Return-Path: Received: (qmail 3126 invoked from network); 7 Apr 2003 14:45:38 -0000 Content-Transfer-Encoding: 8bit Received: from netmail02.services.quay.plus.net (212.159.14.221) by mailstore with SMTP; 7 Apr 2003 14:45:38 -0000 Received: (qmail 5368 invoked by uid 10001); 7 Apr 2003 14:45:37 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: from post.thorcom.com (193.82.116.70) by netmail02.services.quay.plus.net with SMTP; 7 Apr 2003 14:45:37 -0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 X-SQ: A Received: from majordom by post.thorcom.com with local (Exim 4.14) id 192Xqv-0005Eg-4M for rsgb_lf_group-outgoing@blacksheep.org; Mon, 07 Apr 2003 15:44:05 +0100 Received: from [147.197.200.9] (helo=hestia.herts.ac.uk) by post.thorcom.com with esmtp (Exim 4.14) id 192Xqp-0005EX-B1 for rsgb_lf_group@blacksheep.org; Mon, 07 Apr 2003 15:43:59 +0100 Received: from gemini ([147.197.200.44] helo=gemini.herts.ac.uk) by hestia.herts.ac.uk with esmtp (Exim 3.22 #1) id 192Xhd-0006fb-00 for rsgb_lf_group@blacksheep.org; Mon, 07 Apr 2003 15:34:29 +0100 Received: from [147.197.232.252] (helo=rsch-15.herts.ac.uk) by gemini.herts.ac.uk with esmtp (Exim 3.33 #1) id 192Xhb-0006er-00 for rsgb_lf_group@blacksheep.org; Mon, 07 Apr 2003 15:34:27 +0100 Message-ID: <5.1.0.14.0.20030407151723.00a9ea90@gemini.herts.ac.uk> X-Sender: mj9ar@gemini.herts.ac.uk X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Mon, 07 Apr 2003 15:28:02 +0100 To: rsgb_lf_group@blacksheep.org From: "James Moritz" In-reply-to: <191xgH-175wvYC@fwd10.sul.t-online.com> References: <5.1.0.14.2.20030404222503.009f79d0@mail.pncl.co.uk> MIME-Version: 1.0 X-MailScanner: No Virus detected Subject: Re: LF: TX coil screening Content-Type: text/plain; charset=us-ascii; format=flowed X-Spam-Status: No, hits=-22.9 required=5.0tests=EMAIL_ATTRIBUTION,IN_REP_TO,REFERENCES,REPLY_WITH_QUOTESversion=2.51 X-Spam-Checker-Version: SpamAssassin 2.51 (1.174.2.5-2003-03-20-exp) X-SA-Exim-Scanned: Yes Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rsgb_lf_group-outgoing@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false Dear Uwe, LF Group, At 02:06 06/04/2003 +0200, you wrote: >Hi Walter, > >quotation Terman ed.1943, page 128 on shielded air coils: > >...shields... increase the losses as a result of energy dissipated >in the shield by the eddy currents. However, if clearence between >the coil and its shield is everywhere at least equal to the radius >of the coil, the reduction in inductance and in the decrease in >coil Q by a copper or aluminium shield will not be serious. > >end of quotation. > >grounded or not grounded, the eddy currents in the shield cause losses >(short-circuit). Adding a conducting shield will inevitably lead to some additional eddy current losses - however, it will also shield the electric field of the coil, which will reduce dielectric losses in the environment outside the shield. Because of the high voltage/high impedance nature of most amateur LF antennas, dielectric losses may well make up a large proportion of the total loss associated with the coil - so adding the shield could well reduce total loss. Cheers, Jim Moritz 73 de M0BMU