Return-Path: Received: (qmail 2085 invoked from network); 14 Jul 2001 12:38:02 -0000 Received: from unknown (HELO murphys-inbound.servers.plus.net) (212.159.14.225) by excalibur-qfe1-smtp-plusnet.harl.plus.net with SMTP; 14 Jul 2001 12:38:02 -0000 Received: (qmail 29739 invoked from network); 14 Jul 2001 12:36:59 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by murphys with SMTP; 14 Jul 2001 12:36:59 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: from majordom by post.thorcom.com with local (Exim 3.16 #2) id 15LOYr-0005ay-00 for rsgb_lf_group-outgoing@blacksheep.org; Sat, 14 Jul 2001 13:30:17 +0100 Received: from [204.202.140.199] (helo=webmailmta.go.com) by post.thorcom.com with esmtp (Exim 3.16 #2) id 15LOYn-0005at-00 for rsgb_lf_group@blacksheep.org; Sat, 14 Jul 2001 13:30:13 +0100 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from gomailjtp01 ([10.212.0.161]) by mta07.seamail.go.com (Sun Internet Mail Server sims.4.0.2000.10.12.16.25.p8) with ESMTP id <0GGG00DN7QO74X@mta07.seamail.go.com> for rsgb_lf_group@blacksheep.org; Sat, 14 Jul 2001 05:28:55 -0700 (PDT) Date: Sat, 14 Jul 2001 05:28:57 -0700 (PDT) From: "John Sexton" Subject: LF: Near field effects To: rsgb_lf_group@blacksheep.org Message-ID: <1689887.995113737131.JavaMail.computernetworks@gomailjtp01> MIME-Version: 1.0 X-Mailer: GoMail 3.0.0 Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Hi All, Trying to find out more about near field effects, I have read that the transition from near to far field happens at the "Rayleigh distance", sometimes called the "far field distance". An estimate for this distance is given by the formula (2 d^2)/(lambda) where d is the maximum dimension of the radiating structure. (See for example: http://www.ee.surrey.ac.uk/Personal/D.Jefferies/antennas.html) For a quarter wave vertical this gives a value of one eighth of a wavelength, not too different from the 1/2Pi formula quoted elsewhere. However for a typical amateur 10 metre vertical, the formula gives a ridiculously small value of less than 0.1 of a metre at 137 kHz. It seems that the Rayleigh distance is commonly used when considering microwave antennas, dishes, horns and the like. Can anyone explain to me why it is not appropriate for our proportionally small antennas? In particular are the Near Field effects of our small antennas over-estimated when we suppose them to be still significant out to lambda/(2Pi)? Is the latter formula only appropriate to professional antennas? 73, John, G4CNN email: computernetworks@go.com web page: http://www.g4cnn.f2s.com ___________________________________________________ GO.com Mail Get Your Free, Private E-mail at http://mail.go.com