Return-Path: Received: from mtain-dd01.r1000.mx.aol.com (mtain-dd01.r1000.mx.aol.com [172.29.64.141]) by air-md06.mail.aol.com (v129.4) with ESMTP id MAILINMD062-8b894d879c84256; Mon, 21 Mar 2011 14:44:20 -0400 Received: from post.thorcom.com (post.thorcom.com [195.171.43.25]) by mtain-dd01.r1000.mx.aol.com (Internet Inbound) with ESMTP id 6A2B3380000C6; Mon, 21 Mar 2011 14:44:18 -0400 (EDT) Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1Q1k47-00063f-6n for rs_out_1@blacksheep.org; Mon, 21 Mar 2011 18:42:55 +0000 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1Q1k45-00063W-UQ for rsgb_lf_group@blacksheep.org; Mon, 21 Mar 2011 18:42:53 +0000 Received: from imr-da02.mx.aol.com ([205.188.105.144]) by relay1.thorcom.net with esmtp (Exim 4.63) (envelope-from ) id 1Q1k43-0001PB-TC for rsgb_lf_group@blacksheep.org; Mon, 21 Mar 2011 18:42:53 +0000 Received: from imo-ma01.mx.aol.com (imo-ma01.mx.aol.com [64.12.78.136]) by imr-da02.mx.aol.com (8.14.1/8.14.1) with ESMTP id p2LIgoRG029450 for ; Mon, 21 Mar 2011 14:42:50 -0400 Received: from MarkusVester@aol.com by imo-ma01.mx.aol.com (mail_out_v42.9.) id l.fdc.df340f6 (34905) for ; Mon, 21 Mar 2011 14:42:46 -0400 (EDT) Received: from smtprly-db01.mx.aol.com (smtprly-db01.mx.aol.com [205.188.249.152]) by cia-da02.mx.aol.com (v129.9) with ESMTP id MAILCIADA022-5bc94d879c221ca; Mon, 21 Mar 2011 14:42:46 -0400 Received: from webmail-m142 (webmail-m142.sim.aol.com [149.174.9.10]) by smtprly-db01.mx.aol.com (v129.9) with ESMTP id MAILSMTPRLYDB018-5bc94d879c221ca; Mon, 21 Mar 2011 14:42:42 -0400 References: <59F87B1BA5D04A2F98902CF94C38DB30@JimPC> To: rsgb_lf_group@blacksheep.org Date: Mon, 21 Mar 2011 14:42:42 -0400 In-Reply-To: <59F87B1BA5D04A2F98902CF94C38DB30@JimPC> X-MB-Message-Source: WebUI MIME-Version: 1.0 From: Markus Vester X-MB-Message-Type: User X-Mailer: AOL Webmail 33426-STANDARD Received: from 194.138.39.54 by webmail-m142.sysops.aol.com (149.174.9.10) with HTTP (WebMailUI); Mon, 21 Mar 2011 14:42:41 -0400 Message-Id: <8CDB60EE4F8971C-1AD0-3512@webmail-m142.sysops.aol.com> X-Spam-Flag:NO X-AOL-SENDER: MarkusVester@aol.com X-Spam-Score: 1.2 (+) X-Spam-Report: autolearn=disabled,FORGED_AOL_TAGS=0.281,HTML_10_20=0.945,HTML_MESSAGE=0.001,UNPARSEABLE_RELAY=0.001 Subject: Re: LF: Re: Loop TX antennas at VLF? Content-Type: multipart/alternative; boundary="--------MB_8CDB60EE550AA90_1AD0_8159_webmail-m142.sysops.aol.com" X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.1 required=5.0 tests=FORGED_AOL_TAGS, HTML_FONTCOLOR_UNKNOWN,HTML_MESSAGE autolearn=no version=2.63 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-aol-global-disposition: G x-aol-sid: 3039ac1d408d4d879c822651 X-AOL-IP: 195.171.43.25 X-AOL-SPF: domain : blacksheep.org SPF : none ----------MB_8CDB60EE550AA90_1AD0_8159_webmail-m142.sysops.aol.com Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Dear Roger, Jim, and others, =20 regarding the ongoing "VLF loop vs Marconi" discussion, I just can't resis= t to add in my five cents... In principle, both electric and magnetic antennas follow the same scaling.= The Chu-Wheeler bandwidth limit basically states that the maximum possibl= e efficiency of a small antenna is proportional to Q times an effective vo= lume of the antenna, divided by wavelength cubed. So if similar Q-factors= can be achieved, both antenna types should be equally effective.=20 In practice, the Q factor of an electric antenna is limited by capacitivel= y coupled losses from the environment. On the other hand, a magnetic loop= is a low impedance device and less affected, but you will have to put muc= h more copper up into the air. For my small Maconi, I have measured an effective height of 9 m at LF, and= my estimate is that this is reduced to 7 m at VLF due to increased shunt= effects from nearby trees. Thus it would have a radiation resistance of= 7 milliohms at 9 kHz. The total loss resistance is about 700 ohms, leadin= g to an efficiency of 1E-7 (-70dB). About half of the resistance is locate= d in the 1.3 henry loading coil which has an unloaded Q of 220. It consist= s of 2.3 km of 0.4 mm copper wire, a mass of 2.6 kg. For a loop above ground, the radiation resistance would actually be double= d by the image current, giving 500 picoohms for the 10x10 m^2 example. To= achieve the same -70 dB efficiency as the above Marconi, total loss resis= tance would need to be less than 5 milliohms. Eddy current losses in the ground will scale as frequency squared, so I wo= uld expect little contribution from this effect at VLF. Capacitor ESR loss= es can be kept low by using several in parallel. We could avoid skin effec= t by multiple thin strands. Then efficiency ends up being proportional to= loop area times copper mass. In the example, we would need a cross sectio= n of about 140 mm^2, resulting in 50 kg of copper. Aluminium wire could be= a lighter and cheaper alternative. Of course it is always possible to trade efficiency for transmit power. Fo= r a Marconi, it is relatively easy to reach a voltage limit due to the ons= et of corona discharges (about 22 kV for 60 W input to my antenna). For a= loop, current density and wire heating would impose a different, typicall= y higher limit. As the antenna is likely to be built in an inhabited area, you may also wa= nt to consider that magnetic fields are not shielded very well by most hou= sing construction materials. They will also penetrate our bodies, with an= (at least theoretical) possibility of nerve stimulation in the immediate= vicinity of very large AC currents. Best regards, Markus (DF6NM) -----Urspr=C3=BCngliche Mitteilung-----=20 Von: James Moritz An: rsgb_lf_group@blacksheep.org Verschickt: Fr., 18. Mrz. 2011, 16:26 Thema: LF: Re: Loop TX antennas at VLF? Dear Roger, LF Group,=20 =20 > Just wondering if anyone has done the maths to work out what sort of ERP= =20 > could be expected at 8.97kHz with, say, 100W to a smallish loop antenna= in=20 > the garden?=20 =20 A 10m x 10m, 100m^2 loop of "thickish" 3mm dia solid wire would have a res= istance at 9kHz of roughly 0.1ohm. With 100W available, 32A antenna curren= t should be possible, assuming negligible tuning capacitor losses. Inducta= nce would be of the order of 40uH. A tuning capacitor of roughly 8uF would= be needed.=20 =20 The radiation resistance of an electrically small loop is:=20 =20 320 * pi^4 * A^2 / (lambda)^4, where A =3D area, lambda =3D wavelength=20 =20 for 100m^2 at 9kHz, Rrad is about 250 pico-ohms (!)=20 =20 The ERP is then 1.8 * I^2 * Rrad, about 0.45uW=20 =20 So pretty low, but with a bit bigger loop and a bit more power, it would= seem to be competitive with small verticals of a similar size. This is pe= rhaps mainly because of the serious losses present in loading coils that= people have been able to make for verticals, combined with high voltage= limitations of fairly short wire antennas, and high environmental losses= of various types also due to high electric fields. The voltage in this ex= ample would only be about 70V. So might be worth trying for "back garden"= experiments (assuming your antenna masts can support thick enought wire!)= , although I think it would not be competitive for bigger balloon/kite sup= ported vertical antennas.=20 =20 Cheers, Jim Moritz=20 73 de M0BMU =20 ----------MB_8CDB60EE550AA90_1AD0_8159_webmail-m142.sysops.aol.com Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset="utf-8"
Dear Roger, Jim, and others, 
 
regarding the ongoing "VLF loop vs Marconi" discussion, I just can't= resist to add in my five cents...
 
In principle, both electric and magnetic antennas follow the same sca= ling. The Chu-Wheeler bandwidth limit basically states that the maximum po= ssible efficiency of a small antenna is proportional to Q times an effecti= ve volume of the antenna, divided by wavelength cubed. So if similar Q-fac= tors can be achieved, both antenna types should be equally effective.
 
In practice, the Q factor of an electric antenna is limited by capaci= tively coupled losses from the environment. On the other hand, a magnetic= loop is a low impedance device and less affected, but you will have to pu= t much more copper up into the air.
 
For my small Maconi, I have measured an effective height of 9 m at LF= , and my estimate is that this is reduced to 7 m at VLF due to increased= shunt effects from nearby trees. Thus it would have a radiation resistanc= e of 7 milliohms at 9 kHz. The total loss resistance is about 700 ohms, le= ading to an efficiency of 1E-7 (-70dB). About half of the resistance is lo= cated in the 1.3 henry loading coil which has an unloaded Q of 220. It con= sists of 2.3 km of 0.4 mm copper wire, a mass of 2.6 kg.
 
For a loop above ground, the radiation resistance would actually be= doubled by the image current, giving 500 picoohms for the 10x10 m^2= example. To achieve the same -70 dB efficiency as the above Marconi, tota= l loss resistance would need to be less than 5 milliohms.
 
Eddy current losses in the ground will scale as frequency squared, so= I would expect little contribution from this effect at VLF. Capacitor ESR= losses can be kept low by using several in parallel. We could avoid skin= effect by multiple thin strands. Then efficiency ends up being proportion= al to loop area times copper mass. In the example, we would need a cross= section of about 140 mm^2, resulting in 50 kg of copper. Aluminium wire= could be a lighter and cheaper alternative.
 
Of course it is always possible to trade efficiency for transmit powe= r. For a Marconi, it is relatively easy to reach a voltage limit due to th= e onset of corona discharges (about 22 kV for 60 W input to my antenna).= For a loop, current density and wire heating would impose a different, ty= pically higher limit.
 
As the antenna is likely to be built in an inhabited area, you may al= so want to consider that magnetic fields are not shielded very well by mos= t housing construction materials. They will also penetrate our bodies, wit= h an (at least theoretical) possibility of nerve stimulation in the immedi= ate vicinity of very large AC currents.
 
Best regards,
Markus (DF6NM)

 
-----Urspr=C3=BCngliche Mitteilung-----
Von: James Moritz <james.moritz@btopenworld.com>
An: rsgb_lf_group@blacksheep.org
Verschickt: Fr., 18. Mrz. 2011, 16:26
Thema: LF: Re: Loop TX antennas at VLF?

Dear Roger, LF Group, 
 
> Just wondering if anyone has done the maths to work out what sort of= ERP 
> could be expected at 8.97kHz with, say, 100W to a smallish loop anten= na in 
> the garden? 
 
A 10m x 10m, 100m^2 loop of "thickish" 3mm dia solid wire would have a res= istance at 9kHz of roughly 0.1ohm. With 100W available, 32A antenna curren= t should be possible, assuming negligible tuning capacitor losses. Inducta= nce would be of the order of 40uH. A tuning capacitor of roughly 8uF would= be needed. 
 
The radiation resistance of an electrically small loop is: 
 
320 * pi^4 * A^2 / (lambda)^4, where A =3D area, lambda =3D wavelength&nbs= p;
 
for 100m^2 at 9kHz, Rrad is about 250 pico-ohms (!) 
 
The ERP is then 1.8 * I^2 * Rrad, about 0.45uW 
 
So pretty low, but with a bit bigger loop and a bit more power, it would= seem to be competitive with small verticals of a similar size. This is pe= rhaps mainly because of the serious losses present in loading coils that= people have been able to make for verticals, combined with high voltage= limitations of fairly short wire antennas, and high environmental losses= of various types also due to high electric fields. The voltage in this ex= ample would only be about 70V. So might be worth trying for "back garden"= experiments (assuming your antenna masts can support thick enought wire!)= , although I think it would not be competitive for bigger balloon/kite sup= ported vertical antennas. 
 
Cheers, Jim Moritz 
73 de M0BMU  
 ----------MB_8CDB60EE550AA90_1AD0_8159_webmail-m142.sysops.aol.com--