Return-Path: Received: from rly-yb03.mx.aol.com (rly-yb03.mail.aol.com [172.18.205.135]) by air-yb03.mail.aol.com (v119.9) with ESMTP id MAILINYB33-18846fd200a5a; Fri, 28 Sep 2007 11:39:10 -0400 Received: from post.thorcom.com (post.thorcom.com [193.82.116.20]) by rly-yb03.mx.aol.com (v119.9) with ESMTP id MAILRELAYINYB32-18846fd200a5a; Fri, 28 Sep 2007 11:38:52 -0400 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1IbHvB-0003br-0L for rs_out_1@blacksheep.org; Fri, 28 Sep 2007 16:38:29 +0100 Received: from [83.244.159.144] (helo=relay3.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1IbHvA-0003bi-6T for rsgb_lf_group@blacksheep.org; Fri, 28 Sep 2007 16:38:28 +0100 Received: from smtpout05-04.prod.mesa1.secureserver.net ([64.202.165.221] helo=smtpout05.prod.mesa1.secureserver.net) by relay3.thorcom.net with smtp (Exim 4.63) (envelope-from ) id 1IbHv6-0002lR-US for rsgb_lf_group@blacksheep.org; Fri, 28 Sep 2007 16:38:28 +0100 Received: (qmail 28026 invoked from network); 28 Sep 2007 15:38:18 -0000 Received: from unknown (208.37.242.34) by smtpout05-04.prod.mesa1.secureserver.net (64.202.165.221) with ESMTP; 28 Sep 2007 15:38:17 -0000 Message-ID: <46FD1FBC.7080008@w1tag.com> Date: Fri, 28 Sep 2007 11:37:32 -0400 From: John Andrews User-Agent: Thunderbird 2.0.0.6 (Windows/20070728) MIME-Version: 1.0 To: rsgb_lf_group@blacksheep.org References: <000a01c80132$aae29df0$83e41a52@enigma> <46FC071D.2090202@w1tag.com> <000601c801ae$38cc27a0$0fee1a52@enigma> <001d01c801c6$a918b420$0fee1a52@enigma> <000001c801dd$a0ecc1d0$7900a8c0@athlon1200> In-Reply-To: <000001c801dd$a0ecc1d0$7900a8c0@athlon1200> X-Spam-Score: 0.0 (/) X-Spam-Report: autolearn=disabled,none Subject: Re: LF: A question of calibration Content-Type: text/plain; charset=ISO-8859-1; format=flowed 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 X-AOL-SCOLL-AUTHENTICATION: listenair ; SPF_helo : X-AOL-SCOLL-AUTHENTICATION: listenair ; SPF_822_from : X-Mailer: Unknown (No Version) Andy, all, Last year, I did a MathCad analysis of the fields from a small vertical and a small magnetic loop at 137 kHz. The program calculated the vector E and H fields, and the magnitude of E/H for distances from 1 to 10 km. For the vertical, there are three terms to consider for the E field, and two for the H field. The situation is reversed with the loop, with three terms for the H field, and two for the E field. Comparing the calculated |E/H| values with the "far field" value of 377 ohms, the error was about 12% at 1 km and 1% at 3 km for both types of antenna. The errors at 1 km were in opposite directions, as one would expect, favoring the E field for the vertical, and the H field for the loop. If you grant a measurement error of 1%, then it would be best to take readings no closer than 3 km from a small 137 kHz antenna, which would be lambda/(0.73). If higher errors are permissible, then you can move in, but I'd have trouble recommending the 1 km figure at that frequency. As Andy points out, there is at least one other far-field concept that doesn't apply to us LF hobbyists. That only refers to large antennas, where the height of a vertical, for example, is significant compared to the distance. A corollary occurs with multi-element arrays, where the directional pattern is distorted if the spacing between the towers is significant compared to the distance. John Andrews, W1TAG