Return-Path: Received: from mtain-dd05.r1000.mx.aol.com (mtain-dd05.r1000.mx.aol.com [172.29.64.145]) by air-mc01.mail.aol.com (v129.4) with ESMTP id MAILINMC012-a8464d885ed6c3; Tue, 22 Mar 2011 04:33:26 -0400 Received: from post.thorcom.com (post.thorcom.com [195.171.43.25]) by mtain-dd05.r1000.mx.aol.com (Internet Inbound) with ESMTP id 2D28C380000E4; Tue, 22 Mar 2011 04:33:24 -0400 (EDT) Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1Q1x0z-0000pv-Er for rs_out_1@blacksheep.org; Tue, 22 Mar 2011 08:32:33 +0000 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1Q1x0y-0000pm-P5 for rsgb_lf_group@blacksheep.org; Tue, 22 Mar 2011 08:32:32 +0000 Received: from out1.ip08ir2.opaltelecom.net ([62.24.128.244]) by relay1.thorcom.net with esmtp (Exim 4.63) (envelope-from ) id 1Q1x0v-0006Np-Ti for rsgb_lf_group@blacksheep.org; Tue, 22 Mar 2011 08:32:32 +0000 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AuAAAJv7h01cEYB5/2dsb2JhbACCYYFkj0YDg3YOjTF3iE2oUpE8hG13BIF3jmA X-IronPort-AV: E=Sophos;i="4.63,224,1299456000"; d="scan'208,217";a="484074073" Received: from unknown (HELO xphd97xgq27nyf) ([92.17.128.121]) by out1.ip08ir2.opaltelecom.net with SMTP; 22 Mar 2011 08:32:23 +0000 Message-ID: <000001cbe86b$a741e790$0401a8c0@xphd97xgq27nyf> From: "mal hamilton" To: References: <59F87B1BA5D04A2F98902CF94C38DB30@JimPC> <8CDB60EE4F8971C-1AD0-3512@webmail-m142.sysops.aol.com> Date: Mon, 21 Mar 2011 18:54:21 -0000 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2600.0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2600.0000 X-Spam-Score: 0.9 (/) X-Spam-Report: autolearn=disabled,DATE_IN_PAST_12_24=0.881,HTML_MESSAGE=0.001 Subject: Re: LF: Re: Loop TX antennas at VLF? Content-Type: multipart/alternative; boundary="----=_NextPart_000_002A_01CBE7F9.63D00040" X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.5 required=5.0 tests=DATE_IN_PAST_12_24, 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: 3039ac1d40914d885ed421cc X-AOL-IP: 195.171.43.25 X-AOL-SPF: domain : blacksheep.org SPF : none ------=_NextPart_000_002A_01CBE7F9.63D00040 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Markus On health grounds I will leave loops alone. Your closing paragraph ref= ers de mal/g3kev ----- Original Message -----=20 From: Markus Vester=20 To: rsgb_lf_group@blacksheep.org=20 Sent: Monday, March 21, 2011 6:42 PM Subject: Re: LF: Re: Loop TX antennas at VLF? Dear Roger, Jim, and others, =20 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 sc= aling. The Chu-Wheeler bandwidth limit basically states that the maxim= um possible efficiency of a small antenna is proportional to Q times= an effective volume of the antenna, divided by wavelength cubed. So= if similar Q-factors can be achieved, both antenna types should be eq= ually effective.=20 In practice, the Q factor of an electric antenna is limited by capac= itively coupled losses from the environment. On the other hand, a magn= etic loop is a low impedance device and less affected, but you will ha= ve to put 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 incr= eased shunt effects from nearby trees. Thus it would have a radiation= resistance of 7 milliohms at 9 kHz. The total loss resistance is abou= t 700 ohms, leading to an efficiency of 1E-7 (-70dB). About half of th= e resistance is located in the 1.3 henry loading coil which has an unl= oaded Q of 220. It consists 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,= total 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. Capaci= tor 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 proportional 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 pow= er. For a Marconi, it is relatively easy to reach a voltage limit due= to the 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, typically higher limit. As the antenna is likely to be built in an inhabited area, you may= also want to consider that magnetic fields are not shielded very well= by most housing construction materials. They will also penetrate our= bodies, with an (at least theoretical) possibility of nerve stimulati= on 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 an= tenna in=20 > the garden?=20 =20 A 10m x 10m, 100m^2 loop of "thickish" 3mm dia solid wire would have= a resistance at 9kHz of roughly 0.1ohm. With 100W available, 32A ante= nna current should be possible, assuming negligible tuning capacitor= losses. Inductance 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 waveleng= th=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 perhaps mainly because of the serious losses present in loadi= ng coils that people have been able to make for verticals, combined wi= th high voltage limitations of fairly short wire antennas, and high en= vironmental losses of various types also due to high electric fields.= The voltage in this example would only be about 70V. So might be wort= h trying for "back garden" experiments (assuming your antenna masts ca= n support thick enought wire!), although I think it would not be compe= titive for bigger balloon/kite supported vertical antennas.=20 =20 Cheers, Jim Moritz=20 73 de M0BMU =20 ------=_NextPart_000_002A_01CBE7F9.63D00040 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Markus
On health grounds I will leave loops alone. Yo= ur closing=20 paragraph refers
de mal/g3kev
 
----- Original Message -----
Sent: Monday, March 21, 2011= 6:42=20 PM
Subject: Re: LF: Re: Loop TX= antennas at=20 VLF?

Dear Roger, Jim, and others, 
 
regarding the ongoing "VLF loop vs Marconi" discussion, I just= can't=20 resist to add in my five cents...
 
In principle, both electric and magnetic antennas follow the sa= me=20 scaling. The Chu-Wheeler bandwidth limit basically states that the= maximum=20 possible efficiency of a small antenna is proportional to Q times an= effective=20 volume of the antenna, divided by wavelength cubed. So if similar Q-= factors=20 can be achieved, both antenna types should be equally effective.
 
In practice, the Q factor of an electric antenna is limited by= =20 capacitively coupled losses from the environment. On the other hand,= a=20 magnetic loop is a low impedance device and less affected, but you= will have=20 to put much more copper up into the air.
 
For my small Maconi, I have measured an effective height of 9= m at LF,=20 and my estimate is that this is reduced to 7 m at VLF due to increas= ed shunt=20 effects from nearby trees. Thus it would have a radiation resistance= of 7=20 milliohms at 9 kHz. The total loss resistance is about 700 ohms, lea= ding to an=20 efficiency of 1E-7 (-70dB). About half of the resistance is located= in the 1.3=20 henry loading coil which has an unloaded Q of 220. It consists of 2.= 3 km of=20 0.4 mm copper wire, a mass of 2.6 kg.
 
For a loop above ground, the radiation resistance would actuall= y be=20 doubled by the image current, giving 500 picoohms for the 10x10= m^2=20 example. To achieve the same -70 dB efficiency as the above Marconi,= total=20 loss resistance would need to be less than 5 milliohms.
 
Eddy current losses in the ground will scale as frequency squar= ed, so I=20 would expect little contribution from this effect at VLF. Capacitor= ESR losses=20 can be kept low by using several in parallel. We could avoid skin ef= fect by=20 multiple thin strands. Then efficiency ends up being proportional to= loop area=20 times copper mass. In the example, we would need a cross section of= about 140=20 mm^2, resulting in 50 kg of copper. Aluminium wire could be a lighte= r and=20 cheaper alternative.
 
Of course it is always possible to trade efficiency for transmi= t power.=20 For a Marconi, it is relatively easy to reach a voltage limit due to= the onset=20 of corona discharges (about 22 kV for 60 W input to my antenna). For= a loop,=20 current density and wire heating would impose a different, typically= higher=20 limit.
 
As the antenna is likely to be built in an inhabited area, you= may also=20 want to consider that magnetic fields are not shielded very well by= most=20 housing construction materials. They will also penetrate our bodies,= with an=20 (at least theoretical) possibility of nerve stimulation in the immed= iate=20 vicinity of very large AC currents.
 
Best regards,
Markus (DF6NM)

 
-----Urspr=C3=BCngliche=20 Mitteilung-----
Von: James Moritz=20 <james.moritz@btopenworld.com>
An:=20 rsgb_lf_group@blacksheep.org
Verschickt: Fr., 18. Mrz. 2011,=20 16:26
Thema: LF: Re: Loop TX antennas at VLF?

Dear=20 Roger, LF Group, 
 
> Just wondering if anyone ha= s done the=20 maths to work out what sort of ERP 
> could be expected= at 8.97kHz=20 with, say, 100W to a smallish loop antenna in 
> the=20 garden? 
 
A 10m x 10m, 100m^2 loop of "thickish" 3m= m dia=20 solid wire would have a resistance at 9kHz of roughly 0.1ohm. With= 100W=20 available, 32A antenna current should be possible, assuming negligib= le tuning=20 capacitor losses. Inductance would be of the order of 40uH. A tuning= capacitor=20 of roughly 8uF would be needed. 
 
The radiation res= istance of=20 an electrically small loop is: 
 
320 * pi^4 * A^2= /=20 (lambda)^4, where A =3D area, lambda =3D wavelength 
 <= BR>for 100m^2=20 at 9kHz, Rrad is about 250 pico-ohms (!) 
 
The ERP= is then=20 1.8 * I^2 * Rrad, about 0.45uW 
 
So pretty low, but= with a=20 bit bigger loop and a bit more power, it would seem to be competitiv= e with=20 small verticals of a similar size. This is perhaps mainly because of= the=20 serious losses present in loading coils that people have been able= to make for=20 verticals, combined with high voltage limitations of fairly short wi= re=20 antennas, and high environmental losses of various types also due to= high=20 electric fields. The voltage in this example would only be about 70V= . So might=20 be worth trying for "back garden" experiments (assuming your antenna= masts can=20 support thick enought wire!), although I think it would not be compe= titive for=20 bigger balloon/kite supported vertical antennas. 
 
= Cheers,=20 Jim Moritz 
73 de M0BMU  
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