Return-Path: Received: (qmail 16528 invoked from network); 9 Jan 2000 18:43:59 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by bells.core.plus.net.uk with SMTP; 9 Jan 2000 18:43:59 -0000 Received: from majordom by post.thorcom.com with local (Exim 3.02 #1) id 127NC8-0001yl-00 for rsgb_lf_group-outgoing@blacksheep.org; Sun, 09 Jan 2000 18:36:04 +0000 Received: from irwell.zetnet.co.uk ([194.247.47.48] ident=root) by post.thorcom.com with esmtp (Exim 3.02 #1) id 127NC7-0001yg-00 for rsgb_lf_group@blacksheep.org; Sun, 09 Jan 2000 18:36:03 +0000 Received: from oemcomputer (man-010.dialup.zetnet.co.uk [194.247.41.12]) by irwell.zetnet.co.uk (8.9.3/8.9.3/Debian/GNU) with SMTP id SAA31163 for ; Sun, 9 Jan 2000 18:35:26 GMT Message-ID: <000001bf5ace$a807a7a0$0c29f7c2@oemcomputer> From: "John Rabson" To: rsgb_lf_group@blacksheep.org References: <29188.200001071140@gemini> <3875F52A.3AF2D67@bellatlantic.net> <00dc01bf594c$7f1a9ba0$0600a8c0@main> <3876554B.72D98BF3@bellatlantic.net> <3877AF3D.47A84B20@netscapeonline.co.uk> <3877A137.5F0D@xtra.co.nz> Subject: Re: LF: Re: AMRAD Antenna ? Date: Sun, 9 Jan 2000 10:12:18 -0000 MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: 8bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.00.2615.200 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: We use this technique of grounding both ends of a wire in Cave Radio work as it enables us to communicate to a greater depth than does the conventional method, which uses resonant loops a metre or two in diameter. It has also been used on 73 kHz at a location in Oxfordshire - the station was heard at a distance of 250 kilometres. This technique has been referred to as an earth bipole. I understand from Graham Naylor that during the recent cave rescue operation in south-western France, where this technique was used, the rescue party developed a new variation at the surface station. Instead of feeding the wire in a symmetrical manner at its centre, the two halves of the bipole were strapped together and fed against an earth connection near the mid-point of the wire. This seemed to produce equally good results as far as underground communication was concerned and produced a useful reduction in the level of interference from LORAN. This technique has been dubbed the earth tripole. 73 de John Rabson G3PAI ----- Original Message ----- From: vernall To: Sent: Saturday, January 08, 2000 8:42 PM Subject: Re: LF: Re: AMRAD Antenna ? > Hi all, > > The "dipole" with outer ends each grounded is used by some ZL stations > for LF receiving. It has been called a ground loop, after the name > given to it by Andrew ZL2BBJ in a local article. The LF ground path > goes "underground" as skin depth is many metres at LF in typical soil, > so the loop area is significantly more than the visible area above > ground. > > The ground path (actually multiple paths) between "dipole" ends is lossy > compared to copper wire, so the circulating current is generally lower > than if ground wires were used, however the aperture is greater if no > ground conductors are used, and aperture is what is good for receiving > (the noise figure is basically set by signal to external QRN ratio). In > use for transmitting, the ground loop can be expected to be fairly lossy > compared to higher Q antennas (DX QSOs need absolute radiated power). > > Bob ZL2CA > > g3kev wrote: > > > > Hello All. > > Your comment about your antenna maybe operating like a LOOP is probably correct. > > I would suggest it is performing like a grounded quad. Similar systems used on 160 > > and 80 metres where one cannot get a full size quad up. > > At 1600 ft long and 50 ft high, think that is what you said, its natural resonant > > frequency used as a grounded quad would be 296 khz. I expect there is some sort of > > loading to resonate on 137 khz. > > It would be interesting to check whether it radiates better as a loop or a long wire > > with the grounded far end disconnected. Judging by experiments in the past using > > loops v verticlals, I think the vertical/long wire approach would be better for low > > angle. > > I have tried a variety of loops in the past for 160 metres ie 40 m loop resonated on > > 160 and although it was quieter than my full size quarter wave on 160, it was not as > > sensitive and did not pull in the long haul low angle dx, in fact there were signals > > that I could not hear that I was able to copy solid on the vertical, although at > > times probably noiser. Small loops for short/medium distances of several hundred > > miles are acceptable but for low angle long haul poor on mf/hf. > > A full size loop ie quad or delta etc resonant at the operating frequency and > > preferably at least a quarter wave above ground is a totally different story. > > In the UK stations using loops have poor signals compared to those using verticals, > > even low verticals heights with top loading. A couple of stations that have been > > using loops have changed over to verticals and although not very high made a hugh > > difference to their signals received at my qth. > > The so called long wire, just a few feet above ground and fed with a drop wire is > > really a top loaded vertical or inv L. > > The above comments are a result of experiments and observations, especially on 137 > > khz and 1800 khz bands > > 73 de Mal/G3KEV > > Andre' Kesteloot wrote: > > > > > Wooops, > > > I guess I did not express myself quite clearly enough. > > > The far end of the wire terminates in a field , (and specifically near a pond) > > > visited by many cows. In order to avoid any possible unpleasantness (wire > > > falling on the ground if broken by the wind, etc.), we decided to ground that > > > end. > > > It may well be that the whole thing operates as a loop of sorts, as there is a > > > non-zero resistance between the two grounds (the one at the Tx site, and the one > > > at the pond end) > > > 73 > > > Andre' > > > > > > Dave wrote: > > > > > > > Surely the Voltage gradient is just the same but the other way round? High > > > > current point at the earthed end and high Voltage point at the TX site as it > > > > is about a quarter wave.... > > > > The "earthed at the far end" idea has been used with topband antennas for > > > > years in order to get the current into the vertical drop. > > > > > > > > 73 Dave G3YXM. > > > >