Return-Path: Received: (qmail 29657 invoked from network); 25 Aug 1999 01:19:27 +0100 Received: from magnus.plus.net.uk (HELO magnus.force9.net) (195.166.128.27) by guiness.force9.net with SMTP; 25 Aug 1999 01:19:27 +0100 Received: (qmail 26323 invoked from network); 25 Aug 1999 00:17:55 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by magnus.plus.net.uk with SMTP; 25 Aug 1999 00:17:55 -0000 Received: from majordom by post.thorcom.com with local (Exim 3.02 #1) id 11JQe9-0000RV-00 for rsgb_lf_group-outgoing@blacksheep.org; Wed, 25 Aug 1999 01:10:33 +0100 X-Priority: 3 X-MSMail-Priority: Normal Received: from mta1-rme.xtra.co.nz ([203.96.92.1]) by post.thorcom.com with esmtp (Exim 3.02 #1) id 11JQe7-0000Q7-00 for rsgb_lf_group@blacksheep.org; Wed, 25 Aug 1999 01:10:32 +0100 Received: from [203.96.101.230] by mta1-rme.xtra.co.nz (InterMail v4.01.01.00 201-229-111) with SMTP id <19990824235603.UORW2529197.mta1-rme@[203.96.101.230]> for ; Wed, 25 Aug 1999 11:56:03 +1200 Message-ID: <37C32E50.16E3@xtra.co.nz> X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Date: Wed, 25 Aug 1999 11:44:16 +1200 From: "vernall" X-Mailer: Mozilla 3.01C-XTRA (Win95; I) MIME-Version: 1.0 To: rsgb_lf_group@blacksheep.org Subject: Re: LF: Multiple Tuned Vertical References: <199908242207.IAA12994@godzilla.zeta.org.au> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Here is my two cents worth ... done on the fly ... The multiple feedpoints in a multiple tuned antenna give an apparent increase in the real component (resistance) by transformer action (currents divide by the number of separate vertical feeds). However, that does not alter the basic radiation characteristic of THE SYSTEM AS A WHOLE. The radiation resistance of the whole system is still related to overall physical dimensions. Multiple tuning is just a "trick" to get an apparent increase in radiation resistance, but it does not trick the laws of physics! Where gains could be had is with reduced current density at the earth mat around each feedpoint. For a given resistivity earth mat, the power loss is I squared R. As the I values are divided according to vertical feeds, then the current densities and I squared R losses are all much lower than having one feedpoint for the given antenna structure, taking full current. Reducing the ground loss component of an electrically small antenna is an effective way of improving gain (as the radiation resistance is swamped by loss resistance). In an electrically large system (like commercial systems) running high power, the multiple feeds, each with earth mats, disperse the ground current much better than for a single feedpoint system. I have read somewhere that there have been ground systems that have literally "burnt out" trying to work with applied high power. In summary, for a typical amateur system, size limited, I do not see any advantages in trying multiple tunes systems. Neither it seems do the commercial NDB users, where "Marconi T" LF antennas are standard installations. They do however invest in rather good ground radial systems .... Regards, Bob ZL2CA