Envelope-to: dave@picks.force9.co.uk Delivery-date: Mon, 06 Feb 2006 07:42:29 +0000 Received: by ptb-mxcore01.plus.net with spam-scanned (PlusNet MXCore v2.00) id 1F6111-0006V2-AI for dave@picks.force9.co.uk; Mon, 06 Feb 2006 07:42:29 +0000 Received: from post.thorcom.com ([193.82.116.20]) by ptb-mxcore01.plus.net with esmtp (PlusNet MXCore v2.00) id 1F6110-0006UU-Um for dave@picks.force9.co.uk; Mon, 06 Feb 2006 07:42:27 +0000 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1F610k-0000PI-Sb for rs_out_1@blacksheep.org; Mon, 06 Feb 2006 07:42:10 +0000 Received: from [193.82.59.130] (helo=relay2.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1F610k-0000P9-9C for rsgb_lf_group@blacksheep.org; Mon, 06 Feb 2006 07:42:10 +0000 Received: from ams-iport-1.cisco.com ([144.254.224.140]) by relay2.thorcom.net with esmtp (Exim 4.51) id 1F61ks-0007Zg-Ik for rsgb_lf_group@blacksheep.org; Mon, 06 Feb 2006 08:29:51 +0000 Received: from ams-core-1.cisco.com ([144.254.224.150]) by ams-iport-1.cisco.com with ESMTP; 06 Feb 2006 08:40:43 +0100 Received: from cisco.com (mrwint.cisco.com [64.103.71.48]) by ams-core-1.cisco.com (8.12.10/8.12.6) with ESMTP id k167ee5i008280 for ; Mon, 6 Feb 2006 08:40:41 +0100 (MET) Received: from [10.61.80.27] (ams-clip-vpn-dhcp4124.cisco.com [10.61.80.27]) by cisco.com (8.8.8-Cisco List Logging/8.8.8) with ESMTP id HAA25954 for ; Mon, 6 Feb 2006 07:40:38 GMT Message-ID: <43E6FD76.4060304@g3ysx.org.uk> Date: Mon, 06 Feb 2006 07:40:38 +0000 From: Stewart Bryant User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.2) Gecko/20040804 Netscape/7.2 (ax) X-Accept-Language: en-us, en MIME-Version: 1.0 To: rsgb_lf_group@blacksheep.org References: <007001c62a34$97e808b0$2101a8c0@AUG2004> In-Reply-To: <007001c62a34$97e808b0$2101a8c0@AUG2004> Subject: Re: LF: {Spam?} Active_antennas Content-Type: text/html; charset=windows-1252 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-PN-SpamFiltered: by PlusNet MXCore (v2.00) Content-transfer-encoding: 8bit Isn't there also a shielding effect that needs to be considered?

Once the antenna is the highest object in the area, I can see
that extra height might be a marginal effect. However as the
antenna is lowered the field is shielded from it by the
surroundings. It's like lowering the antenna into a 5
sided Faraday box, with the open side much less than a
wavelength across.

- Stewart G3YSX

Walter Blanchard wrote:
The theory behind the "height gain" observed using small LF active antennas at different heights has to do with compression of the near-earth LF potential gradient caused by the grounded "mast" holding the antenna.
The essential bit is that there is a grounded connection between the antenna and receiver, which may be just the outer of the co-ax cable. It would be an interesting experiment to repeat the "height gain" experiment without any connection to ground. This could be done by building an active antenna with a little transmitter to re-radiate the received LF signal (on 2.4 GHz?) and poking it up using a fibreglass mast. If the theory is right then there wouldn't be any height gain. Might do it myself sometime but anyone else interested?
 
 Walter G3JKV.