Return-Path: Received: (qmail 25757 invoked from network); 25 Jun 1999 11:31:57 +0100 Received: from magnet.plus.net.uk (HELO magnet.force9.net) (195.166.128.26) by guiness.force9.net with SMTP; 25 Jun 1999 11:31:57 +0100 Received: (qmail 3064 invoked from network); 25 Jun 1999 10:32:12 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by magnet.plus.net.uk with SMTP; 25 Jun 1999 10:32:12 -0000 X-Priority: 3 Received: from majordom by post.thorcom.com with local (Exim 3.02 #1) id 10xT5a-00036e-00 for rsgb_lf_group-outgoing@blacksheep.org; Fri, 25 Jun 1999 11:20:06 +0100 X-MSMail-Priority: Normal Received: from mail2.hamilton-standard.com ([153.4.57.12]) by post.thorcom.com with esmtp (Exim 3.02 #1) id 10xT5Z-000368-00 for rsgb_lf_group@blacksheep.org; Fri, 25 Jun 1999 11:20:05 +0100 Received: from curly.hsd.utc.com by mail2.hamilton-standard.com; (8.9.1/) id GAA31258; Fri, 25 Jun 1999 06:25:14 -0400 (EDT) X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from umrsvr.hsd.utc.com by curly.hsd.utc.com; (5.65v3.2/1.1.8.2/27Feb96-0334PM) id AA10453; Fri, 25 Jun 1999 06:19:02 -0400 Received: from nmex01nt.hsd.utc.com by umrsvr.hsd.utc.com; (8.9.3/) id GAA13654; Fri, 25 Jun 1999 06:16:51 -0400 (EDT) Received: by nmex01nt.hsd.utc.com with Internet Mail Service (5.5.2448.0) id ; Fri, 25 Jun 1999 12:16:14 +0100 Message-ID: From: "Soegiono, Gamal" <"\"Soegiono, Gamal\"soegiono"@nmex01nt.hsd.utc.com> To: "'rsgb_lf_group@blacksheep.org'" <"\"'rsgb_lf_group@blacksheep.org'\"rsgb_lf_group"@blacksheep.org> Subject: LF: calculating ERP from field strength measurements Date: Fri, 25 Jun 1999 12:16:09 +0100 X-Mailer: Internet Mail Service (5.5.2448.0) Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org Sender: Content-type: text/plain; charset=windows-1252; format=flowed MIME-Version: 1.0 Content-transfer-encoding: 8bit Hello Toni, hello all, > From HB9ASB, JN36pt > 2. I have the means to measure the fieldstrenght (V/m) close to my > antenna (up to 100m away). Is there a reliable method to calculate the > ERP from the fieldstrenght? Yes and no. Yes - you could measure the field strength at a known distance from your antenna and calculate the ERP from that. The equation is: EIRP = [D * E]^2/30 EIRP = effective isotropic radiated power [W] D = Distance [m] E = electrical fieldstrength [V/m] No - you cannot deduce ERP from taking Fieldstrength at a distance of 100m, as you are still in the reactive near field of your antenna at 137kHz and equation above is valid ONLY in the far field. The nearfield/farfield transition is said to occur at 1/(2*PI) the wavelength. This would be 350m for 137kHz alias 2200m wavelength. As I saw in articles regarding nearfield/farfield transition distance, actual measurements should be made at distances which are a function of the radiators sizes related to the wavelength for the results to be reliable. Quater wave verticals require at least 1 wavelength distance. Half wave verticals require at least 1.5 wavelength distance. For shorter than 1/4 wavelength verticals the recommended measuring distance is 1 wavelength. If you cannot take measurements more remotely, then you may think of using numerical methods (NEC) to calculate near field values of electrical field strength. Assume a ERP value, go through the NEC procedure, if resultant field strength is lower than measured one, then increase assumed ERP etc. Best 73 Gamal Soegiono (SWL) 50N01 / 08E27 / JO 40 fa soegiono@nmex01nt.hsd.utc.com