mail.ird.yahoo.com ([217.146.188.62]) by relay1.thorcom.net with smtp (Exim 4.63) (envelope-from ) id 1M8NbV-0002eM-VP for rsgb_lf_group@blacksheep.org; Mon, 25 May 2009 00:59:48 +0100 Received: (qmail 35262 invoked from network); 24 May 2009 23:59:38 -0000 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=btinternet.com; h=Received:X-YMail-OSG:X-Yahoo-Newman-Property:Message-ID:From:To:References:Subject:Date:MIME-Version:Content-Type:Content-Transfer-Encoding:X-Priority:X-MSMail-Priority:X-Mailer:X-MimeOLE; b=YQlIarCrs9zaLRpRKrJP3BMaA1MMYZoDFJiBCFWW/I+XIyBD4XHdKvKeTtDASVmC11sGkdIjGkJsUY5BJp+7zxDnEzrCUdiA4xenw3Ktop3dB6IHHwV77XaM7hVKIBH2LpBGAiflSLm4aqJ1Q8K/4xYN3jXl7z2M20SR39QYX0Y= ; Received: from unknown (HELO Lark) (alan.melia@213.122.38.229 with login) by smtp802.mail.ird.yahoo.com with SMTP; 24 May 2009 23:59:37 -0000 X-YMail-OSG: zyqLxdcVM1kLbGNLkcWG3.xYh0ySRhlWAZMRVwb78Icn6CqFUu8OmVbmOgBLvzVNLLd2nYMtyM_N1kfbGJd2eWFvv6ezGGLROA_NOGtwFUifA_AfVVnR0nP9q0wwpD.f1beZWdRdNEjufTQv7VNRcLqyJr6SSEyc1UBTUxcrOEtfhRu8YlDujSOgo6ieQMcta_VB55YgVVLRKpDmHUYdQytiT4QYpOmiN73p2NuI_kMIXpQlL3zTHtcZqOSZP5PoGxURSo3q2Ckz8Ul8VfTm6B7KukSHNhngj8U- X-Yahoo-Newman-Property: ymail-3 Message-ID: <007a01c9dccb$b3021620$0900a8c0@AM> From: "Alan Melia" To: References: <4A199D1F.7070201@btconnect.com> Date: Mon, 25 May 2009 00:59:35 +0100 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2600.0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2600.0000 X-Karma: unknown: DomainKey-Status: good (testing) X-Spam-Score: 0.0 (/) X-Spam-Report: autolearn=disabled,none Subject: LF: Re: Modelling aerials Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.0 required=5.0 tests=none autolearn=no version=2.63 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-AOL-IP: 193.82.116.20 Hi again Richard. I think the answer is dont waste your time. LF/MF aerials are so short and depend so much on ground and none of the systems model them very well at all. Roy Llewellin admitted as much at an Internet lecture a few years ago at the HF Convention. Laurie G3AQC and Peter G3LDO have done some work with EZINEC. It would be a disaster for instance, to fix tuning component values on the basis of a model. In particular the programs dont seem to model the reduction in ground loss with increased top capacity, which is the effect you may have with a short low aerial. I can imagine that modelling ground strata of a rocky hill top will be almost impossible. At our level LF and MF aerial work has proved that empirical techniques are the best. Hence my cry of "measure it with an RX bridge", I have found that is the only way to really find out what is happening, and quickly assess changes. Graham mentions the increase in gain from top loading but it is very small about 1dB. the main advantage of top inductive loading is to reduce the voltage on the vertical section so that the loss which is coupled into to surrounding environment, trees and buildings, is reduced. You never see it in commercial aerial systems because they just dont have that effect. The thought of you abseiling down a cliff gives a whole new meaning to aerial rigging. Yes you could increse the capacity of the lower end to ground with insulated (from ground) radials. By the time the current gets down there it has traversed the Radiation resistance and an increased capacity with reduce the ground loss. It could work quite well!! The top feed with be just the same as a normal vertical feed. Dave G0MRF made the 1st trans-atlantic 136kHz crossing using an "urban cliff".... throwing two wires out of the top-floor flat of a tower block in West London, and feeding it again the water-distribution system (I believe) !! LF stands for "Loadsa Fun" and MF for "More Fun" :-)) Best Wishes Alan G3NYK ----- Original Message ----- From: "Richard Newstead" To: Sent: Sunday, May 24, 2009 8:16 PM Subject: LF: Modelling aerials > Dear MFers > > I am trying to model various aerial configurations for 500kHz using NEC. > I have set the antenna 0.5m above ground as NEC2 does not like radials > on the ground. I have used "real ground". Has anyone compared the > calculated Source impedances to real-world measurements at 500kHz? I am > trying to understand the likely range of impedances that I need to match > over and hence the ATU types I need to employ and the components needed. > > Any modelling tips will be welcome. > > 73 > > Richard > G3CWI >