Return-Path: Received: (qmail 2904 invoked from network); 2 Jun 2000 17:53:11 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by teachers.core.plus.net.uk with SMTP; 2 Jun 2000 17:53:11 -0000 Received: from majordom by post.thorcom.com with local (Exim 3.02 #1) id 12xvRo-0002T5-00 for rsgb_lf_group-outgoing@blacksheep.org; Fri, 02 Jun 2000 18:41:28 +0100 Received: from helios.herts.ac.uk ([147.197.200.2]) by post.thorcom.com with esmtp (Exim 3.02 #1) id 12xvRn-0002T0-00 for rsgb_lf_group@blacksheep.org; Fri, 02 Jun 2000 18:41:27 +0100 X-Priority: 3 X-MSMail-Priority: Normal Received: from [147.197.200.44] (helo=gemini) by helios.herts.ac.uk with esmtp (Exim 3.11 #1) id 12xvRk-0006Bq-00 for rsgb_lf_group@blacksheep.org; Fri, 02 Jun 2000 18:41:24 +0100 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Message-ID: <19213.200006021741@gemini> From: "James Moritz" Organization: University of Hertfordshire To: rsgb_lf_group@blacksheep.org Date: Fri, 2 Jun 2000 18:48:27 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 8bit Subject: LF: Re: Aerial tests X-Mailer: Pegasus Mail for Win32 (v3.11) Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Dear LF Group, Some comments on G3JKV's recent E-mail - I calculated the ERP on the basis of the measured antenna current and estimated radiation resistance of the two antennas. Part of the reasoning behind this was that the antenna efficiency would not be an issue; the radiated power is just I squared times the radiation resistance, and the calculated radiation resistance is a function only of the antenna geometry, and not it's losses. In principle at least, the only result of improved efficiency due to the Decca earth mat would be to reduce the amount of transmitter power required to produce the measured value of antenna current. So antenna efficiency did not actually enter into the calculation of ERP, and the presence or absence of the Decca earth would not have affected the result of the calculation. Having said that, clearly the ERP calculations are wrong, because there was a significant difference in signal strength, and so by definition ERP, between the two antennas, where the calculations said they should be the same. Unfortunately, my field strength measuring equipment is not accurate enough to say for certain that this was because the small antenna was producing more ERP than it should, or that the Decca antenna was producing less than it should. However, it is probably easier to believe the former, since the assumptions on which the calculations are based are more nearly met by the Decca antenna than the small inverted L. I don't know if this experiment has much to tell us about the presence or absence of horizontally polarised signals; however, the ferrite rod antenna I used to measure field strengths showed the difference in field strength of roughly 4dB observed by nearly everyone else, and should not have been sensitive to horizontally polarised signals (ie. vertical H field). Cheers, Jim Moritz 73 de M0BMU